packages/flutter/lib/src/material/app_bar.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:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';

import 'app_bar_theme.dart';
import 'back_button.dart';
import 'constants.dart';
import 'debug.dart';
import 'flexible_space_bar.dart';
import 'icon_button.dart';
import 'icons.dart';
import 'material.dart';
import 'material_localizations.dart';
import 'scaffold.dart';
import 'tabs.dart';
import 'text_theme.dart';
import 'theme.dart';

const double _kLeadingWidth = kToolbarHeight; // So the leading button is square.
const double _kMaxTitleTextScaleFactor = 1.34; // TODO(perc): Add link to Material spec when available, https://github.com/flutter/flutter/issues/58769.

// Bottom justify the toolbarHeight child which may overflow the top.
class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
  const _ToolbarContainerLayout(this.toolbarHeight);

  final double toolbarHeight;

  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    return constraints.tighten(height: toolbarHeight);
  }

  @override
  Size getSize(BoxConstraints constraints) {
    return Size(constraints.maxWidth, toolbarHeight);
  }

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    return Offset(0.0, size.height - childSize.height);
  }

  @override
  bool shouldRelayout(_ToolbarContainerLayout oldDelegate) =>
      toolbarHeight != oldDelegate.toolbarHeight;
}

// TODO(eseidel): Toolbar needs to change size based on orientation:
// https://material.io/design/components/app-bars-top.html#specs
// Mobile Landscape: 48dp
// Mobile Portrait: 56dp
// Tablet/Desktop: 64dp

/// A material design app bar.
///
/// An app bar consists of a toolbar and potentially other widgets, such as a
/// [TabBar] and a [FlexibleSpaceBar]. App bars typically expose one or more
/// common [actions] with [IconButton]s which are optionally followed by a
/// [PopupMenuButton] for less common operations (sometimes called the "overflow
/// menu").
///
/// App bars are typically used in the [Scaffold.appBar] property, which places
/// the app bar as a fixed-height widget at the top of the screen. For a scrollable
/// app bar, see [SliverAppBar], which embeds an [AppBar] in a sliver for use in
/// a [CustomScrollView].
///
/// The AppBar displays the toolbar widgets, [leading], [title], and [actions],
/// above the [bottom] (if any). The [bottom] is usually used for a [TabBar]. If
/// a [flexibleSpace] widget is specified then it is stacked behind the toolbar
/// and the bottom widget. The following diagram shows where each of these slots
/// appears in the toolbar when the writing language is left-to-right (e.g.
/// English):
///
/// The [AppBar] insets its content based on the ambient [MediaQuery]'s padding,
/// to avoid system UI intrusions. It's taken care of by [Scaffold] when used in
/// the [Scaffold.appBar] property. When animating an [AppBar], unexpected
/// [MediaQuery] changes (as is common in [Hero] animations) may cause the content
/// to suddenly jump. Wrap the [AppBar] in a [MediaQuery] widget, and adjust its
/// padding such that the animation is smooth.
///
/// ![The leading widget is in the top left, the actions are in the top right,
/// the title is between them. The bottom is, naturally, at the bottom, and the
/// flexibleSpace is behind all of them.](https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.png)
///
/// If the [leading] widget is omitted, but the [AppBar] is in a [Scaffold] with
/// a [Drawer], then a button will be inserted to open the drawer. Otherwise, if
/// the nearest [Navigator] has any previous routes, a [BackButton] is inserted
/// instead. This behavior can be turned off by setting the [automaticallyImplyLeading]
/// to false. In that case a null leading widget will result in the middle/title widget
/// stretching to start.
///
/// {@tool dartpad --template=stateless_widget_material}
///
/// This sample shows an [AppBar] with two simple actions. The first action
/// opens a [SnackBar], while the second action navigates to a new page.
///
/// ```dart preamble
/// final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
/// final SnackBar snackBar = const SnackBar(content: Text('Showing Snackbar'));
///
/// void openPage(BuildContext context) {
///   Navigator.push(context, MaterialPageRoute(
///     builder: (BuildContext context) {
///       return Scaffold(
///         appBar: AppBar(
///           title: const Text('Next page'),
///         ),
///         body: const Center(
///           child: Text(
///             'This is the next page',
///             style: TextStyle(fontSize: 24),
///           ),
///         ),
///       );
///     },
///   ));
/// }
/// ```
///
/// ```dart
/// Widget build(BuildContext context) {
///   return Scaffold(
///     key: scaffoldKey,
///     appBar: AppBar(
///       title: const Text('AppBar Demo'),
///       actions: <Widget>[
///         IconButton(
///           icon: const Icon(Icons.add_alert),
///           tooltip: 'Show Snackbar',
///           onPressed: () {
///             ScaffoldMessenger.of(context).showSnackBar(snackBar);
///           },
///         ),
///         IconButton(
///           icon: const Icon(Icons.navigate_next),
///           tooltip: 'Next page',
///           onPressed: () {
///             openPage(context);
///           },
///         ),
///       ],
///     ),
///     body: const Center(
///       child: Text(
///         'This is the home page',
///         style: TextStyle(fontSize: 24),
///       ),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [Scaffold], which displays the [AppBar] in its [Scaffold.appBar] slot.
///  * [SliverAppBar], which uses [AppBar] to provide a flexible app bar that
///    can be used in a [CustomScrollView].
///  * [TabBar], which is typically placed in the [bottom] slot of the [AppBar]
///    if the screen has multiple pages arranged in tabs.
///  * [IconButton], which is used with [actions] to show buttons on the app bar.
///  * [PopupMenuButton], to show a popup menu on the app bar, via [actions].
///  * [FlexibleSpaceBar], which is used with [flexibleSpace] when the app bar
///    can expand and collapse.
///  * <https://material.io/design/components/app-bars-top.html>
///  * Cookbook: [Place a floating app bar above a list](https://flutter.dev/docs/cookbook/lists/floating-app-bar)
class AppBar extends StatefulWidget implements PreferredSizeWidget {
  /// Creates a material design app bar.
  ///
  /// The arguments [primary], [toolbarOpacity], [bottomOpacity]
  /// and [automaticallyImplyLeading] must not be null. Additionally, if
  /// [elevation] is specified, it must be non-negative.
  ///
  /// If [backgroundColor], [elevation], [shadowColor], [brightness], [iconTheme],
  /// [actionsIconTheme], [textTheme] or [centerTitle] are null, then their
  /// [AppBarTheme] values will be used. If the corresponding [AppBarTheme] property is null,
  /// then the default specified in the property's documentation will be used.
  ///
  /// Typically used in the [Scaffold.appBar] property.
  AppBar({
    Key? key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation,
    this.shadowColor,
    this.shape,
    this.backgroundColor,
    this.brightness,
    this.iconTheme,
    this.actionsIconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.excludeHeaderSemantics = false,
    this.titleSpacing,
    this.toolbarOpacity = 1.0,
    this.bottomOpacity = 1.0,
    this.toolbarHeight,
    this.leadingWidth,
  }) : assert(automaticallyImplyLeading != null),
       assert(elevation == null || elevation >= 0.0),
       assert(primary != null),
       assert(toolbarOpacity != null),
       assert(bottomOpacity != null),
       preferredSize = Size.fromHeight(toolbarHeight ?? kToolbarHeight + (bottom?.preferredSize.height ?? 0.0)),
       super(key: key);

  /// A widget to display before the [title].
  ///
  /// Typically the [leading] widget is an [Icon] or an [IconButton].
  ///
  /// Becomes the leading component of the [NavigationToolbar] built
  /// by this widget. The [leading] widget's width and height are constrained to
  /// be no bigger than [leadingWidth] and [toolbarHeight] respectively.
  ///
  /// If this is null and [automaticallyImplyLeading] is set to true, the
  /// [AppBar] will imply an appropriate widget. For example, if the [AppBar] is
  /// in a [Scaffold] that also has a [Drawer], the [Scaffold] will fill this
  /// widget with an [IconButton] that opens the drawer (using [Icons.menu]). If
  /// there's no [Drawer] and the parent [Navigator] can go back, the [AppBar]
  /// will use a [BackButton] that calls [Navigator.maybePop].
  ///
  /// {@tool snippet}
  ///
  /// The following code shows how the drawer button could be manually specified
  /// instead of relying on [automaticallyImplyLeading]:
  ///
  /// ```dart
  /// AppBar(
  ///   leading: Builder(
  ///     builder: (BuildContext context) {
  ///       return IconButton(
  ///         icon: const Icon(Icons.menu),
  ///         onPressed: () { Scaffold.of(context).openDrawer(); },
  ///         tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
  ///       );
  ///     },
  ///   ),
  /// )
  /// ```
  /// {@end-tool}
  ///
  /// The [Builder] is used in this example to ensure that the `context` refers
  /// to that part of the subtree. That way this code snippet can be used even
  /// inside the very code that is creating the [Scaffold] (in which case,
  /// without the [Builder], the `context` wouldn't be able to see the
  /// [Scaffold], since it would refer to an ancestor of that widget).
  ///
  /// See also:
  ///
  ///  * [Scaffold.appBar], in which an [AppBar] is usually placed.
  ///  * [Scaffold.drawer], in which the [Drawer] is usually placed.
  final Widget? leading;

  /// Controls whether we should try to imply the leading widget if null.
  ///
  /// If true and [leading] is null, automatically try to deduce what the leading
  /// widget should be. If false and [leading] is null, leading space is given to [title].
  /// If leading widget is not null, this parameter has no effect.
  final bool automaticallyImplyLeading;

  /// The primary widget displayed in the app bar.
  ///
  /// Typically a [Text] widget that contains a description of the current
  /// contents of the app.
  ///
  /// Becomes the middle component of the [NavigationToolbar] built by this widget.
  /// The [title]'s width is constrained to fit within the remaining space
  /// between the toolbar's [leading] and [actions] widgets. Its height is
  /// _not_ constrained. The [title] is vertically centered and clipped to fit
  /// within the toolbar, whose height is [toolbarHeight]. Typically this
  /// isn't noticeable because a simple [Text] [title] will fit within the
  /// toolbar by default. On the other hand, it is noticeable when a
  /// widget with an intrinsic height that is greater than [toolbarHeight]
  /// is used as the [title]. For example, when the height of an Image used
  /// as the [title] exceeds [toolbarHeight], it will be centered and
  /// clipped (top and bottom), which may be undesirable. In cases like this
  /// the height of the [title] widget can be constrained. For example:
  ///
  /// ```dart
  /// MaterialApp(
  ///   home: Scaffold(
  ///     appBar: AppBar(
  ///        title: SizedBox(
  ///        height: toolbarHeight,
  ///        child: child: Image.asset(logoAsset),
  ///      ),
  ///      toolbarHeight: toolbarHeight,
  ///   ),
  /// )
  /// ```
  final Widget? title;

  /// Widgets to display in a row after the [title] widget.
  ///
  /// Typically these widgets are [IconButton]s representing common operations.
  /// For less common operations, consider using a [PopupMenuButton] as the
  /// last action.
  ///
  /// The [actions] become the trailing component of the [NavigationToolbar] built
  /// by this widget. The height of each action is constrained to be no bigger
  /// than the [toolbarHeight].
  final List<Widget>? actions;

  /// This widget is stacked behind the toolbar and the tab bar. It's height will
  /// be the same as the app bar's overall height.
  ///
  /// A flexible space isn't actually flexible unless the [AppBar]'s container
  /// changes the [AppBar]'s size. A [SliverAppBar] in a [CustomScrollView]
  /// changes the [AppBar]'s height when scrolled.
  ///
  /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
  final Widget? flexibleSpace;

  /// This widget appears across the bottom of the app bar.
  ///
  /// Typically a [TabBar]. Only widgets that implement [PreferredSizeWidget] can
  /// be used at the bottom of an app bar.
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// The z-coordinate at which to place this app bar relative to its parent.
  ///
  /// This controls the size of the shadow below the app bar.
  ///
  /// The value is non-negative.
  ///
  /// If this property is null, then [AppBarTheme.elevation] of
  /// [ThemeData.appBarTheme] is used. If that is also null, the default value
  /// is 4.
  final double? elevation;

  /// The color to paint the shadow below the app bar.
  ///
  /// If this property is null, then [AppBarTheme.shadowColor] of
  /// [ThemeData.appBarTheme] is used. If that is also null, the default value
  /// is fully opaque black.
  final Color? shadowColor;

  /// The material's shape as well its shadow.
  ///
  /// A shadow is only displayed if the [elevation] is greater than
  /// zero.
  final ShapeBorder? shape;

  /// The color to use for the app bar's material. Typically this should be set
  /// along with [brightness], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.color] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryColor] is used.
  final Color? backgroundColor;

  /// The brightness of the app bar's material. Typically this is set along
  /// with [backgroundColor], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.brightness] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryColorBrightness] is used.
  final Brightness? brightness;

  /// The color, opacity, and size to use for app bar icons. Typically this
  /// is set along with [backgroundColor], [brightness], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.iconTheme] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryIconTheme] is used.
  final IconThemeData? iconTheme;

  /// The color, opacity, and size to use for the icons that appear in the app
  /// bar's [actions]. This should only be used when the [actions] should be
  /// themed differently than the icon that appears in the app bar's [leading]
  /// widget.
  ///
  /// If this property is null, then [AppBarTheme.actionsIconTheme] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then this falls
  /// back to [iconTheme].
  final IconThemeData? actionsIconTheme;

  /// The typographic styles to use for text in the app bar. Typically this is
  /// set along with [brightness] [backgroundColor], [iconTheme].
  ///
  /// If this property is null, then [AppBarTheme.textTheme] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryTextTheme] is used.
  final TextTheme? textTheme;

  /// Whether this app bar is being displayed at the top of the screen.
  ///
  /// If true, the app bar's toolbar elements and [bottom] widget will be
  /// padded on top by the height of the system status bar. The layout
  /// of the [flexibleSpace] is not affected by the [primary] property.
  final bool primary;

  /// Whether the title should be centered.
  ///
  /// If this property is null, then [AppBarTheme.centerTitle] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then value is
  /// adapted to the current [TargetPlatform].
  final bool? centerTitle;

  /// Whether the title should be wrapped with header [Semantics].
  ///
  /// Defaults to false.
  final bool excludeHeaderSemantics;

  /// The spacing around [title] content on the horizontal axis. This spacing is
  /// applied even if there is no [leading] content or [actions]. If you want
  /// [title] to take all the space available, set this value to 0.0.
  ///
  /// Defaults to [NavigationToolbar.kMiddleSpacing].
  final double? titleSpacing;

  /// How opaque the toolbar part of the app bar is.
  ///
  /// A value of 1.0 is fully opaque, and a value of 0.0 is fully transparent.
  ///
  /// Typically, this value is not changed from its default value (1.0). It is
  /// used by [SliverAppBar] to animate the opacity of the toolbar when the app
  /// bar is scrolled.
  final double toolbarOpacity;

  /// How opaque the bottom part of the app bar is.
  ///
  /// A value of 1.0 is fully opaque, and a value of 0.0 is fully transparent.
  ///
  /// Typically, this value is not changed from its default value (1.0). It is
  /// used by [SliverAppBar] to animate the opacity of the toolbar when the app
  /// bar is scrolled.
  final double bottomOpacity;

  /// A size whose height is the sum of [toolbarHeight] and the [bottom] widget's
  /// preferred height.
  ///
  /// [Scaffold] uses this size to set its app bar's height.
  @override
  final Size preferredSize;

  /// Defines the height of the toolbar component of an [AppBar].
  ///
  /// By default, the value of `toolbarHeight` is [kToolbarHeight].
  final double? toolbarHeight;

  /// Defines the width of [leading] widget.
  ///
  /// By default, the value of `leadingWidth` is 56.0.
  final double? leadingWidth;

  bool _getEffectiveCenterTitle(ThemeData theme) {
    if (centerTitle != null)
      return centerTitle!;
    if (theme.appBarTheme.centerTitle != null)
      return theme.appBarTheme.centerTitle!;
    assert(theme.platform != null);
    switch (theme.platform) {
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
      case TargetPlatform.linux:
      case TargetPlatform.windows:
        return false;
      case TargetPlatform.iOS:
      case TargetPlatform.macOS:
        return actions == null || actions!.length < 2;
    }
  }

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

class _AppBarState extends State<AppBar> {
  static const double _defaultElevation = 4.0;
  static const Color _defaultShadowColor = Color(0xFF000000);

  void _handleDrawerButton() {
    Scaffold.of(context)!.openDrawer();
  }

  void _handleDrawerButtonEnd() {
    Scaffold.of(context)!.openEndDrawer();
  }

  @override
  Widget build(BuildContext context) {
    assert(!widget.primary || debugCheckHasMediaQuery(context));
    assert(debugCheckHasMaterialLocalizations(context));
    final ThemeData? theme = Theme.of(context);
    final AppBarTheme appBarTheme = AppBarTheme.of(context);
    final ScaffoldState? scaffold = Scaffold.of(context, nullOk: true);
    final ModalRoute<dynamic>? parentRoute = ModalRoute.of(context);

    final bool hasDrawer = scaffold?.hasDrawer ?? false;
    final bool hasEndDrawer = scaffold?.hasEndDrawer ?? false;
    final bool canPop = parentRoute?.canPop ?? false;
    final bool useCloseButton = parentRoute is PageRoute<dynamic> && parentRoute.fullscreenDialog;

    final double toolbarHeight = widget.toolbarHeight ?? kToolbarHeight;

    IconThemeData overallIconTheme = widget.iconTheme
      ?? appBarTheme.iconTheme
      ?? theme!.primaryIconTheme;
    IconThemeData actionsIconTheme = widget.actionsIconTheme
      ?? appBarTheme.actionsIconTheme
      ?? overallIconTheme;
    TextStyle? centerStyle = widget.textTheme?.headline6
      ?? appBarTheme.textTheme?.headline6
      ?? theme!.primaryTextTheme.headline6;
    TextStyle? sideStyle = widget.textTheme?.bodyText2
      ?? appBarTheme.textTheme?.bodyText2
      ?? theme!.primaryTextTheme.bodyText2;

    if (widget.toolbarOpacity != 1.0) {
      final double opacity = const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn).transform(widget.toolbarOpacity);
      if (centerStyle?.color != null)
        centerStyle = centerStyle!.copyWith(color: centerStyle.color!.withOpacity(opacity));
      if (sideStyle?.color != null)
        sideStyle = sideStyle!.copyWith(color: sideStyle.color!.withOpacity(opacity));
      overallIconTheme = overallIconTheme.copyWith(
        opacity: opacity * (overallIconTheme.opacity ?? 1.0)
      );
      actionsIconTheme = actionsIconTheme.copyWith(
        opacity: opacity * (actionsIconTheme.opacity ?? 1.0)
      );
    }

    Widget? leading = widget.leading;
    if (leading == null && widget.automaticallyImplyLeading) {
      if (hasDrawer) {
        leading = IconButton(
          icon: const Icon(Icons.menu),
          onPressed: _handleDrawerButton,
          tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
        );
      } else {
        if (!hasEndDrawer && canPop)
          leading = useCloseButton ? const CloseButton() : const BackButton();
      }
    }
    if (leading != null) {
      leading = ConstrainedBox(
        constraints: BoxConstraints.tightFor(width: widget.leadingWidth ?? _kLeadingWidth),
        child: leading,
      );
    }

    Widget? title = widget.title;
    if (title != null) {
      bool? namesRoute;
      switch (theme!.platform) {
        case TargetPlatform.android:
        case TargetPlatform.fuchsia:
        case TargetPlatform.linux:
        case TargetPlatform.windows:
          namesRoute = true;
          break;
        case TargetPlatform.iOS:
        case TargetPlatform.macOS:
          break;
      }

      title = _AppBarTitleBox(child: title);
      if (!widget.excludeHeaderSemantics) {
        title = Semantics(
          namesRoute: namesRoute,
          child: title,
          header: true,
        );
      }

      title = DefaultTextStyle(
        style: centerStyle!,
        softWrap: false,
        overflow: TextOverflow.ellipsis,
        child: title,
      );

      // Set maximum text scale factor to [_kMaxTitleTextScaleFactor] for the
      // title to keep the visual hierarchy the same even with larger font
      // sizes. To opt out, wrap the [title] widget in a [MediaQuery] widget
      // with [MediaQueryData.textScaleFactor] set to
      // `MediaQuery.textScaleFactorOf(context)`.
      final MediaQueryData mediaQueryData = MediaQuery.of(context)!;
      title = MediaQuery(
        data: mediaQueryData.copyWith(
          textScaleFactor: math.min(
            mediaQueryData.textScaleFactor,
            _kMaxTitleTextScaleFactor,
          ),
        ),
        child: title,
      );
    }

    Widget? actions;
    if (widget.actions != null && widget.actions!.isNotEmpty) {
      actions = Row(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: widget.actions!,
      );
    } else if (hasEndDrawer) {
      actions = IconButton(
        icon: const Icon(Icons.menu),
        onPressed: _handleDrawerButtonEnd,
        tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
      );
    }

    // Allow the trailing actions to have their own theme if necessary.
    if (actions != null) {
      actions = IconTheme.merge(
        data: actionsIconTheme,
        child: actions,
      );
    }

    final Widget toolbar = NavigationToolbar(
      leading: leading,
      middle: title,
      trailing: actions,
      centerMiddle: widget._getEffectiveCenterTitle(theme!),
      middleSpacing: widget.titleSpacing ?? appBarTheme.titleSpacing ?? NavigationToolbar.kMiddleSpacing,
    );

    // If the toolbar is allocated less than toolbarHeight make it
    // appear to scroll upwards within its shrinking container.
    Widget appBar = ClipRect(
      child: CustomSingleChildLayout(
        delegate: _ToolbarContainerLayout(toolbarHeight),
        child: IconTheme.merge(
          data: overallIconTheme,
          child: DefaultTextStyle(
            style: sideStyle!,
            child: toolbar,
          ),
        ),
      ),
    );
    if (widget.bottom != null) {
      appBar = Column(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: <Widget>[
          Flexible(
            child: ConstrainedBox(
              constraints: BoxConstraints(maxHeight: toolbarHeight),
              child: appBar,
            ),
          ),
          if (widget.bottomOpacity == 1.0)
            widget.bottom!
          else
            Opacity(
              opacity: const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn).transform(widget.bottomOpacity),
              child: widget.bottom,
            ),
        ],
      );
    }

    // The padding applies to the toolbar and tabbar, not the flexible space.
    if (widget.primary) {
      appBar = SafeArea(
        bottom: false,
        top: true,
        child: appBar,
      );
    }

    appBar = Align(
      alignment: Alignment.topCenter,
      child: appBar,
    );

    if (widget.flexibleSpace != null) {
      appBar = Stack(
        fit: StackFit.passthrough,
        children: <Widget>[
          Semantics(
            sortKey: const OrdinalSortKey(1.0),
            explicitChildNodes: true,
            child: widget.flexibleSpace,
          ),
          Semantics(
            sortKey: const OrdinalSortKey(0.0),
            explicitChildNodes: true,
            // Creates a material widget to prevent the flexibleSpace from
            // obscuring the ink splashes produced by appBar children.
            child: Material(
              type: MaterialType.transparency,
              child: appBar,
            ),
          ),
        ],
      );
    }
    final Brightness brightness = widget.brightness
      ?? appBarTheme.brightness
      ?? theme.primaryColorBrightness;
    final SystemUiOverlayStyle overlayStyle = brightness == Brightness.dark
      ? SystemUiOverlayStyle.light
      : SystemUiOverlayStyle.dark;

    return Semantics(
      container: true,
      child: AnnotatedRegion<SystemUiOverlayStyle>(
        value: overlayStyle,
        child: Material(
          color: widget.backgroundColor
            ?? appBarTheme.color
            ?? theme.primaryColor,
          elevation: widget.elevation
            ?? appBarTheme.elevation
            ?? _defaultElevation,
          shadowColor: widget.shadowColor
            ?? appBarTheme.shadowColor
            ?? _defaultShadowColor,
          shape: widget.shape,
          child: Semantics(
            explicitChildNodes: true,
            child: appBar,
          ),
        ),
      ),
    );
  }
}

class _FloatingAppBar extends StatefulWidget {
  const _FloatingAppBar({ Key? key, required this.child }) : super(key: key);

  final Widget child;

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

// A wrapper for the widget created by _SliverAppBarDelegate that starts and
// stops the floating app bar's snap-into-view or snap-out-of-view animation.
class _FloatingAppBarState extends State<_FloatingAppBar> {
  ScrollPosition? _position;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    _position = Scrollable.of(context)?.position;
    if (_position != null)
      _position!.isScrollingNotifier.addListener(_isScrollingListener);
  }

  @override
  void dispose() {
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    super.dispose();
  }

  RenderSliverFloatingPersistentHeader? _headerRenderer() {
    return context.findAncestorRenderObjectOfType<RenderSliverFloatingPersistentHeader>();
  }

  void _isScrollingListener() {
    if (_position == null)
      return;

    // When a scroll stops, then maybe snap the appbar into view.
    // Similarly, when a scroll starts, then maybe stop the snap animation.
    final RenderSliverFloatingPersistentHeader? header = _headerRenderer();
    if (_position!.isScrollingNotifier.value)
      header?.maybeStopSnapAnimation(_position!.userScrollDirection);
    else
      header?.maybeStartSnapAnimation(_position!.userScrollDirection);
  }

  @override
  Widget build(BuildContext context) => widget.child;
}

class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverAppBarDelegate({
    required this.leading,
    required this.automaticallyImplyLeading,
    required this.title,
    required this.actions,
    required this.flexibleSpace,
    required this.bottom,
    required this.elevation,
    required this.shadowColor,
    required this.forceElevated,
    required this.backgroundColor,
    required this.brightness,
    required this.iconTheme,
    required this.actionsIconTheme,
    required this.textTheme,
    required this.primary,
    required this.centerTitle,
    required this.excludeHeaderSemantics,
    required this.titleSpacing,
    required this.expandedHeight,
    required this.collapsedHeight,
    required this.topPadding,
    required this.floating,
    required this.pinned,
    required this.vsync,
    required this.snapConfiguration,
    required this.stretchConfiguration,
    required this.showOnScreenConfiguration,
    required this.shape,
    required this.toolbarHeight,
    required this.leadingWidth,
  }) : assert(primary || topPadding == 0.0),
       assert(
         !floating || (snapConfiguration == null && showOnScreenConfiguration == null) || vsync != null,
         'vsync cannot be null when snapConfiguration or showOnScreenConfiguration is not null, and floating is true',
       ),
       _bottomHeight = bottom?.preferredSize.height ?? 0.0;

  final Widget? leading;
  final bool automaticallyImplyLeading;
  final Widget? title;
  final List<Widget>? actions;
  final Widget? flexibleSpace;
  final PreferredSizeWidget? bottom;
  final double? elevation;
  final Color? shadowColor;
  final bool forceElevated;
  final Color? backgroundColor;
  final Brightness? brightness;
  final IconThemeData? iconTheme;
  final IconThemeData? actionsIconTheme;
  final TextTheme? textTheme;
  final bool primary;
  final bool? centerTitle;
  final bool excludeHeaderSemantics;
  final double? titleSpacing;
  final double? expandedHeight;
  final double collapsedHeight;
  final double topPadding;
  final bool floating;
  final bool pinned;
  final ShapeBorder? shape;
  final double? toolbarHeight;
  final double? leadingWidth;

  final double _bottomHeight;

  @override
  double get minExtent => collapsedHeight;

  @override
  double get maxExtent => math.max(topPadding + (expandedHeight ?? (toolbarHeight ?? kToolbarHeight) + _bottomHeight), minExtent);

  @override
  final TickerProvider vsync;

  @override
  final FloatingHeaderSnapConfiguration? snapConfiguration;

  @override
  final OverScrollHeaderStretchConfiguration? stretchConfiguration;

  @override
  final PersistentHeaderShowOnScreenConfiguration? showOnScreenConfiguration;

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
    final double visibleMainHeight = maxExtent - shrinkOffset - topPadding;
    final double extraToolbarHeight = math.max(minExtent - _bottomHeight - topPadding - (toolbarHeight ?? kToolbarHeight), 0.0);
    final double visibleToolbarHeight = visibleMainHeight - _bottomHeight - extraToolbarHeight;

    final bool isPinnedWithOpacityFade = pinned && floating && bottom != null && extraToolbarHeight == 0.0;
    final double toolbarOpacity = !pinned || isPinnedWithOpacityFade
      ? (visibleToolbarHeight / (toolbarHeight ?? kToolbarHeight)).clamp(0.0, 1.0)
      : 1.0;

    final Widget appBar = FlexibleSpaceBar.createSettings(
      minExtent: minExtent,
      maxExtent: maxExtent,
      currentExtent: math.max(minExtent, maxExtent - shrinkOffset),
      toolbarOpacity: toolbarOpacity,
      child: AppBar(
        leading: leading,
        automaticallyImplyLeading: automaticallyImplyLeading,
        title: title,
        actions: actions,
        flexibleSpace: (title == null && flexibleSpace != null && !excludeHeaderSemantics)
          ? Semantics(child: flexibleSpace, header: true)
          : flexibleSpace,
        bottom: bottom,
        elevation: forceElevated || overlapsContent || (pinned && shrinkOffset > maxExtent - minExtent) ? elevation ?? 4.0 : 0.0,
        shadowColor: shadowColor,
        backgroundColor: backgroundColor,
        brightness: brightness,
        iconTheme: iconTheme,
        actionsIconTheme: actionsIconTheme,
        textTheme: textTheme,
        primary: primary,
        centerTitle: centerTitle,
        excludeHeaderSemantics: excludeHeaderSemantics,
        titleSpacing: titleSpacing,
        shape: shape,
        toolbarOpacity: toolbarOpacity,
        bottomOpacity: pinned ? 1.0 : ((visibleMainHeight / _bottomHeight).clamp(0.0, 1.0)),
        toolbarHeight: toolbarHeight,
        leadingWidth: leadingWidth,
      ),
    );
    return floating ? _FloatingAppBar(child: appBar) : appBar;
  }

  @override
  bool shouldRebuild(covariant _SliverAppBarDelegate oldDelegate) {
    return leading != oldDelegate.leading
        || automaticallyImplyLeading != oldDelegate.automaticallyImplyLeading
        || title != oldDelegate.title
        || actions != oldDelegate.actions
        || flexibleSpace != oldDelegate.flexibleSpace
        || bottom != oldDelegate.bottom
        || _bottomHeight != oldDelegate._bottomHeight
        || elevation != oldDelegate.elevation
        || shadowColor != oldDelegate.shadowColor
        || backgroundColor != oldDelegate.backgroundColor
        || brightness != oldDelegate.brightness
        || iconTheme != oldDelegate.iconTheme
        || actionsIconTheme != oldDelegate.actionsIconTheme
        || textTheme != oldDelegate.textTheme
        || primary != oldDelegate.primary
        || centerTitle != oldDelegate.centerTitle
        || titleSpacing != oldDelegate.titleSpacing
        || expandedHeight != oldDelegate.expandedHeight
        || topPadding != oldDelegate.topPadding
        || pinned != oldDelegate.pinned
        || floating != oldDelegate.floating
        || vsync != oldDelegate.vsync
        || snapConfiguration != oldDelegate.snapConfiguration
        || stretchConfiguration != oldDelegate.stretchConfiguration
        || showOnScreenConfiguration != oldDelegate.showOnScreenConfiguration
        || forceElevated != oldDelegate.forceElevated
        || toolbarHeight != oldDelegate.toolbarHeight
        || leadingWidth != leadingWidth;
  }

  @override
  String toString() {
    return '${describeIdentity(this)}(topPadding: ${topPadding.toStringAsFixed(1)}, bottomHeight: ${_bottomHeight.toStringAsFixed(1)}, ...)';
  }
}

/// A material design app bar that integrates with a [CustomScrollView].
///
/// An app bar consists of a toolbar and potentially other widgets, such as a
/// [TabBar] and a [FlexibleSpaceBar]. App bars typically expose one or more
/// common actions with [IconButton]s which are optionally followed by a
/// [PopupMenuButton] for less common operations.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=R9C5KMJKluE}
///
/// Sliver app bars are typically used as the first child of a
/// [CustomScrollView], which lets the app bar integrate with the scroll view so
/// that it can vary in height according to the scroll offset or float above the
/// other content in the scroll view. For a fixed-height app bar at the top of
/// the screen see [AppBar], which is used in the [Scaffold.appBar] slot.
///
/// The AppBar displays the toolbar widgets, [leading], [title], and
/// [actions], above the [bottom] (if any). If a [flexibleSpace] widget is
/// specified then it is stacked behind the toolbar and the bottom widget.
///
/// {@tool snippet}
///
/// This is an example that could be included in a [CustomScrollView]'s
/// [CustomScrollView.slivers] list:
///
/// ```dart
/// SliverAppBar(
///   expandedHeight: 150.0,
///   flexibleSpace: const FlexibleSpaceBar(
///     title: Text('Available seats'),
///   ),
///   actions: <Widget>[
///     IconButton(
///       icon: const Icon(Icons.add_circle),
///       tooltip: 'Add new entry',
///       onPressed: () { /* ... */ },
///     ),
///   ]
/// )
/// ```
/// {@end-tool}
///
/// ## Animated Examples
///
/// The following animations show how app bars with different configurations
/// behave when a user scrolls up and then down again.
///
/// * App bar with [floating]: false, [pinned]: false, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
///
/// * App bar with [floating]: true, [pinned]: false, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
///
/// * App bar with [floating]: true, [pinned]: false, [snap]: true:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating_snap.mp4}
///
/// * App bar with [floating]: true, [pinned]: true, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned_floating.mp4}
///
/// * App bar with [floating]: true, [pinned]: true, [snap]: true:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned_floating_snap.mp4}
///
/// * App bar with [floating]: false, [pinned]: true, [snap]: false:
///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned.mp4}
///
/// The property [snap] can only be set to true if [floating] is also true.
///
/// See also:
///
///  * [CustomScrollView], which integrates the [SliverAppBar] into its
///    scrolling.
///  * [AppBar], which is a fixed-height app bar for use in [Scaffold.appBar].
///  * [TabBar], which is typically placed in the [bottom] slot of the [AppBar]
///    if the screen has multiple pages arranged in tabs.
///  * [IconButton], which is used with [actions] to show buttons on the app bar.
///  * [PopupMenuButton], to show a popup menu on the app bar, via [actions].
///  * [FlexibleSpaceBar], which is used with [flexibleSpace] when the app bar
///    can expand and collapse.
///  * <https://material.io/design/components/app-bars-top.html>
class SliverAppBar extends StatefulWidget {
  /// Creates a material design app bar that can be placed in a [CustomScrollView].
  ///
  /// The arguments [forceElevated], [primary], [floating], [pinned], [snap]
  /// and [automaticallyImplyLeading] must not be null.
  const SliverAppBar({
    Key? key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation,
    this.shadowColor,
    this.forceElevated = false,
    this.backgroundColor,
    this.brightness,
    this.iconTheme,
    this.actionsIconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.excludeHeaderSemantics = false,
    this.titleSpacing,
    this.collapsedHeight,
    this.expandedHeight,
    this.floating = false,
    this.pinned = false,
    this.snap = false,
    this.stretch = false,
    this.stretchTriggerOffset = 100.0,
    this.onStretchTrigger,
    this.shape,
    this.toolbarHeight = kToolbarHeight,
    this.leadingWidth,
  }) : assert(automaticallyImplyLeading != null),
       assert(forceElevated != null),
       assert(primary != null),
       assert(floating != null),
       assert(pinned != null),
       assert(snap != null),
       assert(stretch != null),
       assert(toolbarHeight != null),
       assert(floating || !snap, 'The "snap" argument only makes sense for floating app bars.'),
       assert(stretchTriggerOffset > 0.0),
       assert(collapsedHeight == null || collapsedHeight > toolbarHeight, 'The "collapsedHeight" argument has to be larger than [toolbarHeight].'),
       super(key: key);

  /// A widget to display before the [title].
  ///
  /// If this is null and [automaticallyImplyLeading] is set to true, the [AppBar] will
  /// imply an appropriate widget. For example, if the [AppBar] is in a [Scaffold]
  /// that also has a [Drawer], the [Scaffold] will fill this widget with an
  /// [IconButton] that opens the drawer. If there's no [Drawer] and the parent
  /// [Navigator] can go back, the [AppBar] will use a [BackButton] that calls
  /// [Navigator.maybePop].
  final Widget? leading;

  /// Controls whether we should try to imply the leading widget if null.
  ///
  /// If true and [leading] is null, automatically try to deduce what the leading
  /// widget should be. If false and [leading] is null, leading space is given to [title].
  /// If leading widget is not null, this parameter has no effect.
  final bool automaticallyImplyLeading;

  /// The primary widget displayed in the app bar.
  ///
  /// Typically a [Text] widget containing a description of the current contents
  /// of the app.
  final Widget? title;

  /// Widgets to display after the [title] widget.
  ///
  /// Typically these widgets are [IconButton]s representing common operations.
  /// For less common operations, consider using a [PopupMenuButton] as the
  /// last action.
  ///
  /// {@tool snippet}
  ///
  /// ```dart
  /// Scaffold(
  ///   body: CustomScrollView(
  ///     primary: true,
  ///     slivers: <Widget>[
  ///       SliverAppBar(
  ///         title: Text('Hello World'),
  ///         actions: <Widget>[
  ///           IconButton(
  ///             icon: Icon(Icons.shopping_cart),
  ///             tooltip: 'Open shopping cart',
  ///             onPressed: () {
  ///               // handle the press
  ///             },
  ///           ),
  ///         ],
  ///       ),
  ///       // ...rest of body...
  ///     ],
  ///   ),
  /// )
  /// ```
  /// {@end-tool}
  final List<Widget>? actions;

  /// This widget is stacked behind the toolbar and the tab bar. It's height will
  /// be the same as the app bar's overall height.
  ///
  /// When using [SliverAppBar.flexibleSpace], the [SliverAppBar.expandedHeight]
  /// must be large enough to accommodate the [SliverAppBar.flexibleSpace] widget.
  ///
  /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
  final Widget? flexibleSpace;

  /// This widget appears across the bottom of the app bar.
  ///
  /// Typically a [TabBar]. Only widgets that implement [PreferredSizeWidget] can
  /// be used at the bottom of an app bar.
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// The z-coordinate at which to place this app bar when it is above other
  /// content. This controls the size of the shadow below the app bar.
  ///
  /// If this property is null, then [AppBarTheme.elevation] of
  /// [ThemeData.appBarTheme] is used, if that is also null, the default value
  /// is 4.
  ///
  /// If [forceElevated] is false, the elevation is ignored when the app bar has
  /// no content underneath it. For example, if the app bar is [pinned] but no
  /// content is scrolled under it, or if it scrolls with the content, then no
  /// shadow is drawn, regardless of the value of [elevation].
  final double? elevation;

  /// The color to paint the shadow below the app bar. Typically this should be set
  /// along with [elevation].
  ///
  /// If this property is null, then [AppBarTheme.shadowColor] of
  /// [ThemeData.appBarTheme] is used, if that is also null, the default value
  /// is fully opaque black.
  final Color? shadowColor;

  /// Whether to show the shadow appropriate for the [elevation] even if the
  /// content is not scrolled under the [AppBar].
  ///
  /// Defaults to false, meaning that the [elevation] is only applied when the
  /// [AppBar] is being displayed over content that is scrolled under it.
  ///
  /// When set to true, the [elevation] is applied regardless.
  ///
  /// Ignored when [elevation] is zero.
  final bool forceElevated;

  /// The color to use for the app bar's material. Typically this should be set
  /// along with [brightness], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.color] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryColor] is used.
  final Color? backgroundColor;

  /// The brightness of the app bar's material. Typically this is set along
  /// with [backgroundColor], [iconTheme], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.brightness] of
  /// [ThemeData.appBarTheme] is used. If that is also null, then
  /// [ThemeData.primaryColorBrightness] is used.
  final Brightness? brightness;

  /// The color, opacity, and size to use for app bar icons. Typically this
  /// is set along with [backgroundColor], [brightness], [textTheme].
  ///
  /// If this property is null, then [AppBarTheme.iconTheme] of
  /// [ThemeData.appBarTheme] is used, if that is also null, then
  /// [ThemeData.primaryIconTheme] is used.
  final IconThemeData? iconTheme;

  /// The color, opacity, and size to use for trailing app bar icons. This
  /// should only be used when the trailing icons should be themed differently
  /// than the leading icons.
  ///
  /// If this property is null, then [AppBarTheme.actionsIconTheme] of
  /// [ThemeData.appBarTheme] is used, if that is also null, then this falls
  /// back to [iconTheme].
  final IconThemeData? actionsIconTheme;

  /// The typographic styles to use for text in the app bar. Typically this is
  /// set along with [brightness] [backgroundColor], [iconTheme].
  ///
  /// If this property is null, then [AppBarTheme.textTheme] of
  /// [ThemeData.appBarTheme] is used, if that is also null, then
  /// [ThemeData.primaryTextTheme] is used.
  final TextTheme? textTheme;

  /// Whether this app bar is being displayed at the top of the screen.
  ///
  /// If this is true, the top padding specified by the [MediaQuery] will be
  /// added to the top of the toolbar.
  final bool primary;

  /// Whether the title should be centered.
  ///
  /// Defaults to being adapted to the current [TargetPlatform].
  final bool? centerTitle;

  /// Whether the title should be wrapped with header [Semantics].
  ///
  /// Defaults to false.
  final bool excludeHeaderSemantics;

  /// The spacing around [title] content on the horizontal axis. This spacing is
  /// applied even if there is no [leading] content or [actions]. If you want
  /// [title] to take all the space available, set this value to 0.0.
  ///
  /// Defaults to [NavigationToolbar.kMiddleSpacing].
  final double? titleSpacing;

  /// Defines the height of the app bar when it is collapsed.
  ///
  /// By default, the collapsed height is [toolbarHeight]. If [bottom] widget is
  /// specified, then its height from [PreferredSizeWidget.preferredSize] is
  /// added to the height. If [primary] is true, then the [MediaQuery] top
  /// padding, [EdgeInsets.top] of [MediaQueryData.padding], is added as well.
  ///
  /// If [pinned] and [floating] are true, with [bottom] set, the default
  /// collapsed height is only the height of [PreferredSizeWidget.preferredSize]
  /// with the [MediaQuery] top padding.
  final double? collapsedHeight;

  /// The size of the app bar when it is fully expanded.
  ///
  /// By default, the total height of the toolbar and the bottom widget (if
  /// any). If a [flexibleSpace] widget is specified this height should be big
  /// enough to accommodate whatever that widget contains.
  ///
  /// This does not include the status bar height (which will be automatically
  /// included if [primary] is true).
  final double? expandedHeight;

  /// Whether the app bar should become visible as soon as the user scrolls
  /// towards the app bar.
  ///
  /// Otherwise, the user will need to scroll near the top of the scroll view to
  /// reveal the app bar.
  ///
  /// If [snap] is true then a scroll that exposes the app bar will trigger an
  /// animation that slides the entire app bar into view. Similarly if a scroll
  /// dismisses the app bar, the animation will slide it completely out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [floating] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [floating] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [pinned] and [snap].
  final bool floating;

  /// Whether the app bar should remain visible at the start of the scroll view.
  ///
  /// The app bar can still expand and contract as the user scrolls, but it will
  /// remain visible rather than being scrolled out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [pinned] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [pinned] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [floating].
  final bool pinned;

  /// The material's shape as well as its shadow.
  ///
  /// A shadow is only displayed if the [elevation] is greater than zero.
  final ShapeBorder? shape;

  /// If [snap] and [floating] are true then the floating app bar will "snap"
  /// into view.
  ///
  /// If [snap] is true then a scroll that exposes the floating app bar will
  /// trigger an animation that slides the entire app bar into view. Similarly
  /// if a scroll dismisses the app bar, the animation will slide the app bar
  /// completely out of view. Additionally, setting [snap] to true will fully
  /// expand the floating app bar when the framework tries to reveal the
  /// contents of the app bar by calling [RenderObject.showOnScreen]. For
  /// example, when a [TextField] in the floating app bar gains focus, if [snap]
  /// is true, the framework will always fully expand the floating app bar, in
  /// order to reveal the focused [TextField].
  ///
  /// Snapping only applies when the app bar is floating, not when the app bar
  /// appears at the top of its scroll view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [snap] set to false:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  /// * App bar with [snap] set to true:
  ///   {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating_snap.mp4}
  ///
  /// See also:
  ///
  ///  * [SliverAppBar] for more animated examples of how this property changes the
  ///    behavior of the app bar in combination with [pinned] and [floating].
  final bool snap;

  /// Whether the app bar should stretch to fill the over-scroll area.
  ///
  /// The app bar can still expand and contract as the user scrolls, but it will
  /// also stretch when the user over-scrolls.
  final bool stretch;

  /// The offset of overscroll required to activate [onStretchTrigger].
  ///
  /// This defaults to 100.0.
  final double stretchTriggerOffset;

  /// The callback function to be executed when a user over-scrolls to the
  /// offset specified by [stretchTriggerOffset].
  final AsyncCallback? onStretchTrigger;

  /// Defines the height of the toolbar component of an [AppBar].
  ///
  /// By default, the value of `toolbarHeight` is [kToolbarHeight].
  final double toolbarHeight;

  /// Defines the width of [leading] widget.
  ///
  /// By default, the value of `leadingWidth` is 56.0.
  final double? leadingWidth;

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

// This class is only Stateful because it owns the TickerProvider used
// by the floating appbar snap animation (via FloatingHeaderSnapConfiguration).
class _SliverAppBarState extends State<SliverAppBar> with TickerProviderStateMixin {
  FloatingHeaderSnapConfiguration? _snapConfiguration;
  OverScrollHeaderStretchConfiguration? _stretchConfiguration;
  PersistentHeaderShowOnScreenConfiguration? _showOnScreenConfiguration;

  void _updateSnapConfiguration() {
    if (widget.snap && widget.floating) {
      _snapConfiguration = FloatingHeaderSnapConfiguration(
        curve: Curves.easeOut,
        duration: const Duration(milliseconds: 200),
      );
    } else {
      _snapConfiguration = null;
    }

    _showOnScreenConfiguration = widget.floating & widget.snap
      ? const PersistentHeaderShowOnScreenConfiguration(minShowOnScreenExtent: double.infinity)
      : null;
  }

  void _updateStretchConfiguration() {
    if (widget.stretch) {
      _stretchConfiguration = OverScrollHeaderStretchConfiguration(
        stretchTriggerOffset: widget.stretchTriggerOffset,
        onStretchTrigger: widget.onStretchTrigger,
      );
    } else {
      _stretchConfiguration = null;
    }
  }

  @override
  void initState() {
    super.initState();
    _updateSnapConfiguration();
    _updateStretchConfiguration();
  }

  @override
  void didUpdateWidget(SliverAppBar oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.snap != oldWidget.snap || widget.floating != oldWidget.floating)
      _updateSnapConfiguration();
    if (widget.stretch != oldWidget.stretch)
      _updateStretchConfiguration();
  }

  @override
  Widget build(BuildContext context) {
    assert(!widget.primary || debugCheckHasMediaQuery(context));
    final double bottomHeight = widget.bottom?.preferredSize.height ?? 0.0;
    final double topPadding = widget.primary ? MediaQuery.of(context)!.padding.top : 0.0;
    final double collapsedHeight = (widget.pinned && widget.floating && widget.bottom != null)
      ? (widget.collapsedHeight ?? 0.0) + bottomHeight + topPadding
      : (widget.collapsedHeight ?? widget.toolbarHeight) + bottomHeight + topPadding;

    return MediaQuery.removePadding(
      context: context,
      removeBottom: true,
      child: SliverPersistentHeader(
        floating: widget.floating,
        pinned: widget.pinned,
        delegate: _SliverAppBarDelegate(
          vsync: this,
          leading: widget.leading,
          automaticallyImplyLeading: widget.automaticallyImplyLeading,
          title: widget.title,
          actions: widget.actions,
          flexibleSpace: widget.flexibleSpace,
          bottom: widget.bottom,
          elevation: widget.elevation,
          shadowColor: widget.shadowColor,
          forceElevated: widget.forceElevated,
          backgroundColor: widget.backgroundColor,
          brightness: widget.brightness,
          iconTheme: widget.iconTheme,
          actionsIconTheme: widget.actionsIconTheme,
          textTheme: widget.textTheme,
          primary: widget.primary,
          centerTitle: widget.centerTitle,
          excludeHeaderSemantics: widget.excludeHeaderSemantics,
          titleSpacing: widget.titleSpacing,
          expandedHeight: widget.expandedHeight,
          collapsedHeight: collapsedHeight,
          topPadding: topPadding,
          floating: widget.floating,
          pinned: widget.pinned,
          shape: widget.shape,
          snapConfiguration: _snapConfiguration,
          stretchConfiguration: _stretchConfiguration,
          showOnScreenConfiguration: _showOnScreenConfiguration,
          toolbarHeight: widget.toolbarHeight,
          leadingWidth: widget.leadingWidth,
        ),
      ),
    );
  }
}

// Layout the AppBar's title with unconstrained height, vertically
// center it within its (NavigationToolbar) parent, and allow the
// parent to constrain the title's actual height.
class _AppBarTitleBox extends SingleChildRenderObjectWidget {
  const _AppBarTitleBox({ Key? key, required Widget child }) : assert(child != null), super(key: key, child: child);

  @override
  _RenderAppBarTitleBox createRenderObject(BuildContext context) {
    return _RenderAppBarTitleBox(
      textDirection: Directionality.of(context),
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderAppBarTitleBox renderObject) {
    renderObject.textDirection = Directionality.of(context);
  }
}

class _RenderAppBarTitleBox extends RenderAligningShiftedBox {
  _RenderAppBarTitleBox({
    RenderBox? child,
    TextDirection? textDirection,
  }) : super(child: child, alignment: Alignment.center, textDirection: textDirection);

  @override
  void performLayout() {
    final BoxConstraints innerConstraints = constraints.copyWith(maxHeight: double.infinity);
    child!.layout(innerConstraints, parentUsesSize: true);
    size = constraints.constrain(child!.size);
    alignChild();
  }
}