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' show clampDouble;
 import 'package:flutter/rendering.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.
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=mSc7qFzxHDw}
 ///
 /// Most commonly used 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. When using [SliverAppBar.flexibleSpace], the [SliverAppBar.expandedHeight]
 /// must be large enough to accommodate the [SliverAppBar.flexibleSpace] widget.
 ///
 /// 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}
 /// 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.
 ///
 /// ** See code in examples/api/lib/material/flexible_space_bar/flexible_space_bar.0.dart **
 /// {@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({
     super.key,
     this.title,
     this.background,
     this.centerTitle,
     this.titlePadding,
     this.collapseMode = CollapseMode.parallax,
     this.stretchModes = const <StretchMode>[StretchMode.zoomBackground],
     this.expandedTitleScale = 1.5,
   }) : assert(expandedTitleScale >= 1);

   /// 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
   /// 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;

   /// Defines how much the title is scaled when the FlexibleSpaceBar is expanded
   /// due to the user scrolling downwards. The title is scaled uniformly on the
   /// x and y axes while maintaining its bottom-left position (bottom-center if
   /// [centerTitle] is true).
   ///
   /// Defaults to 1.5 and must be greater than 1.
   final double expandedTitleScale;

   /// 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. `scrolledUnder` is true if the [FlexibleSpaceBar]
   /// overlaps the app's primary scrollable, false if it does not, and null
   /// if the caller has not determined as much.
   /// 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,
     bool? isScrolledUnder,
     required double currentExtent,
     required Widget child,
   }) {
     return FlexibleSpaceBarSettings(
       toolbarOpacity: toolbarOpacity ?? 1.0,
       minExtent: minExtent ?? currentExtent,
       maxExtent: maxExtent ?? currentExtent,
       isScrolledUnder: isScrolledUnder,
       currentExtent: currentExtent,
       child: child,
     );
   }

   @override
   State<FlexibleSpaceBar> createState() => _FlexibleSpaceBarState();
 }

 class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
   bool _getEffectiveCenterTitle(ThemeData theme) {
     if (widget.centerTitle != null) {
       return widget.centerTitle!;
     }
     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;
     }
   }

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

   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);
     }
   }

   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(
       builder: (BuildContext context, BoxConstraints constraints) {
         final FlexibleSpaceBarSettings settings = context.dependOnInheritedWidgetOfExactType<FlexibleSpaceBarSettings>()!;

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

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

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

         // background
         if (widget.background != null) {
           final double fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
           const double fadeEnd = 1.0;
           assert(fadeStart <= fadeEnd);
           // If the min and max extent are the same, the app bar cannot collapse
           // and the content should be visible, so opacity = 1.
           final double opacity = settings.maxExtent == settings.minExtent
               ? 1.0
               : 1.0 - Interval(fadeStart, fadeEnd).transform(t);
           double height = settings.maxExtent;

           // StretchMode.zoomBackground
           if (widget.stretchModes.contains(StretchMode.zoomBackground) &&
             constraints.maxHeight > height) {
             height = constraints.maxHeight;
           }
           final double topPadding = _getCollapsePadding(t, settings);
           children.add(Positioned(
             top: topPadding,
             left: 0.0,
             right: 0.0,
             height: height,
             child: _FlexibleSpaceHeaderOpacity(
               // IOS is relying on this semantics node to correctly traverse
               // through the app bar when it is collapsed.
               alwaysIncludeSemantics: true,
               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(
                 filter: ui.ImageFilter.blur(
                   sigmaX: blurAmount,
                   sigmaY: blurAmount,
                 ),
                 child: Container(
                   color: Colors.transparent,
                 ),
               ),
             ));
           }
         }

         // 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;
             case TargetPlatform.android:
             case TargetPlatform.fuchsia:
             case TargetPlatform.linux:
             case TargetPlatform.windows:
               title = Semantics(
                 namesRoute: true,
                 child: widget.title,
               );
           }

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

           final double opacity = settings.toolbarOpacity;
           if (opacity > 0.0) {
             TextStyle titleStyle = theme.primaryTextTheme.titleLarge!;
             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: widget.expandedTitleScale, 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({
     super.key,
     required this.toolbarOpacity,
     required this.minExtent,
     required this.maxExtent,
     required this.currentExtent,
     required super.child,
     this.isScrolledUnder,
   }) : assert(minExtent >= 0),
        assert(maxExtent >= 0),
        assert(currentExtent >= 0),
        assert(toolbarOpacity >= 0.0),
        assert(minExtent <= maxExtent),
        assert(minExtent <= currentExtent),
        assert(currentExtent <= maxExtent);

   /// 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;

   /// True if the FlexibleSpaceBar overlaps the primary scrollable's contents.
   ///
   /// This value is used by the [AppBar] to resolve
   /// [AppBar.backgroundColor] against [MaterialState.scrolledUnder],
   /// i.e. to enable apps to specify different colors when content
   /// has been scrolled up and behind the app bar.
   ///
   /// Null if the caller hasn't determined if the FlexibleSpaceBar
   /// overlaps the primary scrollable's contents.
   final bool? isScrolledUnder;

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

 // We need the child widget to repaint, however both the opacity
 // and potentially `widget.background` can be constant which won't
 // lead to repainting.
 // see: https://github.com/flutter/flutter/issues/127836
 class _FlexibleSpaceHeaderOpacity extends SingleChildRenderObjectWidget {
   const _FlexibleSpaceHeaderOpacity({required this.opacity, required super.child, required this.alwaysIncludeSemantics});

   final double opacity;
   final bool alwaysIncludeSemantics;

   @override
   RenderObject createRenderObject(BuildContext context) {
     return _RenderFlexibleSpaceHeaderOpacity(opacity: opacity, alwaysIncludeSemantics: alwaysIncludeSemantics);
   }

   @override
   void updateRenderObject(BuildContext context, covariant _RenderFlexibleSpaceHeaderOpacity renderObject) {
     renderObject
       ..alwaysIncludeSemantics = alwaysIncludeSemantics
       ..opacity = opacity;
   }
 }

 class _RenderFlexibleSpaceHeaderOpacity extends RenderOpacity {
   _RenderFlexibleSpaceHeaderOpacity({super.opacity, super.alwaysIncludeSemantics});

   @override
   bool get isRepaintBoundary => false;
 }
	// 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' show clampDouble;
	import 'package:flutter/rendering.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.
	///
	/// {@youtube 560 315 https://www.youtube.com/watch?v=mSc7qFzxHDw}
	///
	/// Most commonly used 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. When using [SliverAppBar.flexibleSpace], the [SliverAppBar.expandedHeight]
	/// must be large enough to accommodate the [SliverAppBar.flexibleSpace] widget.
	///
	/// 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}
	/// 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.
	///
	/// See code in examples/api/lib/material/flexible_space_bar/flexible_space_bar.0.dart
	/// {@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({
	super.key,
	this.title,
	this.background,
	this.centerTitle,
	this.titlePadding,
	this.collapseMode = CollapseMode.parallax,
	this.stretchModes = const <StretchMode>[StretchMode.zoomBackground],
	this.expandedTitleScale = 1.5,
	}) : assert(expandedTitleScale >= 1);

	/// 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
	/// 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;

	/// Defines how much the title is scaled when the FlexibleSpaceBar is expanded
	/// due to the user scrolling downwards. The title is scaled uniformly on the
	/// x and y axes while maintaining its bottom-left position (bottom-center if
	/// [centerTitle] is true).
	///
	/// Defaults to 1.5 and must be greater than 1.
	final double expandedTitleScale;

	/// 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. `scrolledUnder` is true if the [FlexibleSpaceBar]
	/// overlaps the app's primary scrollable, false if it does not, and null
	/// if the caller has not determined as much.
	/// 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,
	bool? isScrolledUnder,
	required double currentExtent,
	required Widget child,
	}) {
	return FlexibleSpaceBarSettings(
	toolbarOpacity: toolbarOpacity ?? 1.0,
	minExtent: minExtent ?? currentExtent,
	maxExtent: maxExtent ?? currentExtent,
	isScrolledUnder: isScrolledUnder,
	currentExtent: currentExtent,
	child: child,
	);
	}

	@override
	State<FlexibleSpaceBar> createState() => _FlexibleSpaceBarState();
	}

	class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
	bool _getEffectiveCenterTitle(ThemeData theme) {
	if (widget.centerTitle != null) {
	return widget.centerTitle!;
	}
	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;
	}
	}

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

	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);
	}
	}

	@override
	Widget build(BuildContext context) {
	return LayoutBuilder(
	builder: (BuildContext context, BoxConstraints constraints) {
	final FlexibleSpaceBarSettings settings = context.dependOnInheritedWidgetOfExactType<FlexibleSpaceBarSettings>()!;

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

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

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

	// background
	if (widget.background != null) {
	final double fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
	const double fadeEnd = 1.0;
	assert(fadeStart <= fadeEnd);
	// If the min and max extent are the same, the app bar cannot collapse
	// and the content should be visible, so opacity = 1.
	final double opacity = settings.maxExtent == settings.minExtent
	? 1.0
	: 1.0 - Interval(fadeStart, fadeEnd).transform(t);
	double height = settings.maxExtent;

	// StretchMode.zoomBackground
	if (widget.stretchModes.contains(StretchMode.zoomBackground) &&
	constraints.maxHeight > height) {
	height = constraints.maxHeight;
	}
	final double topPadding = _getCollapsePadding(t, settings);
	children.add(Positioned(
	top: topPadding,
	left: 0.0,
	right: 0.0,
	height: height,
	child: _FlexibleSpaceHeaderOpacity(
	// IOS is relying on this semantics node to correctly traverse
	// through the app bar when it is collapsed.
	alwaysIncludeSemantics: true,
	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(
	filter: ui.ImageFilter.blur(
	sigmaX: blurAmount,
	sigmaY: blurAmount,
	),
	child: Container(
	color: Colors.transparent,
	),
	),
	));
	}
	}

	// 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;
	case TargetPlatform.android:
	case TargetPlatform.fuchsia:
	case TargetPlatform.linux:
	case TargetPlatform.windows:
	title = Semantics(
	namesRoute: true,
	child: widget.title,
	);
	}

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

	final double opacity = settings.toolbarOpacity;
	if (opacity > 0.0) {
	TextStyle titleStyle = theme.primaryTextTheme.titleLarge!;
	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: widget.expandedTitleScale, 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({
	super.key,
	required this.toolbarOpacity,
	required this.minExtent,
	required this.maxExtent,
	required this.currentExtent,
	required super.child,
	this.isScrolledUnder,
	}) : assert(minExtent >= 0),
	assert(maxExtent >= 0),
	assert(currentExtent >= 0),
	assert(toolbarOpacity >= 0.0),
	assert(minExtent <= maxExtent),
	assert(minExtent <= currentExtent),
	assert(currentExtent <= maxExtent);

	/// 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;

	/// True if the FlexibleSpaceBar overlaps the primary scrollable's contents.
	///
	/// This value is used by the [AppBar] to resolve
	/// [AppBar.backgroundColor] against [MaterialState.scrolledUnder],
	/// i.e. to enable apps to specify different colors when content
	/// has been scrolled up and behind the app bar.
	///
	/// Null if the caller hasn't determined if the FlexibleSpaceBar
	/// overlaps the primary scrollable's contents.
	final bool? isScrolledUnder;

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

	// We need the child widget to repaint, however both the opacity
	// and potentially `widget.background` can be constant which won't
	// lead to repainting.
	// see: https://github.com/flutter/flutter/issues/127836
	class _FlexibleSpaceHeaderOpacity extends SingleChildRenderObjectWidget {
	const _FlexibleSpaceHeaderOpacity({required this.opacity, required super.child, required this.alwaysIncludeSemantics});

	final double opacity;
	final bool alwaysIncludeSemantics;

	@override
	RenderObject createRenderObject(BuildContext context) {
	return _RenderFlexibleSpaceHeaderOpacity(opacity: opacity, alwaysIncludeSemantics: alwaysIncludeSemantics);
	}

	@override
	void updateRenderObject(BuildContext context, covariant _RenderFlexibleSpaceHeaderOpacity renderObject) {
	renderObject
	..alwaysIncludeSemantics = alwaysIncludeSemantics
	..opacity = opacity;
	}
	}

	class _RenderFlexibleSpaceHeaderOpacity extends RenderOpacity {
	_RenderFlexibleSpaceHeaderOpacity({super.opacity, super.alwaysIncludeSemantics});

	@override
	bool get isRepaintBoundary => false;
	}