packages/flutter/lib/src/material/flexible_space_bar.dart - mirrors/flutter - Git at Google

 // 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 'dart:ui' as ui;

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

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

 /// The collapsing effect while the space bar collapses from its full size.
 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 stretching effect while the space bar stretches beyond its full size.
 enum StretchMode {
   /// The background widget will expand to fill the extra space.
   zoomBackground,

   /// The background will blur using a [ImageFilter.blur] effect.
   blurBackground,

   /// The title will fade away as the user over-scrolls.
   fadeTitle,
 }

 /// The part of a material design [AppBar] that expands, collapses, and
 /// stretches.
 ///
 /// 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. Furthermore is included functionality for stretch behavior. When
 /// [SliverAppBar.stretch] is true, and your [ScrollPhysics] allow for
 /// overscroll, this space will stretch with the overscroll.
 ///
 /// The widget that sizes the [AppBar] must wrap it in the widget returned by
 /// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
 /// [FlexibleSpaceBar].
 ///
 /// {@tool dartpad --template=freeform}
 /// This sample application demonstrates the different features of the
 /// [FlexibleSpaceBar] when used in a [SliverAppBar]. This app bar is configured
 /// to stretch into the overscroll space, and uses the
 /// [FlexibleSpaceBar.stretchModes] to apply `fadeTitle`, `blurBackground` and
 /// `zoomBackground`. The app bar also makes use of [CollapseMode.parallax] by
 /// default.
 ///
 /// ```dart imports
 /// import 'package:flutter/material.dart';
 /// ```
 /// ```dart
 /// void main() => runApp(MaterialApp(home: MyApp()));
 ///
 /// class MyApp extends StatelessWidget {
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return Scaffold(
 ///       body: CustomScrollView(
 ///         physics: const BouncingScrollPhysics(),
 ///         slivers: <Widget>[
 ///           SliverAppBar(
 ///             stretch: true,
 ///             onStretchTrigger: () {
 ///               // Function callback for stretch
 ///               return;
 ///             },
 ///             expandedHeight: 300.0,
 ///             flexibleSpace: FlexibleSpaceBar(
 ///               stretchModes: <StretchMode>[
 ///                 StretchMode.zoomBackground,
 ///                 StretchMode.blurBackground,
 ///                 StretchMode.fadeTitle,
 ///               ],
 ///               centerTitle: true,
 ///               title: const Text('Flight Report'),
 ///               background: Stack(
 ///                 fit: StackFit.expand,
 ///                 children: [
 ///                   Image.network(
 ///                     'https://flutter.github.io/assets-for-api-docs/assets/widgets/owl-2.jpg',
 ///                     fit: BoxFit.cover,
 ///                   ),
 ///                   const DecoratedBox(
 ///                     decoration: BoxDecoration(
 ///                       gradient: LinearGradient(
 ///                         begin: Alignment(0.0, 0.5),
 ///                         end: Alignment(0.0, 0.0),
 ///                         colors: <Color>[
 ///                           Color(0x60000000),
 ///                           Color(0x00000000),
 ///                         ],
 ///                       ),
 ///                     ),
 ///                   ),
 ///                 ],
 ///               ),
 ///             ),
 ///           ),
 ///           SliverList(
 ///             delegate: SliverChildListDelegate([
 ///               ListTile(
 ///                 leading: Icon(Icons.wb_sunny),
 ///                 title: Text('Sunday'),
 ///                 subtitle: Text('sunny, h: 80, l: 65'),
 ///               ),
 ///               ListTile(
 ///                 leading: Icon(Icons.wb_sunny),
 ///                 title: Text('Monday'),
 ///                 subtitle: Text('sunny, h: 80, l: 65'),
 ///               ),
 ///               // ListTiles++
 ///             ]),
 ///           ),
 ///         ],
 ///       ),
 ///     );
 ///   }
 /// }
 ///
 /// ```
 /// {@end-tool}
 ///
 /// 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,
     this.stretchModes = const <StretchMode>[StretchMode.zoomBackground],
   }) : 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] or [TargetPlatform.macOS], false otherwise.
   final bool centerTitle;

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

   /// Stretch effect while over-scrolling,
   ///
   /// Defaults to include [StretchMode.zoomBackground].
   final List<StretchMode> stretchModes;

   /// 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:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
         return false;
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:
         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) {
     return LayoutBuilder(
       builder: (BuildContext context, BoxConstraints constraints) {
         final FlexibleSpaceBarSettings settings = context.dependOnInheritedWidgetOfExactType<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) as double;

         // background
         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) {
             double height = settings.maxExtent;

             // StretchMode.zoomBackground
             if (widget.stretchModes.contains(StretchMode.zoomBackground) &&
               constraints.maxHeight > height) {
               height = constraints.maxHeight;
             }

             children.add(Positioned(
               top: _getCollapsePadding(t, settings),
               left: 0.0,
               right: 0.0,
               height: height,
               child: Opacity(
                 opacity: opacity,
                 child: widget.background,
               ),
             ));

             // StretchMode.blurBackground
             if (widget.stretchModes.contains(StretchMode.blurBackground) &&
               constraints.maxHeight > settings.maxExtent) {
               final double blurAmount = (constraints.maxHeight - settings.maxExtent) / 10;
               children.add(Positioned.fill(
                 child: BackdropFilter(
                   child: Container(
                     color: Colors.transparent,
                   ),
                   filter: ui.ImageFilter.blur(
                     sigmaX: blurAmount,
                     sigmaY: blurAmount,
                   )
                 )
               ));
             }
           }
         }

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

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

           // StretchMode.fadeTitle
           if (widget.stretchModes.contains(StretchMode.fadeTitle) &&
             constraints.maxHeight > settings.maxExtent) {
             final double stretchOpacity = 1 -
               (((constraints.maxHeight - settings.maxExtent) / 100).clamp(0.0, 1.0) as double);
             title = Opacity(
               opacity: stretchOpacity,
               child: title,
             );
           }

           final double opacity = settings.toolbarOpacity;
           if (opacity > 0.0) {
             TextStyle titleStyle = theme.primaryTextTheme.headline6;
             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: LayoutBuilder(
                       builder: (BuildContext context, BoxConstraints constraints) {
                         return Container(
                           width: constraints.maxWidth / scaleValue,
                           alignment: titleAlignment,
                           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 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 'dart:ui' as ui;

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

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

	/// The collapsing effect while the space bar collapses from its full size.
	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 stretching effect while the space bar stretches beyond its full size.
	enum StretchMode {
	/// The background widget will expand to fill the extra space.
	zoomBackground,

	/// The background will blur using a [ImageFilter.blur] effect.
	blurBackground,

	/// The title will fade away as the user over-scrolls.
	fadeTitle,
	}

	/// The part of a material design [AppBar] that expands, collapses, and
	/// stretches.
	///
	/// 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. Furthermore is included functionality for stretch behavior. When
	/// [SliverAppBar.stretch] is true, and your [ScrollPhysics] allow for
	/// overscroll, this space will stretch with the overscroll.
	///
	/// The widget that sizes the [AppBar] must wrap it in the widget returned by
	/// [FlexibleSpaceBar.createSettings], to convey sizing information down to the
	/// [FlexibleSpaceBar].
	///
	/// {@tool dartpad --template=freeform}
	/// This sample application demonstrates the different features of the
	/// [FlexibleSpaceBar] when used in a [SliverAppBar]. This app bar is configured
	/// to stretch into the overscroll space, and uses the
	/// [FlexibleSpaceBar.stretchModes] to apply `fadeTitle`, `blurBackground` and
	/// `zoomBackground`. The app bar also makes use of [CollapseMode.parallax] by
	/// default.
	///
	/// ```dart imports
	/// import 'package:flutter/material.dart';
	/// ```
	/// ```dart
	/// void main() => runApp(MaterialApp(home: MyApp()));
	///
	/// class MyApp extends StatelessWidget {
	/// @override
	/// Widget build(BuildContext context) {
	/// return Scaffold(
	/// body: CustomScrollView(
	/// physics: const BouncingScrollPhysics(),
	/// slivers: <Widget>[
	/// SliverAppBar(
	/// stretch: true,
	/// onStretchTrigger: () {
	/// // Function callback for stretch
	/// return;
	/// },
	/// expandedHeight: 300.0,
	/// flexibleSpace: FlexibleSpaceBar(
	/// stretchModes: <StretchMode>[
	/// StretchMode.zoomBackground,
	/// StretchMode.blurBackground,
	/// StretchMode.fadeTitle,
	/// ],
	/// centerTitle: true,
	/// title: const Text('Flight Report'),
	/// background: Stack(
	/// fit: StackFit.expand,
	/// children: [
	/// Image.network(
	/// 'https://flutter.github.io/assets-for-api-docs/assets/widgets/owl-2.jpg',
	/// fit: BoxFit.cover,
	/// ),
	/// const DecoratedBox(
	/// decoration: BoxDecoration(
	/// gradient: LinearGradient(
	/// begin: Alignment(0.0, 0.5),
	/// end: Alignment(0.0, 0.0),
	/// colors: <Color>[
	/// Color(0x60000000),
	/// Color(0x00000000),
	/// ],
	/// ),
	/// ),
	/// ),
	/// ],
	/// ),
	/// ),
	/// ),
	/// SliverList(
	/// delegate: SliverChildListDelegate([
	/// ListTile(
	/// leading: Icon(Icons.wb_sunny),
	/// title: Text('Sunday'),
	/// subtitle: Text('sunny, h: 80, l: 65'),
	/// ),
	/// ListTile(
	/// leading: Icon(Icons.wb_sunny),
	/// title: Text('Monday'),
	/// subtitle: Text('sunny, h: 80, l: 65'),
	/// ),
	/// // ListTiles++
	/// ]),
	/// ),
	/// ],
	/// ),
	/// );
	/// }
	/// }
	///
	/// ```
	/// {@end-tool}
	///
	/// 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,
	this.stretchModes = const <StretchMode>[StretchMode.zoomBackground],
	}) : 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] or [TargetPlatform.macOS], false otherwise.
	final bool centerTitle;

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

	/// Stretch effect while over-scrolling,
	///
	/// Defaults to include [StretchMode.zoomBackground].
	final List<StretchMode> stretchModes;

	/// 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:
	case TargetPlatform.linux:
	case TargetPlatform.windows:
	return false;
	case TargetPlatform.iOS:
	case TargetPlatform.macOS:
	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) {
	return LayoutBuilder(
	builder: (BuildContext context, BoxConstraints constraints) {
	final FlexibleSpaceBarSettings settings = context.dependOnInheritedWidgetOfExactType<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) as double;

	// background
	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) {
	double height = settings.maxExtent;

	// StretchMode.zoomBackground
	if (widget.stretchModes.contains(StretchMode.zoomBackground) &&
	constraints.maxHeight > height) {
	height = constraints.maxHeight;
	}

	children.add(Positioned(
	top: _getCollapsePadding(t, settings),
	left: 0.0,
	right: 0.0,
	height: height,
	child: Opacity(
	opacity: opacity,
	child: widget.background,
	),
	));

	// StretchMode.blurBackground
	if (widget.stretchModes.contains(StretchMode.blurBackground) &&
	constraints.maxHeight > settings.maxExtent) {
	final double blurAmount = (constraints.maxHeight - settings.maxExtent) / 10;
	children.add(Positioned.fill(
	child: BackdropFilter(
	child: Container(
	color: Colors.transparent,
	),
	filter: ui.ImageFilter.blur(
	sigmaX: blurAmount,
	sigmaY: blurAmount,
	)
	)
	));
	}
	}
	}

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

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

	// StretchMode.fadeTitle
	if (widget.stretchModes.contains(StretchMode.fadeTitle) &&
	constraints.maxHeight > settings.maxExtent) {
	final double stretchOpacity = 1 -
	(((constraints.maxHeight - settings.maxExtent) / 100).clamp(0.0, 1.0) as double);
	title = Opacity(
	opacity: stretchOpacity,
	child: title,
	);
	}

	final double opacity = settings.toolbarOpacity;
	if (opacity > 0.0) {
	TextStyle titleStyle = theme.primaryTextTheme.headline6;
	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: LayoutBuilder(
	builder: (BuildContext context, BoxConstraints constraints) {
	return Container(
	width: constraints.maxWidth / scaleValue,
	alignment: titleAlignment,
	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;
	}
	}