packages/flutter/lib/src/material/flexible_space_bar.dart - external/github.com/flutter/flutter - Git at Google

 // Copyright 2016 The Chromium 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/widgets.dart';

 import 'constants.dart';
 import 'theme.dart';

 /// The collapsing effect while the space bar expands or collapses.
 enum CollapseMode {
   /// The background widget will scroll in a parallax fashion.
   parallax,

   /// The background widget pin in place until it reaches the min extent.
   pin,

   /// The background widget will act as normal with no collapsing effect.
   none,
 }

 /// The part of a material design [AppBar] that expands and collapses.
 ///
 /// Most commonly used in in the [SliverAppBar.flexibleSpace] field, a flexible
 /// space bar expands and contracts as the app scrolls so that the [AppBar]
 /// reaches from the top of the app to the top of the scrolling contents of the
 /// app.
 ///
 /// The widget that sizes the [AppBar] must wrap it in the widget returned by
 /// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
 /// [FlexibleSpaceBar].
 ///
 /// See also:
 ///
 ///  * [SliverAppBar], which implements the expanding and contracting.
 ///  * [AppBar], which is used by [SliverAppBar].
 ///  * <https://material.io/design/components/app-bars-top.html#behavior>
 class FlexibleSpaceBar extends StatefulWidget {
   /// Creates a flexible space bar.
   ///
   /// Most commonly used in the [AppBar.flexibleSpace] field.
   const FlexibleSpaceBar({
     Key key,
     this.title,
     this.background,
     this.centerTitle,
     this.titlePadding,
     this.collapseMode = CollapseMode.parallax,
   }) : assert(collapseMode != null),
        super(key: key);

   /// The primary contents of the flexible space bar when expanded.
   ///
   /// Typically a [Text] widget.
   final Widget title;

   /// Shown behind the [title] when expanded.
   ///
   /// Typically an [Image] widget with [Image.fit] set to [BoxFit.cover].
   final Widget background;

   /// Whether the title should be centered.
   ///
   /// By default this property is true if the current target platform
   /// is [TargetPlatform.iOS], false otherwise.
   final bool centerTitle;

   /// Collapse effect while scrolling.
   ///
   /// Defaults to [CollapseMode.parallax].
   final CollapseMode collapseMode;

   /// Defines how far the [title] is inset from either the widget's
   /// bottom-left or its center.
   ///
   /// Typically this property is used to adjust how far the title is
   /// is inset from the bottom-left and it is specified along with
   /// [centerTitle] false.
   ///
   /// By default the value of this property is
   /// `EdgeInsetsDirectional.only(start: 72, bottom: 16)` if the title is
   /// not centered, `EdgeInsetsDirectional.only(start 0, bottom: 16)` otherwise.
   final EdgeInsetsGeometry titlePadding;

   /// Wraps a widget that contains an [AppBar] to convey sizing information down
   /// to the [FlexibleSpaceBar].
   ///
   /// Used by [Scaffold] and [SliverAppBar].
   ///
   /// `toolbarOpacity` affects how transparent the text within the toolbar
   /// appears. `minExtent` sets the minimum height of the resulting
   /// [FlexibleSpaceBar] when fully collapsed. `maxExtent` sets the maximum
   /// height of the resulting [FlexibleSpaceBar] when fully expanded.
   /// `currentExtent` sets the scale of the [FlexibleSpaceBar.background] and
   /// [FlexibleSpaceBar.title] widgets of [FlexibleSpaceBar] upon
   /// initialization.
   ///
   /// See also:
   ///
   ///  * [FlexibleSpaceBarSettings] which creates a settings object that can be
   ///    used to specify these settings to a [FlexibleSpaceBar].
   static Widget createSettings({
     double toolbarOpacity,
     double minExtent,
     double maxExtent,
     @required double currentExtent,
     @required Widget child,
   }) {
     assert(currentExtent != null);
     return FlexibleSpaceBarSettings(
       toolbarOpacity: toolbarOpacity ?? 1.0,
       minExtent: minExtent ?? currentExtent,
       maxExtent: maxExtent ?? currentExtent,
       currentExtent: currentExtent,
       child: child,
     );
   }

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

 class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
   bool _getEffectiveCenterTitle(ThemeData theme) {
     if (widget.centerTitle != null)
       return widget.centerTitle;
     assert(theme.platform != null);
     switch (theme.platform) {
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
         return false;
       case TargetPlatform.iOS:
         return true;
     }
     return null;
   }

   Alignment _getTitleAlignment(bool effectiveCenterTitle) {
     if (effectiveCenterTitle)
       return Alignment.bottomCenter;
     final TextDirection textDirection = Directionality.of(context);
     assert(textDirection != null);
     switch (textDirection) {
       case TextDirection.rtl:
         return Alignment.bottomRight;
       case TextDirection.ltr:
         return Alignment.bottomLeft;
     }
     return null;
   }

   double _getCollapsePadding(double t, FlexibleSpaceBarSettings settings) {
     switch (widget.collapseMode) {
       case CollapseMode.pin:
         return -(settings.maxExtent - settings.currentExtent);
       case CollapseMode.none:
         return 0.0;
       case CollapseMode.parallax:
         final double deltaExtent = settings.maxExtent - settings.minExtent;
         return -Tween<double>(begin: 0.0, end: deltaExtent / 4.0).transform(t);
     }
     return null;
   }

   @override
   Widget build(BuildContext context) {
     final FlexibleSpaceBarSettings settings = context.inheritFromWidgetOfExactType(FlexibleSpaceBarSettings);
     assert(settings != null, 'A FlexibleSpaceBar must be wrapped in the widget returned by FlexibleSpaceBar.createSettings().');

     final List<Widget> children = <Widget>[];

     final double deltaExtent = settings.maxExtent - settings.minExtent;

     // 0.0 -> Expanded
     // 1.0 -> Collapsed to toolbar
     final double t = (1.0 - (settings.currentExtent - settings.minExtent) / deltaExtent).clamp(0.0, 1.0);

     // background image
     if (widget.background != null) {
       final double fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
       const double fadeEnd = 1.0;
       assert(fadeStart <= fadeEnd);
       final double opacity = 1.0 - Interval(fadeStart, fadeEnd).transform(t);
       if (opacity > 0.0) {
         children.add(Positioned(
           top: _getCollapsePadding(t, settings),
           left: 0.0,
           right: 0.0,
           height: settings.maxExtent,
           child: Opacity(
             opacity: opacity,
             child: widget.background,
           ),
         ));
       }
     }

     if (widget.title != null) {
       final ThemeData theme = Theme.of(context);

       Widget title;
       switch (theme.platform) {
         case TargetPlatform.iOS:
           title = widget.title;
           break;
         case TargetPlatform.fuchsia:
         case TargetPlatform.android:
           title = Semantics(
             namesRoute: true,
             child: widget.title,
           );
       }

       final double opacity = settings.toolbarOpacity;
       if (opacity > 0.0) {
         TextStyle titleStyle = theme.primaryTextTheme.title;
         titleStyle = titleStyle.copyWith(
           color: titleStyle.color.withOpacity(opacity)
         );
         final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
         final EdgeInsetsGeometry padding = widget.titlePadding ??
           EdgeInsetsDirectional.only(
             start: effectiveCenterTitle ? 0.0 : 72.0,
             bottom: 16.0,
           );
         final double scaleValue = Tween<double>(begin: 1.5, end: 1.0).transform(t);
         final Matrix4 scaleTransform = Matrix4.identity()
           ..scale(scaleValue, scaleValue, 1.0);
         final Alignment titleAlignment = _getTitleAlignment(effectiveCenterTitle);
         children.add(Container(
           padding: padding,
           child: Transform(
             alignment: titleAlignment,
             transform: scaleTransform,
             child: Align(
               alignment: titleAlignment,
               child: DefaultTextStyle(
                 style: titleStyle,
                 child: title,
               ),
             ),
           ),
         ));
       }
     }

     return ClipRect(child: Stack(children: children));
   }
 }

 /// Provides sizing and opacity information to a [FlexibleSpaceBar].
 ///
 /// See also:
 ///
 ///  * [FlexibleSpaceBar] which creates a flexible space bar.
 class FlexibleSpaceBarSettings extends InheritedWidget {
   /// Creates a Flexible Space Bar Settings widget.
   ///
   /// Used by [Scaffold] and [SliverAppBar]. [child] must have a
   /// [FlexibleSpaceBar] widget in its tree for the settings to take affect.
   ///
   /// The required [toolbarOpacity], [minExtent], [maxExtent], [currentExtent],
   /// and [child] parameters must not be null.
   const FlexibleSpaceBarSettings({
     Key key,
     @required this.toolbarOpacity,
     @required this.minExtent,
     @required this.maxExtent,
     @required this.currentExtent,
     @required Widget child,
   }) : assert(toolbarOpacity != null),
        assert(minExtent != null && minExtent >= 0),
        assert(maxExtent != null && maxExtent >= 0),
        assert(currentExtent != null && currentExtent >= 0),
        assert(toolbarOpacity >= 0.0),
        assert(minExtent <= maxExtent),
        assert(minExtent <= currentExtent),
        assert(currentExtent <= maxExtent),
        super(key: key, child: child);

   /// Affects how transparent the text within the toolbar appears.
   final double toolbarOpacity;

   /// Minimum height of the resulting [FlexibleSpaceBar] when fully collapsed.
   final double minExtent;

   /// Maximum height of the resulting [FlexibleSpaceBar] when fully expanded.
   final double maxExtent;

   /// If the [FlexibleSpaceBar.title] or the [FlexibleSpaceBar.background] is
   /// not null, then this value is used to calculate the relative scale of
   /// these elements upon initialization.
   final double currentExtent;

   @override
   bool updateShouldNotify(FlexibleSpaceBarSettings oldWidget) {
     return toolbarOpacity != oldWidget.toolbarOpacity
         || minExtent != oldWidget.minExtent
         || maxExtent != oldWidget.maxExtent
         || currentExtent != oldWidget.currentExtent;
   }
 }
	// Copyright 2016 The Chromium 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/widgets.dart';

	import 'constants.dart';
	import 'theme.dart';

	/// The collapsing effect while the space bar expands or collapses.
	enum CollapseMode {
	/// The background widget will scroll in a parallax fashion.
	parallax,

	/// The background widget pin in place until it reaches the min extent.
	pin,

	/// The background widget will act as normal with no collapsing effect.
	none,
	}

	/// The part of a material design [AppBar] that expands and collapses.
	///
	/// Most commonly used in in the [SliverAppBar.flexibleSpace] field, a flexible
	/// space bar expands and contracts as the app scrolls so that the [AppBar]
	/// reaches from the top of the app to the top of the scrolling contents of the
	/// app.
	///
	/// The widget that sizes the [AppBar] must wrap it in the widget returned by
	/// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
	/// [FlexibleSpaceBar].
	///
	/// See also:
	///
	/// * [SliverAppBar], which implements the expanding and contracting.
	/// * [AppBar], which is used by [SliverAppBar].
	/// * <https://material.io/design/components/app-bars-top.html#behavior>
	class FlexibleSpaceBar extends StatefulWidget {
	/// Creates a flexible space bar.
	///
	/// Most commonly used in the [AppBar.flexibleSpace] field.
	const FlexibleSpaceBar({
	Key key,
	this.title,
	this.background,
	this.centerTitle,
	this.titlePadding,
	this.collapseMode = CollapseMode.parallax,
	}) : assert(collapseMode != null),
	super(key: key);

	/// The primary contents of the flexible space bar when expanded.
	///
	/// Typically a [Text] widget.
	final Widget title;

	/// Shown behind the [title] when expanded.
	///
	/// Typically an [Image] widget with [Image.fit] set to [BoxFit.cover].
	final Widget background;

	/// Whether the title should be centered.
	///
	/// By default this property is true if the current target platform
	/// is [TargetPlatform.iOS], false otherwise.
	final bool centerTitle;

	/// Collapse effect while scrolling.
	///
	/// Defaults to [CollapseMode.parallax].
	final CollapseMode collapseMode;

	/// Defines how far the [title] is inset from either the widget's
	/// bottom-left or its center.
	///
	/// Typically this property is used to adjust how far the title is
	/// is inset from the bottom-left and it is specified along with
	/// [centerTitle] false.
	///
	/// By default the value of this property is
	/// `EdgeInsetsDirectional.only(start: 72, bottom: 16)` if the title is
	/// not centered, `EdgeInsetsDirectional.only(start 0, bottom: 16)` otherwise.
	final EdgeInsetsGeometry titlePadding;

	/// Wraps a widget that contains an [AppBar] to convey sizing information down
	/// to the [FlexibleSpaceBar].
	///
	/// Used by [Scaffold] and [SliverAppBar].
	///
	/// `toolbarOpacity` affects how transparent the text within the toolbar
	/// appears. `minExtent` sets the minimum height of the resulting
	/// [FlexibleSpaceBar] when fully collapsed. `maxExtent` sets the maximum
	/// height of the resulting [FlexibleSpaceBar] when fully expanded.
	/// `currentExtent` sets the scale of the [FlexibleSpaceBar.background] and
	/// [FlexibleSpaceBar.title] widgets of [FlexibleSpaceBar] upon
	/// initialization.
	///
	/// See also:
	///
	/// * [FlexibleSpaceBarSettings] which creates a settings object that can be
	/// used to specify these settings to a [FlexibleSpaceBar].
	static Widget createSettings({
	double toolbarOpacity,
	double minExtent,
	double maxExtent,
	@required double currentExtent,
	@required Widget child,
	}) {
	assert(currentExtent != null);
	return FlexibleSpaceBarSettings(
	toolbarOpacity: toolbarOpacity ?? 1.0,
	minExtent: minExtent ?? currentExtent,
	maxExtent: maxExtent ?? currentExtent,
	currentExtent: currentExtent,
	child: child,
	);
	}

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

	class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
	bool _getEffectiveCenterTitle(ThemeData theme) {
	if (widget.centerTitle != null)
	return widget.centerTitle;
	assert(theme.platform != null);
	switch (theme.platform) {
	case TargetPlatform.android:
	case TargetPlatform.fuchsia:
	return false;
	case TargetPlatform.iOS:
	return true;
	}
	return null;
	}

	Alignment _getTitleAlignment(bool effectiveCenterTitle) {
	if (effectiveCenterTitle)
	return Alignment.bottomCenter;
	final TextDirection textDirection = Directionality.of(context);
	assert(textDirection != null);
	switch (textDirection) {
	case TextDirection.rtl:
	return Alignment.bottomRight;
	case TextDirection.ltr:
	return Alignment.bottomLeft;
	}
	return null;
	}

	double _getCollapsePadding(double t, FlexibleSpaceBarSettings settings) {
	switch (widget.collapseMode) {
	case CollapseMode.pin:
	return -(settings.maxExtent - settings.currentExtent);
	case CollapseMode.none:
	return 0.0;
	case CollapseMode.parallax:
	final double deltaExtent = settings.maxExtent - settings.minExtent;
	return -Tween<double>(begin: 0.0, end: deltaExtent / 4.0).transform(t);
	}
	return null;
	}

	@override
	Widget build(BuildContext context) {
	final FlexibleSpaceBarSettings settings = context.inheritFromWidgetOfExactType(FlexibleSpaceBarSettings);
	assert(settings != null, 'A FlexibleSpaceBar must be wrapped in the widget returned by FlexibleSpaceBar.createSettings().');

	final List<Widget> children = <Widget>[];

	final double deltaExtent = settings.maxExtent - settings.minExtent;

	// 0.0 -> Expanded
	// 1.0 -> Collapsed to toolbar
	final double t = (1.0 - (settings.currentExtent - settings.minExtent) / deltaExtent).clamp(0.0, 1.0);

	// background image
	if (widget.background != null) {
	final double fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
	const double fadeEnd = 1.0;
	assert(fadeStart <= fadeEnd);
	final double opacity = 1.0 - Interval(fadeStart, fadeEnd).transform(t);
	if (opacity > 0.0) {
	children.add(Positioned(
	top: _getCollapsePadding(t, settings),
	left: 0.0,
	right: 0.0,
	height: settings.maxExtent,
	child: Opacity(
	opacity: opacity,
	child: widget.background,
	),
	));
	}
	}

	if (widget.title != null) {
	final ThemeData theme = Theme.of(context);

	Widget title;
	switch (theme.platform) {
	case TargetPlatform.iOS:
	title = widget.title;
	break;
	case TargetPlatform.fuchsia:
	case TargetPlatform.android:
	title = Semantics(
	namesRoute: true,
	child: widget.title,
	);
	}

	final double opacity = settings.toolbarOpacity;
	if (opacity > 0.0) {
	TextStyle titleStyle = theme.primaryTextTheme.title;
	titleStyle = titleStyle.copyWith(
	color: titleStyle.color.withOpacity(opacity)
	);
	final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
	final EdgeInsetsGeometry padding = widget.titlePadding ??
	EdgeInsetsDirectional.only(
	start: effectiveCenterTitle ? 0.0 : 72.0,
	bottom: 16.0,
	);
	final double scaleValue = Tween<double>(begin: 1.5, end: 1.0).transform(t);
	final Matrix4 scaleTransform = Matrix4.identity()
	..scale(scaleValue, scaleValue, 1.0);
	final Alignment titleAlignment = _getTitleAlignment(effectiveCenterTitle);
	children.add(Container(
	padding: padding,
	child: Transform(
	alignment: titleAlignment,
	transform: scaleTransform,
	child: Align(
	alignment: titleAlignment,
	child: DefaultTextStyle(
	style: titleStyle,
	child: title,
	),
	),
	),
	));
	}
	}

	return ClipRect(child: Stack(children: children));
	}
	}

	/// Provides sizing and opacity information to a [FlexibleSpaceBar].
	///
	/// See also:
	///
	/// * [FlexibleSpaceBar] which creates a flexible space bar.
	class FlexibleSpaceBarSettings extends InheritedWidget {
	/// Creates a Flexible Space Bar Settings widget.
	///
	/// Used by [Scaffold] and [SliverAppBar]. [child] must have a
	/// [FlexibleSpaceBar] widget in its tree for the settings to take affect.
	///
	/// The required [toolbarOpacity], [minExtent], [maxExtent], [currentExtent],
	/// and [child] parameters must not be null.
	const FlexibleSpaceBarSettings({
	Key key,
	@required this.toolbarOpacity,
	@required this.minExtent,
	@required this.maxExtent,
	@required this.currentExtent,
	@required Widget child,
	}) : assert(toolbarOpacity != null),
	assert(minExtent != null && minExtent >= 0),
	assert(maxExtent != null && maxExtent >= 0),
	assert(currentExtent != null && currentExtent >= 0),
	assert(toolbarOpacity >= 0.0),
	assert(minExtent <= maxExtent),
	assert(minExtent <= currentExtent),
	assert(currentExtent <= maxExtent),
	super(key: key, child: child);

	/// Affects how transparent the text within the toolbar appears.
	final double toolbarOpacity;

	/// Minimum height of the resulting [FlexibleSpaceBar] when fully collapsed.
	final double minExtent;

	/// Maximum height of the resulting [FlexibleSpaceBar] when fully expanded.
	final double maxExtent;

	/// If the [FlexibleSpaceBar.title] or the [FlexibleSpaceBar.background] is
	/// not null, then this value is used to calculate the relative scale of
	/// these elements upon initialization.
	final double currentExtent;

	@override
	bool updateShouldNotify(FlexibleSpaceBarSettings oldWidget) {
	return toolbarOpacity != oldWidget.toolbarOpacity
	\|\| minExtent != oldWidget.minExtent
	\|\| maxExtent != oldWidget.maxExtent
	\|\| currentExtent != oldWidget.currentExtent;
	}
	}