packages/flutter/lib/src/material/scaffold.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:collection';
import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/gestures.dart' show DragStartBehavior;

import 'app_bar.dart';
import 'bottom_sheet.dart';
import 'button_bar.dart';
import 'colors.dart';
import 'divider.dart';
import 'drawer.dart';
import 'flexible_space_bar.dart';
import 'floating_action_button.dart';
import 'floating_action_button_location.dart';
import 'material.dart';
import 'snack_bar.dart';
import 'snack_bar_theme.dart';
import 'theme.dart';
import 'theme_data.dart';

// Examples can assume:
// TabController tabController;
// void setState(VoidCallback fn) { }
// String appBarTitle;
// int tabCount;
// TickerProvider tickerProvider;

const FloatingActionButtonLocation _kDefaultFloatingActionButtonLocation = FloatingActionButtonLocation.endFloat;
const FloatingActionButtonAnimator _kDefaultFloatingActionButtonAnimator = FloatingActionButtonAnimator.scaling;

// When the top of the BottomSheet crosses this threshold, it will start to
// shrink the FAB and show a scrim.
const double _kBottomSheetDominatesPercentage = 0.3;
const double _kMinBottomSheetScrimOpacity = 0.1;
const double _kMaxBottomSheetScrimOpacity = 0.6;

enum _ScaffoldSlot {
  body,
  appBar,
  bodyScrim,
  bottomSheet,
  snackBar,
  persistentFooter,
  bottomNavigationBar,
  floatingActionButton,
  drawer,
  endDrawer,
  statusBar,
}

/// The geometry of the [Scaffold] after all its contents have been laid out
/// except the [FloatingActionButton].
///
/// The [Scaffold] passes this pre-layout geometry to its
/// [FloatingActionButtonLocation], which produces an [Offset] that the
/// [Scaffold] uses to position the [FloatingActionButton].
///
/// For a description of the [Scaffold]'s geometry after it has
/// finished laying out, see the [ScaffoldGeometry].
@immutable
class ScaffoldPrelayoutGeometry {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const ScaffoldPrelayoutGeometry({
    @required this.bottomSheetSize,
    @required this.contentBottom,
    @required this.contentTop,
    @required this.floatingActionButtonSize,
    @required this.minInsets,
    @required this.scaffoldSize,
    @required this.snackBarSize,
    @required this.textDirection,
  });

  /// The [Size] of [Scaffold.floatingActionButton].
  ///
  /// If [Scaffold.floatingActionButton] is null, this will be [Size.zero].
  final Size floatingActionButtonSize;

  /// The [Size] of the [Scaffold]'s [BottomSheet].
  ///
  /// If the [Scaffold] is not currently showing a [BottomSheet],
  /// this will be [Size.zero].
  final Size bottomSheetSize;

  /// The vertical distance from the Scaffold's origin to the bottom of
  /// [Scaffold.body].
  ///
  /// This is useful in a [FloatingActionButtonLocation] designed to
  /// place the [FloatingActionButton] at the bottom of the screen, while
  /// keeping it above the [BottomSheet], the [Scaffold.bottomNavigationBar],
  /// or the keyboard.
  ///
  /// The [Scaffold.body] is laid out with respect to [minInsets] already. This
  /// means that a [FloatingActionButtonLocation] does not need to factor in
  /// [minInsets.bottom] when aligning a [FloatingActionButton] to
  /// [contentBottom].
  final double contentBottom;

  /// The vertical distance from the [Scaffold]'s origin to the top of
  /// [Scaffold.body].
  ///
  /// This is useful in a [FloatingActionButtonLocation] designed to
  /// place the [FloatingActionButton] at the top of the screen, while
  /// keeping it below the [Scaffold.appBar].
  ///
  /// The [Scaffold.body] is laid out with respect to [minInsets] already. This
  /// means that a [FloatingActionButtonLocation] does not need to factor in
  /// [minInsets.top] when aligning a [FloatingActionButton] to [contentTop].
  final double contentTop;

  /// The minimum padding to inset the [FloatingActionButton] by for it
  /// to remain visible.
  ///
  /// This value is the result of calling [MediaQuery.padding] in the
  /// [Scaffold]'s [BuildContext],
  /// and is useful for insetting the [FloatingActionButton] to avoid features like
  /// the system status bar or the keyboard.
  ///
  /// If [Scaffold.resizeToAvoidBottomInset] is set to false, [minInsets.bottom]
  /// will be 0.0.
  final EdgeInsets minInsets;

  /// The [Size] of the whole [Scaffold].
  ///
  /// If the [Size] of the [Scaffold]'s contents is modified by values such as
  /// [Scaffold.resizeToAvoidBottomInset] or the keyboard opening, then the
  /// [scaffoldSize] will not reflect those changes.
  ///
  /// This means that [FloatingActionButtonLocation]s designed to reposition
  /// the [FloatingActionButton] based on events such as the keyboard popping
  /// up should use [minInsets] to make sure that the [FloatingActionButton] is
  /// inset by enough to remain visible.
  ///
  /// See [minInsets] and [MediaQuery.padding] for more information on the appropriate
  /// insets to apply.
  final Size scaffoldSize;

  /// The [Size] of the [Scaffold]'s [SnackBar].
  ///
  /// If the [Scaffold] is not showing a [SnackBar], this will be [Size.zero].
  final Size snackBarSize;

  /// The [TextDirection] of the [Scaffold]'s [BuildContext].
  final TextDirection textDirection;
}

/// A snapshot of a transition between two [FloatingActionButtonLocation]s.
///
/// [ScaffoldState] uses this to seamlessly change transition animations
/// when a running [FloatingActionButtonLocation] transition is interrupted by a new transition.
@immutable
class _TransitionSnapshotFabLocation extends FloatingActionButtonLocation {

  const _TransitionSnapshotFabLocation(this.begin, this.end, this.animator, this.progress);

  final FloatingActionButtonLocation begin;
  final FloatingActionButtonLocation end;
  final FloatingActionButtonAnimator animator;
  final double progress;

  @override
  Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
    return animator.getOffset(
      begin: begin.getOffset(scaffoldGeometry),
      end: end.getOffset(scaffoldGeometry),
      progress: progress,
    );
  }

  @override
  String toString() {
    return '${objectRuntimeType(this, '_TransitionSnapshotFabLocation')}(begin: $begin, end: $end, progress: $progress)';
  }
}

/// Geometry information for [Scaffold] components after layout is finished.
///
/// To get a [ValueNotifier] for the scaffold geometry of a given
/// [BuildContext], use [Scaffold.geometryOf].
///
/// The ScaffoldGeometry is only available during the paint phase, because
/// its value is computed during the animation and layout phases prior to painting.
///
/// For an example of using the [ScaffoldGeometry], see the [BottomAppBar],
/// which uses the [ScaffoldGeometry] to paint a notch around the
/// [FloatingActionButton].
///
/// For information about the [Scaffold]'s geometry that is used while laying
/// out the [FloatingActionButton], see [ScaffoldPrelayoutGeometry].
@immutable
class ScaffoldGeometry {
  /// Create an object that describes the geometry of a [Scaffold].
  const ScaffoldGeometry({
    this.bottomNavigationBarTop,
    this.floatingActionButtonArea,
  });

  /// The distance from the [Scaffold]'s top edge to the top edge of the
  /// rectangle in which the [Scaffold.bottomNavigationBar] bar is laid out.
  ///
  /// Null if [Scaffold.bottomNavigationBar] is null.
  final double bottomNavigationBarTop;

  /// The [Scaffold.floatingActionButton]'s bounding rectangle.
  ///
  /// This is null when there is no floating action button showing.
  final Rect floatingActionButtonArea;

  ScaffoldGeometry _scaleFloatingActionButton(double scaleFactor) {
    if (scaleFactor == 1.0)
      return this;

    if (scaleFactor == 0.0) {
      return ScaffoldGeometry(
        bottomNavigationBarTop: bottomNavigationBarTop,
      );
    }

    final Rect scaledButton = Rect.lerp(
      floatingActionButtonArea.center & Size.zero,
      floatingActionButtonArea,
      scaleFactor,
    );
    return copyWith(floatingActionButtonArea: scaledButton);
  }

  /// Creates a copy of this [ScaffoldGeometry] but with the given fields replaced with
  /// the new values.
  ScaffoldGeometry copyWith({
    double bottomNavigationBarTop,
    Rect floatingActionButtonArea,
  }) {
    return ScaffoldGeometry(
      bottomNavigationBarTop: bottomNavigationBarTop ?? this.bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonArea ?? this.floatingActionButtonArea,
    );
  }
}

class _ScaffoldGeometryNotifier extends ChangeNotifier implements ValueListenable<ScaffoldGeometry> {
  _ScaffoldGeometryNotifier(this.geometry, this.context)
    : assert (context != null);

  final BuildContext context;
  double floatingActionButtonScale;
  ScaffoldGeometry geometry;

  @override
  ScaffoldGeometry get value {
    assert(() {
      final RenderObject renderObject = context.findRenderObject();
      if (renderObject == null || !renderObject.owner.debugDoingPaint)
        throw FlutterError(
            'Scaffold.geometryOf() must only be accessed during the paint phase.\n'
            'The ScaffoldGeometry is only available during the paint phase, because '
            'its value is computed during the animation and layout phases prior to painting.'
        );
      return true;
    }());
    return geometry._scaleFloatingActionButton(floatingActionButtonScale);
  }

  void _updateWith({
    double bottomNavigationBarTop,
    Rect floatingActionButtonArea,
    double floatingActionButtonScale,
  }) {
    this.floatingActionButtonScale = floatingActionButtonScale ?? this.floatingActionButtonScale;
    geometry = geometry.copyWith(
      bottomNavigationBarTop: bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonArea,
    );
    notifyListeners();
  }
}

// Used to communicate the height of the Scaffold's bottomNavigationBar and
// persistentFooterButtons to the LayoutBuilder which builds the Scaffold's body.
//
// Scaffold expects a _BodyBoxConstraints to be passed to the _BodyBuilder
// widget's LayoutBuilder, see _ScaffoldLayout.performLayout(). The BoxConstraints
// methods that construct new BoxConstraints objects, like copyWith() have not
// been overridden here because we expect the _BodyBoxConstraintsObject to be
// passed along unmodified to the LayoutBuilder. If that changes in the future
// then _BodyBuilder will assert.
class _BodyBoxConstraints extends BoxConstraints {
  const _BodyBoxConstraints({
    double minWidth = 0.0,
    double maxWidth = double.infinity,
    double minHeight = 0.0,
    double maxHeight = double.infinity,
    @required this.bottomWidgetsHeight,
    @required this.appBarHeight,
  }) : assert(bottomWidgetsHeight != null),
       assert(bottomWidgetsHeight >= 0),
       assert(appBarHeight != null),
       assert(appBarHeight >= 0),
       super(minWidth: minWidth, maxWidth: maxWidth, minHeight: minHeight, maxHeight: maxHeight);

  final double bottomWidgetsHeight;
  final double appBarHeight;

  // RenderObject.layout() will only short-circuit its call to its performLayout
  // method if the new layout constraints are not == to the current constraints.
  // If the height of the bottom widgets has changed, even though the constraints'
  // min and max values have not, we still want performLayout to happen.
  @override
  bool operator ==(Object other) {
    if (super != other)
      return false;
    return other is _BodyBoxConstraints
        && other.bottomWidgetsHeight == bottomWidgetsHeight
        && other.appBarHeight == appBarHeight;
  }

  @override
  int get hashCode {
    return hashValues(super.hashCode, bottomWidgetsHeight, appBarHeight);
  }
}

// Used when Scaffold.extendBody is true to wrap the scaffold's body in a MediaQuery
// whose padding accounts for the height of the bottomNavigationBar and/or the
// persistentFooterButtons.
//
// The bottom widgets' height is passed along via the _BodyBoxConstraints parameter.
// The constraints parameter is constructed in_ScaffoldLayout.performLayout().
class _BodyBuilder extends StatelessWidget {
  const _BodyBuilder({
    Key key,
    @required this.extendBody,
    @required this.extendBodyBehindAppBar,
    @required this.body,
  }) : assert(extendBody != null),
       assert(extendBodyBehindAppBar != null),
       assert(body != null),
       super(key: key);

  final Widget body;
  final bool extendBody;
  final bool extendBodyBehindAppBar;

  @override
  Widget build(BuildContext context) {
    if (!extendBody && !extendBodyBehindAppBar)
      return body;

    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        final _BodyBoxConstraints bodyConstraints = constraints as _BodyBoxConstraints;
        final MediaQueryData metrics = MediaQuery.of(context);

        final double bottom = extendBody
          ? math.max(metrics.padding.bottom, bodyConstraints.bottomWidgetsHeight)
          : metrics.padding.bottom;

        final double top = extendBodyBehindAppBar
          ? math.max(metrics.padding.top, bodyConstraints.appBarHeight)
          : metrics.padding.top;

        return MediaQuery(
          data: metrics.copyWith(
            padding: metrics.padding.copyWith(
              top: top,
              bottom: bottom,
            ),
          ),
          child: body,
        );
      },
    );
  }
}

class _ScaffoldLayout extends MultiChildLayoutDelegate {
  _ScaffoldLayout({
    @required this.minInsets,
    @required this.textDirection,
    @required this.geometryNotifier,
    // for floating action button
    @required this.previousFloatingActionButtonLocation,
    @required this.currentFloatingActionButtonLocation,
    @required this.floatingActionButtonMoveAnimationProgress,
    @required this.floatingActionButtonMotionAnimator,
    @required this.isSnackBarFloating,
    @required this.extendBody,
    @required this.extendBodyBehindAppBar,
  }) : assert(minInsets != null),
       assert(textDirection != null),
       assert(geometryNotifier != null),
       assert(previousFloatingActionButtonLocation != null),
       assert(currentFloatingActionButtonLocation != null),
       assert(extendBody != null),
       assert(extendBodyBehindAppBar != null);

  final bool extendBody;
  final bool extendBodyBehindAppBar;
  final EdgeInsets minInsets;
  final TextDirection textDirection;
  final _ScaffoldGeometryNotifier geometryNotifier;

  final FloatingActionButtonLocation previousFloatingActionButtonLocation;
  final FloatingActionButtonLocation currentFloatingActionButtonLocation;
  final double floatingActionButtonMoveAnimationProgress;
  final FloatingActionButtonAnimator floatingActionButtonMotionAnimator;

  final bool isSnackBarFloating;

  @override
  void performLayout(Size size) {
    final BoxConstraints looseConstraints = BoxConstraints.loose(size);

    // This part of the layout has the same effect as putting the app bar and
    // body in a column and making the body flexible. What's different is that
    // in this case the app bar appears _after_ the body in the stacking order,
    // so the app bar's shadow is drawn on top of the body.

    final BoxConstraints fullWidthConstraints = looseConstraints.tighten(width: size.width);
    final double bottom = size.height;
    double contentTop = 0.0;
    double bottomWidgetsHeight = 0.0;
    double appBarHeight = 0.0;

    if (hasChild(_ScaffoldSlot.appBar)) {
      appBarHeight = layoutChild(_ScaffoldSlot.appBar, fullWidthConstraints).height;
      contentTop = extendBodyBehindAppBar ? 0.0 : appBarHeight;
      positionChild(_ScaffoldSlot.appBar, Offset.zero);
    }

    double bottomNavigationBarTop;
    if (hasChild(_ScaffoldSlot.bottomNavigationBar)) {
      final double bottomNavigationBarHeight = layoutChild(_ScaffoldSlot.bottomNavigationBar, fullWidthConstraints).height;
      bottomWidgetsHeight += bottomNavigationBarHeight;
      bottomNavigationBarTop = math.max(0.0, bottom - bottomWidgetsHeight);
      positionChild(_ScaffoldSlot.bottomNavigationBar, Offset(0.0, bottomNavigationBarTop));
    }

    if (hasChild(_ScaffoldSlot.persistentFooter)) {
      final BoxConstraints footerConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: math.max(0.0, bottom - bottomWidgetsHeight - contentTop),
      );
      final double persistentFooterHeight = layoutChild(_ScaffoldSlot.persistentFooter, footerConstraints).height;
      bottomWidgetsHeight += persistentFooterHeight;
      positionChild(_ScaffoldSlot.persistentFooter, Offset(0.0, math.max(0.0, bottom - bottomWidgetsHeight)));
    }

    // Set the content bottom to account for the greater of the height of any
    // bottom-anchored material widgets or of the keyboard or other
    // bottom-anchored system UI.
    final double contentBottom = math.max(0.0, bottom - math.max(minInsets.bottom, bottomWidgetsHeight));

    if (hasChild(_ScaffoldSlot.body)) {
      double bodyMaxHeight = math.max(0.0, contentBottom - contentTop);

      if (extendBody) {
        bodyMaxHeight += bottomWidgetsHeight;
        bodyMaxHeight = bodyMaxHeight.clamp(0.0, looseConstraints.maxHeight - contentTop).toDouble();
        assert(bodyMaxHeight <= math.max(0.0, looseConstraints.maxHeight - contentTop));
      }

      final BoxConstraints bodyConstraints = _BodyBoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: bodyMaxHeight,
        bottomWidgetsHeight: extendBody ? bottomWidgetsHeight : 0.0,
        appBarHeight: appBarHeight,
      );
      layoutChild(_ScaffoldSlot.body, bodyConstraints);
      positionChild(_ScaffoldSlot.body, Offset(0.0, contentTop));
    }

    // The BottomSheet and the SnackBar are anchored to the bottom of the parent,
    // they're as wide as the parent and are given their intrinsic height. The
    // only difference is that SnackBar appears on the top side of the
    // BottomNavigationBar while the BottomSheet is stacked on top of it.
    //
    // If all three elements are present then either the center of the FAB straddles
    // the top edge of the BottomSheet or the bottom of the FAB is
    // kFloatingActionButtonMargin above the SnackBar, whichever puts the FAB
    // the farthest above the bottom of the parent. If only the FAB is has a
    // non-zero height then it's inset from the parent's right and bottom edges
    // by kFloatingActionButtonMargin.

    Size bottomSheetSize = Size.zero;
    Size snackBarSize = Size.zero;
    if (hasChild(_ScaffoldSlot.bodyScrim)) {
      final BoxConstraints bottomSheetScrimConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: contentBottom,
      );
      layoutChild(_ScaffoldSlot.bodyScrim, bottomSheetScrimConstraints);
      positionChild(_ScaffoldSlot.bodyScrim, Offset.zero);
    }

    // Set the size of the SnackBar early if the behavior is fixed so
    // the FAB can be positioned correctly.
    if (hasChild(_ScaffoldSlot.snackBar) && !isSnackBarFloating) {
      snackBarSize = layoutChild(_ScaffoldSlot.snackBar, fullWidthConstraints);
    }

    if (hasChild(_ScaffoldSlot.bottomSheet)) {
      final BoxConstraints bottomSheetConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: math.max(0.0, contentBottom - contentTop),
      );
      bottomSheetSize = layoutChild(_ScaffoldSlot.bottomSheet, bottomSheetConstraints);
      positionChild(_ScaffoldSlot.bottomSheet, Offset((size.width - bottomSheetSize.width) / 2.0, contentBottom - bottomSheetSize.height));
    }

    Rect floatingActionButtonRect;
    if (hasChild(_ScaffoldSlot.floatingActionButton)) {
      final Size fabSize = layoutChild(_ScaffoldSlot.floatingActionButton, looseConstraints);

      // To account for the FAB position being changed, we'll animate between
      // the old and new positions.
      final ScaffoldPrelayoutGeometry currentGeometry = ScaffoldPrelayoutGeometry(
        bottomSheetSize: bottomSheetSize,
        contentBottom: contentBottom,
        contentTop: contentTop,
        floatingActionButtonSize: fabSize,
        minInsets: minInsets,
        scaffoldSize: size,
        snackBarSize: snackBarSize,
        textDirection: textDirection,
      );
      final Offset currentFabOffset = currentFloatingActionButtonLocation.getOffset(currentGeometry);
      final Offset previousFabOffset = previousFloatingActionButtonLocation.getOffset(currentGeometry);
      final Offset fabOffset = floatingActionButtonMotionAnimator.getOffset(
        begin: previousFabOffset,
        end: currentFabOffset,
        progress: floatingActionButtonMoveAnimationProgress,
      );
      positionChild(_ScaffoldSlot.floatingActionButton, fabOffset);
      floatingActionButtonRect = fabOffset & fabSize;
    }

    if (hasChild(_ScaffoldSlot.snackBar)) {
      if (snackBarSize == Size.zero) {
        snackBarSize = layoutChild(_ScaffoldSlot.snackBar, fullWidthConstraints);
      }
      final double snackBarYOffsetBase = floatingActionButtonRect != null && isSnackBarFloating
        ? floatingActionButtonRect.top
        : contentBottom;
      positionChild(_ScaffoldSlot.snackBar, Offset(0.0, snackBarYOffsetBase - snackBarSize.height));
    }

    if (hasChild(_ScaffoldSlot.statusBar)) {
      layoutChild(_ScaffoldSlot.statusBar, fullWidthConstraints.tighten(height: minInsets.top));
      positionChild(_ScaffoldSlot.statusBar, Offset.zero);
    }

    if (hasChild(_ScaffoldSlot.drawer)) {
      layoutChild(_ScaffoldSlot.drawer, BoxConstraints.tight(size));
      positionChild(_ScaffoldSlot.drawer, Offset.zero);
    }

    if (hasChild(_ScaffoldSlot.endDrawer)) {
      layoutChild(_ScaffoldSlot.endDrawer, BoxConstraints.tight(size));
      positionChild(_ScaffoldSlot.endDrawer, Offset.zero);
    }

    geometryNotifier._updateWith(
      bottomNavigationBarTop: bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonRect,
    );
  }

  @override
  bool shouldRelayout(_ScaffoldLayout oldDelegate) {
    return oldDelegate.minInsets != minInsets
        || oldDelegate.textDirection != textDirection
        || oldDelegate.floatingActionButtonMoveAnimationProgress != floatingActionButtonMoveAnimationProgress
        || oldDelegate.previousFloatingActionButtonLocation != previousFloatingActionButtonLocation
        || oldDelegate.currentFloatingActionButtonLocation != currentFloatingActionButtonLocation
        || oldDelegate.extendBody != extendBody
        || oldDelegate.extendBodyBehindAppBar != extendBodyBehindAppBar;
  }
}

/// Handler for scale and rotation animations in the [FloatingActionButton].
///
/// Currently, there are two types of [FloatingActionButton] animations:
///
/// * Entrance/Exit animations, which this widget triggers
///   when the [FloatingActionButton] is added, updated, or removed.
/// * Motion animations, which are triggered by the [Scaffold]
///   when its [FloatingActionButtonLocation] is updated.
class _FloatingActionButtonTransition extends StatefulWidget {
  const _FloatingActionButtonTransition({
    Key key,
    @required this.child,
    @required this.fabMoveAnimation,
    @required this.fabMotionAnimator,
    @required this.geometryNotifier,
    @required this.currentController,
  }) : assert(fabMoveAnimation != null),
       assert(fabMotionAnimator != null),
       assert(currentController != null),
       super(key: key);

  final Widget child;
  final Animation<double> fabMoveAnimation;
  final FloatingActionButtonAnimator fabMotionAnimator;
  final _ScaffoldGeometryNotifier geometryNotifier;

  /// Controls the current child widget.child as it exits.
  final AnimationController currentController;

  @override
  _FloatingActionButtonTransitionState createState() => _FloatingActionButtonTransitionState();
}

class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTransition> with TickerProviderStateMixin {
  // The animations applied to the Floating Action Button when it is entering or exiting.
  // Controls the previous widget.child as it exits.
  AnimationController _previousController;
  Animation<double> _previousScaleAnimation;
  Animation<double> _previousRotationAnimation;
  // The animations to run, considering the widget's fabMoveAnimation and the current/previous entrance/exit animations.
  Animation<double> _currentScaleAnimation;
  Animation<double> _extendedCurrentScaleAnimation;
  Animation<double> _currentRotationAnimation;
  Widget _previousChild;

  @override
  void initState() {
    super.initState();

    _previousController = AnimationController(
      duration: kFloatingActionButtonSegue,
      vsync: this,
    )..addStatusListener(_handlePreviousAnimationStatusChanged);
    _updateAnimations();

    if (widget.child != null) {
      // If we start out with a child, have the child appear fully visible instead
      // of animating in.
      widget.currentController.value = 1.0;
    } else {
      // If we start without a child we update the geometry object with a
      // floating action button scale of 0, as it is not showing on the screen.
      _updateGeometryScale(0.0);
    }
  }

  @override
  void dispose() {
    _previousController.dispose();
    super.dispose();
  }

  @override
  void didUpdateWidget(_FloatingActionButtonTransition oldWidget) {
    super.didUpdateWidget(oldWidget);
    final bool oldChildIsNull = oldWidget.child == null;
    final bool newChildIsNull = widget.child == null;
    if (oldChildIsNull == newChildIsNull && oldWidget.child?.key == widget.child?.key)
      return;
    if (oldWidget.fabMotionAnimator != widget.fabMotionAnimator || oldWidget.fabMoveAnimation != widget.fabMoveAnimation) {
      // Get the right scale and rotation animations to use for this widget.
      _updateAnimations();
    }
    if (_previousController.status == AnimationStatus.dismissed) {
      final double currentValue = widget.currentController.value;
      if (currentValue == 0.0 || oldWidget.child == null) {
        // The current child hasn't started its entrance animation yet. We can
        // just skip directly to the new child's entrance.
        _previousChild = null;
        if (widget.child != null)
          widget.currentController.forward();
      } else {
        // Otherwise, we need to copy the state from the current controller to
        // the previous controller and run an exit animation for the previous
        // widget before running the entrance animation for the new child.
        _previousChild = oldWidget.child;
        _previousController
          ..value = currentValue
          ..reverse();
        widget.currentController.value = 0.0;
      }
    }
  }

  static final Animatable<double> _entranceTurnTween = Tween<double>(
    begin: 1.0 - kFloatingActionButtonTurnInterval,
    end: 1.0,
  ).chain(CurveTween(curve: Curves.easeIn));

  void _updateAnimations() {
    // Get the animations for exit and entrance.
    final CurvedAnimation previousExitScaleAnimation = CurvedAnimation(
      parent: _previousController,
      curve: Curves.easeIn,
    );
    final Animation<double> previousExitRotationAnimation = Tween<double>(begin: 1.0, end: 1.0).animate(
      CurvedAnimation(
        parent: _previousController,
        curve: Curves.easeIn,
      ),
    );

    final CurvedAnimation currentEntranceScaleAnimation = CurvedAnimation(
      parent: widget.currentController,
      curve: Curves.easeIn,
    );
    final Animation<double> currentEntranceRotationAnimation = widget.currentController.drive(_entranceTurnTween);

    // Get the animations for when the FAB is moving.
    final Animation<double> moveScaleAnimation = widget.fabMotionAnimator.getScaleAnimation(parent: widget.fabMoveAnimation);
    final Animation<double> moveRotationAnimation = widget.fabMotionAnimator.getRotationAnimation(parent: widget.fabMoveAnimation);

    // Aggregate the animations.
    _previousScaleAnimation = AnimationMin<double>(moveScaleAnimation, previousExitScaleAnimation);
    _currentScaleAnimation = AnimationMin<double>(moveScaleAnimation, currentEntranceScaleAnimation);
    _extendedCurrentScaleAnimation = _currentScaleAnimation.drive(CurveTween(curve: const Interval(0.0, 0.1)));

    _previousRotationAnimation = TrainHoppingAnimation(previousExitRotationAnimation, moveRotationAnimation);
    _currentRotationAnimation = TrainHoppingAnimation(currentEntranceRotationAnimation, moveRotationAnimation);

    _currentScaleAnimation.addListener(_onProgressChanged);
    _previousScaleAnimation.addListener(_onProgressChanged);
  }

  void _handlePreviousAnimationStatusChanged(AnimationStatus status) {
    setState(() {
      if (status == AnimationStatus.dismissed) {
        assert(widget.currentController.status == AnimationStatus.dismissed);
        if (widget.child != null)
          widget.currentController.forward();
      }
    });
  }

  bool _isExtendedFloatingActionButton(Widget widget) {
    return widget is FloatingActionButton
        && widget.isExtended;
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      alignment: Alignment.centerRight,
      children: <Widget>[
        if (_previousController.status != AnimationStatus.dismissed)
          if (_isExtendedFloatingActionButton(_previousChild))
            FadeTransition(
              opacity: _previousScaleAnimation,
              child: _previousChild,
            )
          else
            ScaleTransition(
              scale: _previousScaleAnimation,
              child: RotationTransition(
                turns: _previousRotationAnimation,
                child: _previousChild,
              ),
            ),
        if (_isExtendedFloatingActionButton(widget.child))
          ScaleTransition(
            scale: _extendedCurrentScaleAnimation,
            child: FadeTransition(
              opacity: _currentScaleAnimation,
              child: widget.child,
            ),
          )
        else
          ScaleTransition(
            scale: _currentScaleAnimation,
            child: RotationTransition(
              turns: _currentRotationAnimation,
              child: widget.child,
            ),
          ),
      ],
    );
  }

  void _onProgressChanged() {
    _updateGeometryScale(math.max(_previousScaleAnimation.value, _currentScaleAnimation.value));
  }

  void _updateGeometryScale(double scale) {
    widget.geometryNotifier._updateWith(
      floatingActionButtonScale: scale,
    );
  }
}

/// Implements the basic material design visual layout structure.
///
/// This class provides APIs for showing drawers, snack bars, and bottom sheets.
///
/// To display a snackbar or a persistent bottom sheet, obtain the
/// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
/// [ScaffoldState.showSnackBar] and [ScaffoldState.showBottomSheet] functions.
///
/// {@tool dartpad --template=stateful_widget_material}
/// This example shows a [Scaffold] with a [body] and [FloatingActionButton].
/// The [body] is a [Text] placed in a [Center] in order to center the text
/// within the [Scaffold]. The [FloatingActionButton] is connected to a
/// callback that increments a counter.
///
/// ![The Scaffold has a white background with a blue AppBar at the top. A blue FloatingActionButton is positioned at the bottom right corner of the Scaffold.](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: const Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.')
///     ),
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() => _count++),
///       tooltip: 'Increment Counter',
///       child: const Icon(Icons.add),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// {@tool dartpad --template=stateful_widget_material}
/// This example shows a [Scaffold] with a blueGrey [backgroundColor], [body]
/// and [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in
/// order to center the text within the [Scaffold]. The [FloatingActionButton]
/// is connected to a callback that increments a counter.
///
/// ![](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold_background_color.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: const Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.')
///     ),
///     backgroundColor: Colors.blueGrey.shade200,
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() => _count++),
///       tooltip: 'Increment Counter',
///       child: const Icon(Icons.add),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// {@tool dartpad --template=stateful_widget_material}
/// This example shows a [Scaffold] with an [AppBar], a [BottomAppBar] and a
/// [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in order
/// to center the text within the [Scaffold]. The [FloatingActionButton] is
/// centered and docked within the [BottomAppBar] using
/// [FloatingActionButtonLocation.centerDocked]. The [FloatingActionButton] is
/// connected to a callback that increments a counter.
///
/// ![](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold_bottom_app_bar.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.'),
///     ),
///     bottomNavigationBar: BottomAppBar(
///       shape: const CircularNotchedRectangle(),
///       child: Container(height: 50.0,),
///     ),
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() {
///         _count++;
///       }),
///       tooltip: 'Increment Counter',
///       child: Icon(Icons.add),
///     ),
///     floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
///   );
/// }
/// ```
/// {@end-tool}
///
/// ## Scaffold layout, the keyboard, and display "notches"
///
/// The scaffold will expand to fill the available space. That usually
/// means that it will occupy its entire window or device screen. When
/// the device's keyboard appears the Scaffold's ancestor [MediaQuery]
/// widget's [MediaQueryData.viewInsets] changes and the Scaffold will
/// be rebuilt. By default the scaffold's [body] is resized to make
/// room for the keyboard. To prevent the resize set
/// [resizeToAvoidBottomInset] to false. In either case the focused
/// widget will be scrolled into view if it's within a scrollable
/// container.
///
/// The [MediaQueryData.padding] value defines areas that might
/// not be completely visible, like the display "notch" on the iPhone
/// X. The scaffold's [body] is not inset by this padding value
/// although an [appBar] or [bottomNavigationBar] will typically
/// cause the body to avoid the padding. The [SafeArea]
/// widget can be used within the scaffold's body to avoid areas
/// like display notches.
///
/// ## Troubleshooting
///
/// ### Nested Scaffolds
///
/// The Scaffold was designed to be the single top level container for
/// a [MaterialApp] and it's typically not necessary to nest
/// scaffolds. For example in a tabbed UI, where the
/// [bottomNavigationBar] is a [TabBar] and the body is a
/// [TabBarView], you might be tempted to make each tab bar view a
/// scaffold with a differently titled AppBar. It would be better to add a
/// listener to the [TabController] that updates the AppBar.
///
/// {@tool snippet}
/// Add a listener to the app's tab controller so that the [AppBar] title of the
/// app's one and only scaffold is reset each time a new tab is selected.
///
/// ```dart
/// TabController(vsync: tickerProvider, length: tabCount)..addListener(() {
///   if (!tabController.indexIsChanging) {
///     setState(() {
///       // Rebuild the enclosing scaffold with a new AppBar title
///       appBarTitle = 'Tab ${tabController.index}';
///     });
///   }
/// })
/// ```
/// {@end-tool}
///
/// Although there are some use cases, like a presentation app that
/// shows embedded flutter content, where nested scaffolds are
/// appropriate, it's best to avoid nesting scaffolds.
///
/// See also:
///
///  * [AppBar], which is a horizontal bar typically shown at the top of an app
///    using the [appBar] property.
///  * [BottomAppBar], which is a horizontal bar typically shown at the bottom
///    of an app using the [bottomNavigationBar] property.
///  * [FloatingActionButton], which is a circular button typically shown in the
///    bottom right corner of the app using the [floatingActionButton] property.
///  * [Drawer], which is a vertical panel that is typically displayed to the
///    left of the body (and often hidden on phones) using the [drawer]
///    property.
///  * [BottomNavigationBar], which is a horizontal array of buttons typically
///    shown along the bottom of the app using the [bottomNavigationBar]
///    property.
///  * [SnackBar], which is a temporary notification typically shown near the
///    bottom of the app using the [ScaffoldState.showSnackBar] method.
///  * [BottomSheet], which is an overlay typically shown near the bottom of the
///    app. A bottom sheet can either be persistent, in which case it is shown
///    using the [ScaffoldState.showBottomSheet] method, or modal, in which case
///    it is shown using the [showModalBottomSheet] function.
///  * [ScaffoldState], which is the state associated with this widget.
///  * <https://material.io/design/layout/responsive-layout-grid.html>
class Scaffold extends StatefulWidget {
  /// Creates a visual scaffold for material design widgets.
  const Scaffold({
    Key key,
    this.appBar,
    this.body,
    this.floatingActionButton,
    this.floatingActionButtonLocation,
    this.floatingActionButtonAnimator,
    this.persistentFooterButtons,
    this.drawer,
    this.endDrawer,
    this.bottomNavigationBar,
    this.bottomSheet,
    this.backgroundColor,
    this.resizeToAvoidBottomPadding,
    this.resizeToAvoidBottomInset,
    this.primary = true,
    this.drawerDragStartBehavior = DragStartBehavior.start,
    this.extendBody = false,
    this.extendBodyBehindAppBar = false,
    this.drawerScrimColor,
    this.drawerEdgeDragWidth,
  }) : assert(primary != null),
       assert(extendBody != null),
       assert(extendBodyBehindAppBar != null),
       assert(drawerDragStartBehavior != null),
       super(key: key);

  /// If true, and [bottomNavigationBar] or [persistentFooterButtons]
  /// is specified, then the [body] extends to the bottom of the Scaffold,
  /// instead of only extending to the top of the [bottomNavigationBar]
  /// or the [persistentFooterButtons].
  ///
  /// If true, a [MediaQuery] widget whose bottom padding matches the
  /// the height of the [bottomNavigationBar] will be added above the
  /// scaffold's [body].
  ///
  /// This property is often useful when the [bottomNavigationBar] has
  /// a non-rectangular shape, like [CircularNotchedRectangle], which
  /// adds a [FloatingActionButton] sized notch to the top edge of the bar.
  /// In this case specifying `extendBody: true` ensures that that scaffold's
  /// body will be visible through the bottom navigation bar's notch.
  ///
  /// See also:
  ///
  ///  * [extendBodyBehindAppBar], which extends the height of the body
  ///    to the top of the scaffold.
  final bool extendBody;

  /// If true, and an [appBar] is specified, then the height of the [body] is
  /// extended to include the height of the app bar and the top of the body
  /// is aligned with the top of the app bar.
  ///
  /// This is useful if the app bar's [AppBar.backgroundColor] is not
  /// completely opaque.
  ///
  /// This property is false by default. It must not be null.
  ///
  /// See also:
  ///
  ///  * [extendBody], which extends the height of the body to the bottom
  ///    of the scaffold.
  final bool extendBodyBehindAppBar;

  /// An app bar to display at the top of the scaffold.
  final PreferredSizeWidget appBar;

  /// The primary content of the scaffold.
  ///
  /// Displayed below the [appBar], above the bottom of the ambient
  /// [MediaQuery]'s [MediaQueryData.viewInsets], and behind the
  /// [floatingActionButton] and [drawer]. If [resizeToAvoidBottomInset] is
  /// false then the body is not resized when the onscreen keyboard appears,
  /// i.e. it is not inset by `viewInsets.bottom`.
  ///
  /// The widget in the body of the scaffold is positioned at the top-left of
  /// the available space between the app bar and the bottom of the scaffold. To
  /// center this widget instead, consider putting it in a [Center] widget and
  /// having that be the body. To expand this widget instead, consider
  /// putting it in a [SizedBox.expand].
  ///
  /// If you have a column of widgets that should normally fit on the screen,
  /// but may overflow and would in such cases need to scroll, consider using a
  /// [ListView] as the body of the scaffold. This is also a good choice for
  /// the case where your body is a scrollable list.
  final Widget body;

  /// A button displayed floating above [body], in the bottom right corner.
  ///
  /// Typically a [FloatingActionButton].
  final Widget floatingActionButton;

  /// Responsible for determining where the [floatingActionButton] should go.
  ///
  /// If null, the [ScaffoldState] will use the default location, [FloatingActionButtonLocation.endFloat].
  final FloatingActionButtonLocation floatingActionButtonLocation;

  /// Animator to move the [floatingActionButton] to a new [floatingActionButtonLocation].
  ///
  /// If null, the [ScaffoldState] will use the default animator, [FloatingActionButtonAnimator.scaling].
  final FloatingActionButtonAnimator floatingActionButtonAnimator;

  /// A set of buttons that are displayed at the bottom of the scaffold.
  ///
  /// Typically this is a list of [FlatButton] widgets. These buttons are
  /// persistently visible, even if the [body] of the scaffold scrolls.
  ///
  /// These widgets will be wrapped in a [ButtonBar].
  ///
  /// The [persistentFooterButtons] are rendered above the
  /// [bottomNavigationBar] but below the [body].
  final List<Widget> persistentFooterButtons;

  /// A panel displayed to the side of the [body], often hidden on mobile
  /// devices. Swipes in from either left-to-right ([TextDirection.ltr]) or
  /// right-to-left ([TextDirection.rtl])
  ///
  /// Typically a [Drawer].
  ///
  /// To open the drawer, use the [ScaffoldState.openDrawer] function.
  ///
  /// To close the drawer, use [Navigator.pop].
  ///
  /// {@tool dartpad --template=stateful_widget_material}
  /// To disable the drawer edge swipe, set the [Scaffold.drawerEdgeWidth] to 0.
  /// Then, use [ScaffoldState.openDrawer] to open the drawer and
  /// [Navigator.pop] to close it.
  ///
  /// ```dart
  /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
  ///
  /// void _openDrawer() {
  ///   _scaffoldKey.currentState.openDrawer();
  /// }
  ///
  /// void _closeDrawer() {
  ///   Navigator.of(context).pop();
  /// }
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     key: _scaffoldKey,
  ///     appBar: AppBar(title: const Text('Drawer Demo')),
  ///     body: Center(
  ///       child: RaisedButton(
  ///         onPressed: _openDrawer,
  ///         child: const Text('Open Drawer'),
  ///       ),
  ///     ),
  ///     drawer: Drawer(
  ///       child: Center(
  ///         child: Column(
  ///           mainAxisAlignment: MainAxisAlignment.center,
  ///           children: <Widget>[
  ///             const Text('This is the Drawer'),
  ///             RaisedButton(
  ///               onPressed: _closeDrawer,
  ///               child: const Text('Close Drawer'),
  ///             ),
  ///           ],
  ///         ),
  ///       ),
  ///     ),
  ///     drawerEdgeDragWidth: 0.0, // Disable opening the drawer with a swipe gesture.
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  final Widget drawer;

  /// A panel displayed to the side of the [body], often hidden on mobile
  /// devices. Swipes in from right-to-left ([TextDirection.ltr]) or
  /// left-to-right ([TextDirection.rtl])
  ///
  /// Typically a [Drawer].
  ///
  /// To open the drawer, use the [ScaffoldState.openEndDrawer] function.
  ///
  /// To close the drawer, use [Navigator.pop].
  ///
  /// {@tool dartpad --template=stateful_widget_material}
  /// To disable the drawer edge swipe, set the [Scaffold.drawerEdgeWidth]
  /// to 0. Then, use [ScaffoldState.openEndDrawer] to open the drawer and
  /// [Navigator.pop] to close it.
  ///
  /// ```dart
  /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
  ///
  /// void _openEndDrawer() {
  ///   _scaffoldKey.currentState.openEndDrawer();
  /// }
  ///
  /// void _closeEndDrawer() {
  ///   Navigator.of(context).pop();
  /// }
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     key: _scaffoldKey,
  ///     appBar: AppBar(title: Text('Drawer Demo')),
  ///     body: Center(
  ///       child: RaisedButton(
  ///         onPressed: _openEndDrawer,
  ///         child: Text('Open End Drawer'),
  ///       ),
  ///     ),
  ///     endDrawer: Drawer(
  ///       child: Center(
  ///         child: Column(
  ///           mainAxisAlignment: MainAxisAlignment.center,
  ///           children: <Widget>[
  ///             const Text('This is the Drawer'),
  ///             RaisedButton(
  ///               onPressed: _closeEndDrawer,
  ///               child: const Text('Close Drawer'),
  ///             ),
  ///           ],
  ///         ),
  ///       ),
  ///     ),
  ///     drawerEdgeDragWidth: 0.0, // Disable opening the drawer with a swipe gesture.
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  final Widget endDrawer;

  /// The color to use for the scrim that obscures primary content while a drawer is open.
  ///
  /// By default, the color is [Colors.black54]
  final Color drawerScrimColor;

  /// The color of the [Material] widget that underlies the entire Scaffold.
  ///
  /// The theme's [ThemeData.scaffoldBackgroundColor] by default.
  final Color backgroundColor;

  /// A bottom navigation bar to display at the bottom of the scaffold.
  ///
  /// Snack bars slide from underneath the bottom navigation bar while bottom
  /// sheets are stacked on top.
  ///
  /// The [bottomNavigationBar] is rendered below the [persistentFooterButtons]
  /// and the [body].
  final Widget bottomNavigationBar;

  /// The persistent bottom sheet to display.
  ///
  /// A persistent bottom sheet shows information that supplements the primary
  /// content of the app. A persistent bottom sheet remains visible even when
  /// the user interacts with other parts of the app.
  ///
  /// A closely related widget is a modal bottom sheet, which is an alternative
  /// to a menu or a dialog and prevents the user from interacting with the rest
  /// of the app. Modal bottom sheets can be created and displayed with the
  /// [showModalBottomSheet] function.
  ///
  /// Unlike the persistent bottom sheet displayed by [showBottomSheet]
  /// this bottom sheet is not a [LocalHistoryEntry] and cannot be dismissed
  /// with the scaffold appbar's back button.
  ///
  /// If a persistent bottom sheet created with [showBottomSheet] is already
  /// visible, it must be closed before building the Scaffold with a new
  /// [bottomSheet].
  ///
  /// The value of [bottomSheet] can be any widget at all. It's unlikely to
  /// actually be a [BottomSheet], which is used by the implementations of
  /// [showBottomSheet] and [showModalBottomSheet]. Typically it's a widget
  /// that includes [Material].
  ///
  /// See also:
  ///
  ///  * [showBottomSheet], which displays a bottom sheet as a route that can
  ///    be dismissed with the scaffold's back button.
  ///  * [showModalBottomSheet], which displays a modal bottom sheet.
  final Widget bottomSheet;

  /// This flag is deprecated, please use [resizeToAvoidBottomInset]
  /// instead.
  ///
  /// Originally the name referred [MediaQueryData.padding]. Now it refers
  /// [MediaQueryData.viewInsets], so using [resizeToAvoidBottomInset]
  /// should be clearer to readers.
  @Deprecated(
    'Use resizeToAvoidBottomInset to specify if the body should resize when the keyboard appears. '
    'This feature was deprecated after v1.1.9.'
  )
  final bool resizeToAvoidBottomPadding;

  /// If true the [body] and the scaffold's floating widgets should size
  /// themselves to avoid the onscreen keyboard whose height is defined by the
  /// ambient [MediaQuery]'s [MediaQueryData.viewInsets] `bottom` property.
  ///
  /// For example, if there is an onscreen keyboard displayed above the
  /// scaffold, the body can be resized to avoid overlapping the keyboard, which
  /// prevents widgets inside the body from being obscured by the keyboard.
  ///
  /// Defaults to true.
  final bool resizeToAvoidBottomInset;

  /// Whether this scaffold is being displayed at the top of the screen.
  ///
  /// If true then the height of the [appBar] will be extended by the height
  /// of the screen's status bar, i.e. the top padding for [MediaQuery].
  ///
  /// The default value of this property, like the default value of
  /// [AppBar.primary], is true.
  final bool primary;

  /// {@macro flutter.material.drawer.dragStartBehavior}
  final DragStartBehavior drawerDragStartBehavior;

  /// The width of the area within which a horizontal swipe will open the
  /// drawer.
  ///
  /// By default, the value used is 20.0 added to the padding edge of
  /// `MediaQuery.of(context).padding` that corresponds to [alignment].
  /// This ensures that the drag area for notched devices is not obscured. For
  /// example, if `TextDirection.of(context)` is set to [TextDirection.ltr],
  /// 20.0 will be added to `MediaQuery.of(context).padding.left`.
  final double drawerEdgeDragWidth;

  /// The state from the closest instance of this class that encloses the given context.
  ///
  /// {@tool dartpad --template=freeform}
  /// Typical usage of the [Scaffold.of] function is to call it from within the
  /// `build` method of a child of a [Scaffold].
  ///
  /// ```dart imports
  /// import 'package:flutter/material.dart';
  /// ```
  ///
  /// ```dart main
  /// void main() => runApp(MyApp());
  /// ```
  ///
  /// ```dart preamble
  /// class MyApp extends StatelessWidget {
  ///   // This widget is the root of your application.
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     return MaterialApp(
  ///       title: 'Flutter Code Sample for Scaffold.of.',
  ///       theme: ThemeData(
  ///         primarySwatch: Colors.blue,
  ///       ),
  ///       home: Scaffold(
  ///         body: MyScaffoldBody(),
  ///         appBar: AppBar(title: Text('Scaffold.of Example')),
  ///       ),
  ///       color: Colors.white,
  ///     );
  ///   }
  /// }
  /// ```
  ///
  /// ```dart
  /// class MyScaffoldBody extends StatelessWidget {
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     return Center(
  ///       child: RaisedButton(
  ///         child: Text('SHOW A SNACKBAR'),
  ///         onPressed: () {
  ///           Scaffold.of(context).showSnackBar(
  ///             SnackBar(
  ///               content: Text('Have a snack!'),
  ///             ),
  ///           );
  ///         },
  ///       ),
  ///     );
  ///   }
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// {@tool dartpad --template=stateless_widget_material}
  /// When the [Scaffold] is actually created in the same `build` function, the
  /// `context` argument to the `build` function can't be used to find the
  /// [Scaffold] (since it's "above" the widget being returned in the widget
  /// tree). In such cases, the following technique with a [Builder] can be used
  /// to provide a new scope with a [BuildContext] that is "under" the
  /// [Scaffold]:
  ///
  /// ```dart
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     appBar: AppBar(
  ///       title: Text('Demo')
  ///     ),
  ///     body: Builder(
  ///       // Create an inner BuildContext so that the onPressed methods
  ///       // can refer to the Scaffold with Scaffold.of().
  ///       builder: (BuildContext context) {
  ///         return Center(
  ///           child: RaisedButton(
  ///             child: Text('SHOW A SNACKBAR'),
  ///             onPressed: () {
  ///               Scaffold.of(context).showSnackBar(SnackBar(
  ///                 content: Text('Have a snack!'),
  ///               ));
  ///             },
  ///           ),
  ///         );
  ///       },
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// A more efficient solution is to split your build function into several
  /// widgets. This introduces a new context from which you can obtain the
  /// [Scaffold]. In this solution, you would have an outer widget that creates
  /// the [Scaffold] populated by instances of your new inner widgets, and then
  /// in these inner widgets you would use [Scaffold.of].
  ///
  /// A less elegant but more expedient solution is assign a [GlobalKey] to the
  /// [Scaffold], then use the `key.currentState` property to obtain the
  /// [ScaffoldState] rather than using the [Scaffold.of] function.
  ///
  /// If there is no [Scaffold] in scope, then this will throw an exception.
  /// To return null if there is no [Scaffold], then pass `nullOk: true`.
  static ScaffoldState of(BuildContext context, { bool nullOk = false }) {
    assert(nullOk != null);
    assert(context != null);
    final ScaffoldState result = context.findAncestorStateOfType<ScaffoldState>();
    if (nullOk || result != null)
      return result;
    throw FlutterError.fromParts(<DiagnosticsNode>[
      ErrorSummary(
        'Scaffold.of() called with a context that does not contain a Scaffold.'
      ),
      ErrorDescription(
        'No Scaffold ancestor could be found starting from the context that was passed to Scaffold.of(). '
        'This usually happens when the context provided is from the same StatefulWidget as that '
        'whose build function actually creates the Scaffold widget being sought.'
      ),
      ErrorHint(
        'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
        'context that is "under" the Scaffold. For an example of this, please see the '
        'documentation for Scaffold.of():\n'
        '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
      ),
      ErrorHint(
        'A more efficient solution is to split your build function into several widgets. This '
        'introduces a new context from which you can obtain the Scaffold. In this solution, '
        'you would have an outer widget that creates the Scaffold populated by instances of '
        'your new inner widgets, and then in these inner widgets you would use Scaffold.of().\n'
        'A less elegant but more expedient solution is assign a GlobalKey to the Scaffold, '
        'then use the key.currentState property to obtain the ScaffoldState rather than '
        'using the Scaffold.of() function.'
      ),
      context.describeElement('The context used was')
    ]);
  }

  /// Returns a [ValueListenable] for the [ScaffoldGeometry] for the closest
  /// [Scaffold] ancestor of the given context.
  ///
  /// The [ValueListenable.value] is only available at paint time.
  ///
  /// Notifications are guaranteed to be sent before the first paint pass
  /// with the new geometry, but there is no guarantee whether a build or
  /// layout passes are going to happen between the notification and the next
  /// paint pass.
  ///
  /// The closest [Scaffold] ancestor for the context might change, e.g when
  /// an element is moved from one scaffold to another. For [StatefulWidget]s
  /// using this listenable, a change of the [Scaffold] ancestor will
  /// trigger a [State.didChangeDependencies].
  ///
  /// A typical pattern for listening to the scaffold geometry would be to
  /// call [Scaffold.geometryOf] in [State.didChangeDependencies], compare the
  /// return value with the previous listenable, if it has changed, unregister
  /// the listener, and register a listener to the new [ScaffoldGeometry]
  /// listenable.
  static ValueListenable<ScaffoldGeometry> geometryOf(BuildContext context) {
    final _ScaffoldScope scaffoldScope = context.dependOnInheritedWidgetOfExactType<_ScaffoldScope>();
    if (scaffoldScope == null)
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary(
          'Scaffold.geometryOf() called with a context that does not contain a Scaffold.'
        ),
        ErrorDescription(
          'This usually happens when the context provided is from the same StatefulWidget as that '
          'whose build function actually creates the Scaffold widget being sought.'
        ),
        ErrorHint(
          'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
          'context that is "under" the Scaffold. For an example of this, please see the '
          'documentation for Scaffold.of():\n'
          '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
        ),
        ErrorHint(
          'A more efficient solution is to split your build function into several widgets. This '
          'introduces a new context from which you can obtain the Scaffold. In this solution, '
          'you would have an outer widget that creates the Scaffold populated by instances of '
          'your new inner widgets, and then in these inner widgets you would use Scaffold.geometryOf().',
        ),
        context.describeElement('The context used was')
      ]);
    return scaffoldScope.geometryNotifier;
  }

  /// Whether the Scaffold that most tightly encloses the given context has a
  /// drawer.
  ///
  /// If this is being used during a build (for example to decide whether to
  /// show an "open drawer" button), set the `registerForUpdates` argument to
  /// true. This will then set up an [InheritedWidget] relationship with the
  /// [Scaffold] so that the client widget gets rebuilt whenever the [hasDrawer]
  /// value changes.
  ///
  /// See also:
  ///
  ///  * [Scaffold.of], which provides access to the [ScaffoldState] object as a
  ///    whole, from which you can show snackbars, bottom sheets, and so forth.
  static bool hasDrawer(BuildContext context, { bool registerForUpdates = true }) {
    assert(registerForUpdates != null);
    assert(context != null);
    if (registerForUpdates) {
      final _ScaffoldScope scaffold = context.dependOnInheritedWidgetOfExactType<_ScaffoldScope>();
      return scaffold?.hasDrawer ?? false;
    } else {
      final ScaffoldState scaffold = context.findAncestorStateOfType<ScaffoldState>();
      return scaffold?.hasDrawer ?? false;
    }
  }

  @override
  ScaffoldState createState() => ScaffoldState();
}

/// State for a [Scaffold].
///
/// Can display [SnackBar]s and [BottomSheet]s. Retrieve a [ScaffoldState] from
/// the current [BuildContext] using [Scaffold.of].
class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {

  // DRAWER API

  final GlobalKey<DrawerControllerState> _drawerKey = GlobalKey<DrawerControllerState>();
  final GlobalKey<DrawerControllerState> _endDrawerKey = GlobalKey<DrawerControllerState>();

  /// Whether this scaffold has a non-null [Scaffold.appBar].
  bool get hasAppBar => widget.appBar != null;
  /// Whether this scaffold has a non-null [Scaffold.drawer].
  bool get hasDrawer => widget.drawer != null;
  /// Whether this scaffold has a non-null [Scaffold.endDrawer].
  bool get hasEndDrawer => widget.endDrawer != null;
  /// Whether this scaffold has a non-null [Scaffold.floatingActionButton].
  bool get hasFloatingActionButton => widget.floatingActionButton != null;

  double _appBarMaxHeight;
  /// The max height the [Scaffold.appBar] uses.
  ///
  /// This is based on the appBar preferred height plus the top padding.
  double get appBarMaxHeight => _appBarMaxHeight;
  bool _drawerOpened = false;
  bool _endDrawerOpened = false;

  /// Whether the [Scaffold.drawer] is opened.
  ///
  /// See also:
  ///
  ///  * [ScaffoldState.openDrawer], which opens the [Scaffold.drawer] of a
  ///    [Scaffold].
  bool get isDrawerOpen => _drawerOpened;

  /// Whether the [Scaffold.endDrawer] is opened.
  ///
  /// See also:
  ///
  ///  * [ScaffoldState.openEndDrawer], which opens the [Scaffold.endDrawer] of
  ///    a [Scaffold].
  bool get isEndDrawerOpen => _endDrawerOpened;

  void _drawerOpenedCallback(bool isOpened) {
    setState(() {
      _drawerOpened = isOpened;
    });
  }

  void _endDrawerOpenedCallback(bool isOpened) {
    setState(() {
      _endDrawerOpened = isOpened;
    });
  }

  /// Opens the [Drawer] (if any).
  ///
  /// If the scaffold has a non-null [Scaffold.drawer], this function will cause
  /// the drawer to begin its entrance animation.
  ///
  /// Normally this is not needed since the [Scaffold] automatically shows an
  /// appropriate [IconButton], and handles the edge-swipe gesture, to show the
  /// drawer.
  ///
  /// To close the drawer once it is open, use [Navigator.pop].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  void openDrawer() {
    if (_endDrawerKey.currentState != null && _endDrawerOpened)
      _endDrawerKey.currentState.close();
    _drawerKey.currentState?.open();
  }

  /// Opens the end side [Drawer] (if any).
  ///
  /// If the scaffold has a non-null [Scaffold.endDrawer], this function will cause
  /// the end side drawer to begin its entrance animation.
  ///
  /// Normally this is not needed since the [Scaffold] automatically shows an
  /// appropriate [IconButton], and handles the edge-swipe gesture, to show the
  /// drawer.
  ///
  /// To close the end side drawer once it is open, use [Navigator.pop].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  void openEndDrawer() {
    if (_drawerKey.currentState != null && _drawerOpened)
      _drawerKey.currentState.close();
    _endDrawerKey.currentState?.open();
  }

  // SNACKBAR API

  final Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>> _snackBars = Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>>();
  AnimationController _snackBarController;
  Timer _snackBarTimer;
  bool _accessibleNavigation;

  /// Shows a [SnackBar] at the bottom of the scaffold.
  ///
  /// A scaffold can show at most one snack bar at a time. If this function is
  /// called while another snack bar is already visible, the given snack bar
  /// will be added to a queue and displayed after the earlier snack bars have
  /// closed.
  ///
  /// To control how long a [SnackBar] remains visible, use [SnackBar.duration].
  ///
  /// To remove the [SnackBar] with an exit animation, use [hideCurrentSnackBar]
  /// or call [ScaffoldFeatureController.close] on the returned
  /// [ScaffoldFeatureController]. To remove a [SnackBar] suddenly (without an
  /// animation), use [removeCurrentSnackBar].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackbar) {
    _snackBarController ??= SnackBar.createAnimationController(vsync: this)
      ..addStatusListener(_handleSnackBarStatusChange);
    if (_snackBars.isEmpty) {
      assert(_snackBarController.isDismissed);
      _snackBarController.forward();
    }
    ScaffoldFeatureController<SnackBar, SnackBarClosedReason> controller;
    controller = ScaffoldFeatureController<SnackBar, SnackBarClosedReason>._(
      // We provide a fallback key so that if back-to-back snackbars happen to
      // match in structure, material ink splashes and highlights don't survive
      // from one to the next.
      snackbar.withAnimation(_snackBarController, fallbackKey: UniqueKey()),
      Completer<SnackBarClosedReason>(),
      () {
        assert(_snackBars.first == controller);
        hideCurrentSnackBar(reason: SnackBarClosedReason.hide);
      },
      null, // SnackBar doesn't use a builder function so setState() wouldn't rebuild it
    );
    setState(() {
      _snackBars.addLast(controller);
    });
    return controller;
  }

  void _handleSnackBarStatusChange(AnimationStatus status) {
    switch (status) {
      case AnimationStatus.dismissed:
        assert(_snackBars.isNotEmpty);
        setState(() {
          _snackBars.removeFirst();
        });
        if (_snackBars.isNotEmpty)
          _snackBarController.forward();
        break;
      case AnimationStatus.completed:
        setState(() {
          assert(_snackBarTimer == null);
          // build will create a new timer if necessary to dismiss the snack bar
        });
        break;
      case AnimationStatus.forward:
      case AnimationStatus.reverse:
        break;
    }
  }

  /// Removes the current [SnackBar] (if any) immediately.
  ///
  /// The removed snack bar does not run its normal exit animation. If there are
  /// any queued snack bars, they begin their entrance animation immediately.
  void removeCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.remove }) {
    assert(reason != null);
    if (_snackBars.isEmpty)
      return;
    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
    if (!completer.isCompleted)
      completer.complete(reason);
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
    _snackBarController.value = 0.0;
  }

  /// Removes the current [SnackBar] by running its normal exit animation.
  ///
  /// The closed completer is called after the animation is complete.
  void hideCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.hide }) {
    assert(reason != null);
    if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
      return;
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
    if (mediaQuery.accessibleNavigation) {
      _snackBarController.value = 0.0;
      completer.complete(reason);
    } else {
      _snackBarController.reverse().then<void>((void value) {
        assert(mounted);
        if (!completer.isCompleted)
          completer.complete(reason);
      });
    }
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
  }


  // PERSISTENT BOTTOM SHEET API

  // Contains bottom sheets that may still be animating out of view.
  // Important if the app/user takes an action that could repeatedly show a
  // bottom sheet.
  final List<_StandardBottomSheet> _dismissedBottomSheets = <_StandardBottomSheet>[];
  PersistentBottomSheetController<dynamic> _currentBottomSheet;

  void _maybeBuildPersistentBottomSheet() {
    if (widget.bottomSheet != null && _currentBottomSheet == null) {
      // The new _currentBottomSheet is not a local history entry so a "back" button
      // will not be added to the Scaffold's appbar and the bottom sheet will not
      // support drag or swipe to dismiss.
      final AnimationController animationController = BottomSheet.createAnimationController(this)..value = 1.0;
      LocalHistoryEntry _persistentSheetHistoryEntry;
      bool _persistentBottomSheetExtentChanged(DraggableScrollableNotification notification) {
        if (notification.extent > notification.initialExtent) {
          if (_persistentSheetHistoryEntry == null) {
            _persistentSheetHistoryEntry = LocalHistoryEntry(onRemove: () {
              if (notification.extent > notification.initialExtent) {
                DraggableScrollableActuator.reset(notification.context);
              }
              showBodyScrim(false, 0.0);
              _floatingActionButtonVisibilityValue = 1.0;
              _persistentSheetHistoryEntry = null;
            });
            ModalRoute.of(context).addLocalHistoryEntry(_persistentSheetHistoryEntry);
          }
        } else if (_persistentSheetHistoryEntry != null) {
          ModalRoute.of(context).removeLocalHistoryEntry(_persistentSheetHistoryEntry);
        }
        return false;
      }

      _currentBottomSheet = _buildBottomSheet<void>(
        (BuildContext context) {
          return NotificationListener<DraggableScrollableNotification>(
            onNotification: _persistentBottomSheetExtentChanged,
            child: DraggableScrollableActuator(
              child: widget.bottomSheet,
            ),
          );
        },
        true,
        animationController: animationController,
      );
    }
  }

  void _closeCurrentBottomSheet() {
    if (_currentBottomSheet != null) {
      if (!_currentBottomSheet._isLocalHistoryEntry) {
        _currentBottomSheet.close();
      }
      assert(() {
        _currentBottomSheet?._completer?.future?.whenComplete(() {
          assert(_currentBottomSheet == null);
        });
        return true;
      }());
    }
  }

  PersistentBottomSheetController<T> _buildBottomSheet<T>(
    WidgetBuilder builder,
    bool isPersistent, {
    AnimationController animationController,
    Color backgroundColor,
    double elevation,
    ShapeBorder shape,
    Clip clipBehavior,
  }) {
    assert(() {
      if (widget.bottomSheet != null && isPersistent && _currentBottomSheet != null) {
        throw FlutterError(
          'Scaffold.bottomSheet cannot be specified while a bottom sheet '
          'displayed with showBottomSheet() is still visible.\n'
          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
        );
      }
      return true;
    }());

    final Completer<T> completer = Completer<T>();
    final GlobalKey<_StandardBottomSheetState> bottomSheetKey = GlobalKey<_StandardBottomSheetState>();
    _StandardBottomSheet bottomSheet;

    bool removedEntry = false;
    void _removeCurrentBottomSheet() {
      removedEntry = true;
      if (_currentBottomSheet == null) {
        return;
      }
      assert(_currentBottomSheet._widget == bottomSheet);
      assert(bottomSheetKey.currentState != null);
      _showFloatingActionButton();

      void _closed(void value) {
        setState(() {
          _currentBottomSheet = null;
        });

        if (animationController.status != AnimationStatus.dismissed) {
          _dismissedBottomSheets.add(bottomSheet);
        }
        completer.complete();
      }

      final Future<void> closing = bottomSheetKey.currentState.close();
      if (closing != null) {
        closing.then(_closed);
      } else {
        _closed(null);
      }
    }

    final LocalHistoryEntry entry = isPersistent
      ? null
      : LocalHistoryEntry(onRemove: () {
          if (!removedEntry) {
            _removeCurrentBottomSheet();
          }
        });

    bottomSheet = _StandardBottomSheet(
      key: bottomSheetKey,
      animationController: animationController,
      enableDrag: !isPersistent,
      onClosing: () {
        if (_currentBottomSheet == null) {
          return;
        }
        assert(_currentBottomSheet._widget == bottomSheet);
        if (!isPersistent && !removedEntry) {
          assert(entry != null);
          entry.remove();
          removedEntry = true;
        }
      },
      onDismissed: () {
        if (_dismissedBottomSheets.contains(bottomSheet)) {
          setState(() {
            _dismissedBottomSheets.remove(bottomSheet);
          });
        }
      },
      builder: builder,
      isPersistent: isPersistent,
      backgroundColor: backgroundColor,
      elevation: elevation,
      shape: shape,
      clipBehavior: clipBehavior,
    );

    if (!isPersistent)
      ModalRoute.of(context).addLocalHistoryEntry(entry);

    return PersistentBottomSheetController<T>._(
      bottomSheet,
      completer,
      entry != null
        ? entry.remove
        : _removeCurrentBottomSheet,
      (VoidCallback fn) { bottomSheetKey.currentState?.setState(fn); },
      !isPersistent,
    );
  }

  /// Shows a material design bottom sheet in the nearest [Scaffold]. To show
  /// a persistent bottom sheet, use the [Scaffold.bottomSheet].
  ///
  /// Returns a controller that can be used to close and otherwise manipulate the
  /// bottom sheet.
  ///
  /// To rebuild the bottom sheet (e.g. if it is stateful), call
  /// [PersistentBottomSheetController.setState] on the controller returned by
  /// this method.
  ///
  /// The new bottom sheet becomes a [LocalHistoryEntry] for the enclosing
  /// [ModalRoute] and a back button is added to the app bar of the [Scaffold]
  /// that closes the bottom sheet.
  ///
  /// To create a persistent bottom sheet that is not a [LocalHistoryEntry] and
  /// does not add a back button to the enclosing Scaffold's app bar, use the
  /// [Scaffold.bottomSheet] constructor parameter.
  ///
  /// A persistent bottom sheet shows information that supplements the primary
  /// content of the app. A persistent bottom sheet remains visible even when
  /// the user interacts with other parts of the app.
  ///
  /// A closely related widget is a modal bottom sheet, which is an alternative
  /// to a menu or a dialog and prevents the user from interacting with the rest
  /// of the app. Modal bottom sheets can be created and displayed with the
  /// [showModalBottomSheet] function.
  ///
  /// {@tool dartpad --template=stateless_widget_scaffold}
  ///
  /// This example demonstrates how to use `showBottomSheet` to display a
  /// bottom sheet when a user taps a button. It also demonstrates how to
  /// close a bottom sheet using the Navigator.
  ///
  /// ```dart
  /// Widget build(BuildContext context) {
  ///   return Center(
  ///     child: RaisedButton(
  ///       child: const Text('showBottomSheet'),
  ///       onPressed: () {
  ///         Scaffold.of(context).showBottomSheet<void>(
  ///           (BuildContext context) {
  ///             return Container(
  ///               height: 200,
  ///               color: Colors.amber,
  ///               child: Center(
  ///                 child: Column(
  ///                   mainAxisAlignment: MainAxisAlignment.center,
  ///                   mainAxisSize: MainAxisSize.min,
  ///                   children: <Widget>[
  ///                     const Text('BottomSheet'),
  ///                     RaisedButton(
  ///                       child: const Text('Close BottomSheet'),
  ///                       onPressed: () => Navigator.pop(context),
  ///                     )
  ///                   ],
  ///                 ),
  ///               ),
  ///             );
  ///           },
  ///         );
  ///       },
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  /// See also:
  ///
  ///  * [BottomSheet], which becomes the parent of the widget returned by the
  ///    `builder`.
  ///  * [showBottomSheet], which calls this method given a [BuildContext].
  ///  * [showModalBottomSheet], which can be used to display a modal bottom
  ///    sheet.
  ///  * [Scaffold.of], for information about how to obtain the [ScaffoldState].
  ///  * <https://material.io/design/components/sheets-bottom.html#standard-bottom-sheet>
  PersistentBottomSheetController<T> showBottomSheet<T>(
    WidgetBuilder builder, {
    Color backgroundColor,
    double elevation,
    ShapeBorder shape,
    Clip clipBehavior,
  }) {
    assert(() {
      if (widget.bottomSheet != null) {
        throw FlutterError(
          'Scaffold.bottomSheet cannot be specified while a bottom sheet '
          'displayed with showBottomSheet() is still visible.\n'
          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
        );
      }
      return true;
    }());
    assert(debugCheckHasMediaQuery(context));

    _closeCurrentBottomSheet();
    final AnimationController controller = BottomSheet.createAnimationController(this)..forward();
    setState(() {
      _currentBottomSheet = _buildBottomSheet<T>(
        builder,
        false,
        animationController: controller,
        backgroundColor: backgroundColor,
        elevation: elevation,
        shape: shape,
        clipBehavior: clipBehavior,
      );
    });
    return _currentBottomSheet as PersistentBottomSheetController<T>;
  }

  // Floating Action Button API
  AnimationController _floatingActionButtonMoveController;
  FloatingActionButtonAnimator _floatingActionButtonAnimator;
  FloatingActionButtonLocation _previousFloatingActionButtonLocation;
  FloatingActionButtonLocation _floatingActionButtonLocation;

  AnimationController _floatingActionButtonVisibilityController;

  /// Gets the current value of the visibility animation for the
  /// [Scaffold.floatingActionButton].
  double get _floatingActionButtonVisibilityValue => _floatingActionButtonVisibilityController.value;

  /// Sets the current value of the visibility animation for the
  /// [Scaffold.floatingActionButton].  This value must not be null.
  set _floatingActionButtonVisibilityValue(double newValue) {
    assert(newValue != null);
    _floatingActionButtonVisibilityController.value = newValue.clamp(
      _floatingActionButtonVisibilityController.lowerBound,
      _floatingActionButtonVisibilityController.upperBound,
    ) as double;
  }

  /// Shows the [Scaffold.floatingActionButton].
  TickerFuture _showFloatingActionButton() {
    return _floatingActionButtonVisibilityController.forward();
  }

  // Moves the Floating Action Button to the new Floating Action Button Location.
  void _moveFloatingActionButton(final FloatingActionButtonLocation newLocation) {
    FloatingActionButtonLocation previousLocation = _floatingActionButtonLocation;
    double restartAnimationFrom = 0.0;
    // If the Floating Action Button is moving right now, we need to start from a snapshot of the current transition.
    if (_floatingActionButtonMoveController.isAnimating) {
      previousLocation = _TransitionSnapshotFabLocation(_previousFloatingActionButtonLocation, _floatingActionButtonLocation, _floatingActionButtonAnimator, _floatingActionButtonMoveController.value);
      restartAnimationFrom = _floatingActionButtonAnimator.getAnimationRestart(_floatingActionButtonMoveController.value);
    }

    setState(() {
      _previousFloatingActionButtonLocation = previousLocation;
      _floatingActionButtonLocation = newLocation;
    });

    // Animate the motion even when the fab is null so that if the exit animation is running,
    // the old fab will start the motion transition while it exits instead of jumping to the
    // new position.
    _floatingActionButtonMoveController.forward(from: restartAnimationFrom);
  }

  // iOS FEATURES - status bar tap, back gesture

  // On iOS, tapping the status bar scrolls the app's primary scrollable to the
  // top. We implement this by providing a primary scroll controller and
  // scrolling it to the top when tapped.

  final ScrollController _primaryScrollController = ScrollController();

  void _handleStatusBarTap() {
    if (_primaryScrollController.hasClients) {
      _primaryScrollController.animateTo(
        0.0,
        duration: const Duration(milliseconds: 300),
        curve: Curves.linear, // TODO(ianh): Use a more appropriate curve.
      );
    }
  }

  // INTERNALS

  _ScaffoldGeometryNotifier _geometryNotifier;

  // Backwards compatibility for deprecated resizeToAvoidBottomPadding property
  bool get _resizeToAvoidBottomInset {
    // ignore: deprecated_member_use_from_same_package
    return widget.resizeToAvoidBottomInset ?? widget.resizeToAvoidBottomPadding ?? true;
  }

  @override
  void initState() {
    super.initState();
    _geometryNotifier = _ScaffoldGeometryNotifier(const ScaffoldGeometry(), context);
    _floatingActionButtonLocation = widget.floatingActionButtonLocation ?? _kDefaultFloatingActionButtonLocation;
    _floatingActionButtonAnimator = widget.floatingActionButtonAnimator ?? _kDefaultFloatingActionButtonAnimator;
    _previousFloatingActionButtonLocation = _floatingActionButtonLocation;
    _floatingActionButtonMoveController = AnimationController(
      vsync: this,
      lowerBound: 0.0,
      upperBound: 1.0,
      value: 1.0,
      duration: kFloatingActionButtonSegue * 2,
    );

    _floatingActionButtonVisibilityController = AnimationController(
      duration: kFloatingActionButtonSegue,
      vsync: this,
    );
  }

  @override
  void didUpdateWidget(Scaffold oldWidget) {
    // Update the Floating Action Button Animator, and then schedule the Floating Action Button for repositioning.
    if (widget.floatingActionButtonAnimator != oldWidget.floatingActionButtonAnimator) {
      _floatingActionButtonAnimator = widget.floatingActionButtonAnimator ?? _kDefaultFloatingActionButtonAnimator;
    }
    if (widget.floatingActionButtonLocation != oldWidget.floatingActionButtonLocation) {
      _moveFloatingActionButton(widget.floatingActionButtonLocation ?? _kDefaultFloatingActionButtonLocation);
    }
    if (widget.bottomSheet != oldWidget.bottomSheet) {
      assert(() {
        if (widget.bottomSheet != null && _currentBottomSheet?._isLocalHistoryEntry == true) {
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary(
              'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
              'with showBottomSheet() is still visible.'
            ),
            ErrorHint(
              'Use the PersistentBottomSheetController '
              'returned by showBottomSheet() to close the old bottom sheet before creating '
              'a Scaffold with a (non null) bottomSheet.'
            ),
          ]);
        }
        return true;
      }());
      _closeCurrentBottomSheet();
      _maybeBuildPersistentBottomSheet();
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  void didChangeDependencies() {
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    // If we transition from accessible navigation to non-accessible navigation
    // and there is a SnackBar that would have timed out that has already
    // completed its timer, dismiss that SnackBar. If the timer hasn't finished
    // yet, let it timeout as normal.
    if (_accessibleNavigation == true
      && !mediaQuery.accessibleNavigation
      && _snackBarTimer != null
      && !_snackBarTimer.isActive) {
      hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
    }
    _accessibleNavigation = mediaQuery.accessibleNavigation;
    _maybeBuildPersistentBottomSheet();
    super.didChangeDependencies();
  }

  @override
  void dispose() {
    _snackBarController?.dispose();
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
    _geometryNotifier.dispose();
    for (final _StandardBottomSheet bottomSheet in _dismissedBottomSheets) {
      bottomSheet.animationController?.dispose();
    }
    if (_currentBottomSheet != null) {
      _currentBottomSheet._widget.animationController?.dispose();
    }
    _floatingActionButtonMoveController.dispose();
    _floatingActionButtonVisibilityController.dispose();
    super.dispose();
  }

  void _addIfNonNull(
    List<LayoutId> children,
    Widget child,
    Object childId, {
    @required bool removeLeftPadding,
    @required bool removeTopPadding,
    @required bool removeRightPadding,
    @required bool removeBottomPadding,
    bool removeBottomInset = false,
    bool maintainBottomViewPadding = false,
  }) {
    MediaQueryData data = MediaQuery.of(context).removePadding(
      removeLeft: removeLeftPadding,
      removeTop: removeTopPadding,
      removeRight: removeRightPadding,
      removeBottom: removeBottomPadding,
    );
    if (removeBottomInset)
      data = data.removeViewInsets(removeBottom: true);

    if (maintainBottomViewPadding && data.viewInsets.bottom != 0.0) {
      data = data.copyWith(
        padding: data.padding.copyWith(bottom: data.viewPadding.bottom)
      );
    }

    if (child != null) {
      children.add(
        LayoutId(
          id: childId,
          child: MediaQuery(data: data, child: child),
        ),
      );
    }
  }

  void _buildEndDrawer(List<LayoutId> children, TextDirection textDirection) {
    if (widget.endDrawer != null) {
      assert(hasEndDrawer);
      _addIfNonNull(
        children,
        DrawerController(
          key: _endDrawerKey,
          alignment: DrawerAlignment.end,
          child: widget.endDrawer,
          drawerCallback: _endDrawerOpenedCallback,
          dragStartBehavior: widget.drawerDragStartBehavior,
          scrimColor: widget.drawerScrimColor,
          edgeDragWidth: widget.drawerEdgeDragWidth,
        ),
        _ScaffoldSlot.endDrawer,
        // remove the side padding from the side we're not touching
        removeLeftPadding: textDirection == TextDirection.ltr,
        removeTopPadding: false,
        removeRightPadding: textDirection == TextDirection.rtl,
        removeBottomPadding: false,
      );
    }
  }

  void _buildDrawer(List<LayoutId> children, TextDirection textDirection) {
    if (widget.drawer != null) {
      assert(hasDrawer);
      _addIfNonNull(
        children,
        DrawerController(
          key: _drawerKey,
          alignment: DrawerAlignment.start,
          child: widget.drawer,
          drawerCallback: _drawerOpenedCallback,
          dragStartBehavior: widget.drawerDragStartBehavior,
          scrimColor: widget.drawerScrimColor,
          edgeDragWidth: widget.drawerEdgeDragWidth,
        ),
        _ScaffoldSlot.drawer,
        // remove the side padding from the side we're not touching
        removeLeftPadding: textDirection == TextDirection.rtl,
        removeTopPadding: false,
        removeRightPadding: textDirection == TextDirection.ltr,
        removeBottomPadding: false,
      );
    }
  }

  bool _showBodyScrim = false;
  Color _bodyScrimColor = Colors.black;

  /// Whether to show a [ModalBarrier] over the body of the scaffold.
  ///
  /// The `value` parameter must not be null.
  void showBodyScrim(bool value, double opacity) {
    assert(value != null);
    if (_showBodyScrim == value && _bodyScrimColor.opacity == opacity) {
      return;
    }
    setState(() {
      _showBodyScrim = value;
      _bodyScrimColor = Colors.black.withOpacity(opacity);
    });
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMediaQuery(context));
    assert(debugCheckHasDirectionality(context));
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    final ThemeData themeData = Theme.of(context);
    final TextDirection textDirection = Directionality.of(context);
    _accessibleNavigation = mediaQuery.accessibleNavigation;

    if (_snackBars.isNotEmpty) {
      final ModalRoute<dynamic> route = ModalRoute.of(context);
      if (route == null || route.isCurrent) {
        if (_snackBarController.isCompleted && _snackBarTimer == null) {
          final SnackBar snackBar = _snackBars.first._widget;
          _snackBarTimer = Timer(snackBar.duration, () {
            assert(_snackBarController.status == AnimationStatus.forward ||
                   _snackBarController.status == AnimationStatus.completed);
            // Look up MediaQuery again in case the setting changed.
            final MediaQueryData mediaQuery = MediaQuery.of(context);
            if (mediaQuery.accessibleNavigation && snackBar.action != null)
              return;
            hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
          });
        }
      } else {
        _snackBarTimer?.cancel();
        _snackBarTimer = null;
      }
    }

    final List<LayoutId> children = <LayoutId>[];
    _addIfNonNull(
      children,
      widget.body == null ? null : _BodyBuilder(
        extendBody: widget.extendBody,
        extendBodyBehindAppBar: widget.extendBodyBehindAppBar,
        body: widget.body,
      ),
      _ScaffoldSlot.body,
      removeLeftPadding: false,
      removeTopPadding: widget.appBar != null,
      removeRightPadding: false,
      removeBottomPadding: widget.bottomNavigationBar != null || widget.persistentFooterButtons != null,
      removeBottomInset: _resizeToAvoidBottomInset,
    );
    if (_showBodyScrim) {
      _addIfNonNull(
        children,
        ModalBarrier(
          dismissible: false,
          color: _bodyScrimColor,
        ),
        _ScaffoldSlot.bodyScrim,
        removeLeftPadding: true,
        removeTopPadding: true,
        removeRightPadding: true,
        removeBottomPadding: true,
      );
    }

    if (widget.appBar != null) {
      final double topPadding = widget.primary ? mediaQuery.padding.top : 0.0;
      _appBarMaxHeight = widget.appBar.preferredSize.height + topPadding;
      assert(_appBarMaxHeight >= 0.0 && _appBarMaxHeight.isFinite);
      _addIfNonNull(
        children,
        ConstrainedBox(
          constraints: BoxConstraints(maxHeight: _appBarMaxHeight),
          child: FlexibleSpaceBar.createSettings(
            currentExtent: _appBarMaxHeight,
            child: widget.appBar,
          ),
        ),
        _ScaffoldSlot.appBar,
        removeLeftPadding: false,
        removeTopPadding: false,
        removeRightPadding: false,
        removeBottomPadding: true,
      );
    }

    bool isSnackBarFloating = false;
    if (_snackBars.isNotEmpty) {
      final SnackBarBehavior snackBarBehavior = _snackBars.first._widget.behavior
        ?? themeData.snackBarTheme.behavior
        ?? SnackBarBehavior.fixed;
      isSnackBarFloating = snackBarBehavior == SnackBarBehavior.floating;

      _addIfNonNull(
        children,
        _snackBars.first._widget,
        _ScaffoldSlot.snackBar,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: widget.bottomNavigationBar != null || widget.persistentFooterButtons != null,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (widget.persistentFooterButtons != null) {
      _addIfNonNull(
        children,
        Container(
          decoration: BoxDecoration(
            border: Border(
              top: Divider.createBorderSide(context, width: 1.0),
            ),
          ),
          child: SafeArea(
            top: false,
            child: ButtonBar(
              children: widget.persistentFooterButtons,
            ),
          ),
        ),
        _ScaffoldSlot.persistentFooter,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: false,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (widget.bottomNavigationBar != null) {
      _addIfNonNull(
        children,
        widget.bottomNavigationBar,
        _ScaffoldSlot.bottomNavigationBar,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: false,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (_currentBottomSheet != null || _dismissedBottomSheets.isNotEmpty) {
      final Widget stack = Stack(
        alignment: Alignment.bottomCenter,
        children: <Widget>[
          ..._dismissedBottomSheets,
          if (_currentBottomSheet != null) _currentBottomSheet._widget,
        ],
      );
      _addIfNonNull(
        children,
        stack,
        _ScaffoldSlot.bottomSheet,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: _resizeToAvoidBottomInset,
      );
    }

    _addIfNonNull(
      children,
      _FloatingActionButtonTransition(
        child: widget.floatingActionButton,
        fabMoveAnimation: _floatingActionButtonMoveController,
        fabMotionAnimator: _floatingActionButtonAnimator,
        geometryNotifier: _geometryNotifier,
        currentController: _floatingActionButtonVisibilityController,
      ),
      _ScaffoldSlot.floatingActionButton,
      removeLeftPadding: true,
      removeTopPadding: true,
      removeRightPadding: true,
      removeBottomPadding: true,
    );

    switch (themeData.platform) {
      case TargetPlatform.iOS:
      case TargetPlatform.macOS:
        _addIfNonNull(
          children,
          GestureDetector(
            behavior: HitTestBehavior.opaque,
            onTap: _handleStatusBarTap,
            // iOS accessibility automatically adds scroll-to-top to the clock in the status bar
            excludeFromSemantics: true,
          ),
          _ScaffoldSlot.statusBar,
          removeLeftPadding: false,
          removeTopPadding: true,
          removeRightPadding: false,
          removeBottomPadding: true,
        );
        break;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        break;
    }

    if (_endDrawerOpened) {
      _buildDrawer(children, textDirection);
      _buildEndDrawer(children, textDirection);
    } else {
      _buildEndDrawer(children, textDirection);
      _buildDrawer(children, textDirection);
    }

    // The minimum insets for contents of the Scaffold to keep visible.
    final EdgeInsets minInsets = mediaQuery.padding.copyWith(
      bottom: _resizeToAvoidBottomInset ? mediaQuery.viewInsets.bottom : 0.0,
    );

    // extendBody locked when keyboard is open
    final bool _extendBody = minInsets.bottom <= 0 && widget.extendBody;

    return _ScaffoldScope(
      hasDrawer: hasDrawer,
      geometryNotifier: _geometryNotifier,
      child: PrimaryScrollController(
        controller: _primaryScrollController,
        child: Material(
          color: widget.backgroundColor ?? themeData.scaffoldBackgroundColor,
          child: AnimatedBuilder(animation: _floatingActionButtonMoveController, builder: (BuildContext context, Widget child) {
            return CustomMultiChildLayout(
              children: children,
              delegate: _ScaffoldLayout(
                extendBody: _extendBody,
                extendBodyBehindAppBar: widget.extendBodyBehindAppBar,
                minInsets: minInsets,
                currentFloatingActionButtonLocation: _floatingActionButtonLocation,
                floatingActionButtonMoveAnimationProgress: _floatingActionButtonMoveController.value,
                floatingActionButtonMotionAnimator: _floatingActionButtonAnimator,
                geometryNotifier: _geometryNotifier,
                previousFloatingActionButtonLocation: _previousFloatingActionButtonLocation,
                textDirection: textDirection,
                isSnackBarFloating: isSnackBarFloating,
              ),
            );
          }),
        ),
      ),
    );
  }
}

/// An interface for controlling a feature of a [Scaffold].
///
/// Commonly obtained from [ScaffoldState.showSnackBar] or [ScaffoldState.showBottomSheet].
class ScaffoldFeatureController<T extends Widget, U> {
  const ScaffoldFeatureController._(this._widget, this._completer, this.close, this.setState);
  final T _widget;
  final Completer<U> _completer;

  /// Completes when the feature controlled by this object is no longer visible.
  Future<U> get closed => _completer.future;

  /// Remove the feature (e.g., bottom sheet or snack bar) from the scaffold.
  final VoidCallback close;

  /// Mark the feature (e.g., bottom sheet or snack bar) as needing to rebuild.
  final StateSetter setState;
}

class _StandardBottomSheet extends StatefulWidget {
  const _StandardBottomSheet({
    Key key,
    this.animationController,
    this.enableDrag = true,
    this.onClosing,
    this.onDismissed,
    this.builder,
    this.isPersistent = false,
    this.backgroundColor,
    this.elevation,
    this.shape,
    this.clipBehavior,
  }) : super(key: key);

  final AnimationController animationController; // we control it, but it must be disposed by whoever created it.
  final bool enableDrag;
  final VoidCallback onClosing;
  final VoidCallback onDismissed;
  final WidgetBuilder builder;
  final bool isPersistent;
  final Color backgroundColor;
  final double elevation;
  final ShapeBorder shape;
  final Clip clipBehavior;

  @override
  _StandardBottomSheetState createState() => _StandardBottomSheetState();
}

class _StandardBottomSheetState extends State<_StandardBottomSheet> {
  @override
  void initState() {
    super.initState();
    assert(widget.animationController != null);
    assert(widget.animationController.status == AnimationStatus.forward
        || widget.animationController.status == AnimationStatus.completed);
    widget.animationController.addStatusListener(_handleStatusChange);
  }

  @override
  void didUpdateWidget(_StandardBottomSheet oldWidget) {
    super.didUpdateWidget(oldWidget);
    assert(widget.animationController == oldWidget.animationController);
  }

  Future<void> close() {
    assert(widget.animationController != null);
    widget.animationController.reverse();
    if (widget.onClosing != null) {
      widget.onClosing();
    }
    return null;
  }

  void _handleStatusChange(AnimationStatus status) {
    if (status == AnimationStatus.dismissed && widget.onDismissed != null) {
      widget.onDismissed();
    }
  }

  bool extentChanged(DraggableScrollableNotification notification) {
    final double extentRemaining = 1.0 - notification.extent;
    final ScaffoldState scaffold = Scaffold.of(context);
    if (extentRemaining < _kBottomSheetDominatesPercentage) {
      scaffold._floatingActionButtonVisibilityValue = extentRemaining * _kBottomSheetDominatesPercentage * 10;
      scaffold.showBodyScrim(true,  math.max(
        _kMinBottomSheetScrimOpacity,
        _kMaxBottomSheetScrimOpacity - scaffold._floatingActionButtonVisibilityValue,
      ));
    } else {
      scaffold._floatingActionButtonVisibilityValue = 1.0;
      scaffold.showBodyScrim(false, 0.0);
    }
    // If the Scaffold.bottomSheet != null, we're a persistent bottom sheet.
    if (notification.extent == notification.minExtent && scaffold.widget.bottomSheet == null) {
      close();
    }
    return false;
  }

  Widget _wrapBottomSheet(Widget bottomSheet) {
    return Semantics(
      container: true,
      onDismiss: close,
      child:  NotificationListener<DraggableScrollableNotification>(
        onNotification: extentChanged,
        child: bottomSheet,
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    if (widget.animationController != null) {
      return AnimatedBuilder(
        animation: widget.animationController,
        builder: (BuildContext context, Widget child) {
          return Align(
            alignment: AlignmentDirectional.topStart,
            heightFactor: widget.animationController.value,
            child: child,
          );
        },
        child: _wrapBottomSheet(
          BottomSheet(
            animationController: widget.animationController,
            enableDrag: widget.enableDrag,
            onClosing: widget.onClosing,
            builder: widget.builder,
            backgroundColor: widget.backgroundColor,
            elevation: widget.elevation,
            shape: widget.shape,
            clipBehavior: widget.clipBehavior,
          ),
        ),
      );
    }

    return _wrapBottomSheet(
      BottomSheet(
        onClosing: widget.onClosing,
        builder: widget.builder,
        backgroundColor: widget.backgroundColor,
      ),
    );
  }

}

/// A [ScaffoldFeatureController] for standard bottom sheets.
///
/// This is the type of objects returned by [ScaffoldState.showBottomSheet].
///
/// This controller is used to display both standard and persistent bottom
/// sheets. A bottom sheet is only persistent if it is set as the
/// [Scaffold.bottomSheet].
class PersistentBottomSheetController<T> extends ScaffoldFeatureController<_StandardBottomSheet, T> {
  const PersistentBottomSheetController._(
    _StandardBottomSheet widget,
    Completer<T> completer,
    VoidCallback close,
    StateSetter setState,
    this._isLocalHistoryEntry,
  ) : super._(widget, completer, close, setState);

  final bool _isLocalHistoryEntry;
}

class _ScaffoldScope extends InheritedWidget {
  const _ScaffoldScope({
    Key key,
    @required this.hasDrawer,
    @required this.geometryNotifier,
    @required Widget child,
  }) : assert(hasDrawer != null),
       super(key: key, child: child);

  final bool hasDrawer;
  final _ScaffoldGeometryNotifier geometryNotifier;

  @override
  bool updateShouldNotify(_ScaffoldScope oldWidget) {
    return hasDrawer != oldWidget.hasDrawer;
  }
}