packages/animations/lib/src/page_transition_switcher.dart - mirrors/packages - Git at Google

 // Copyright 2013 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 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';

 /// An internal representation of a child widget subtree that, now or in the past,
 /// was set on the [PageTransitionSwitcher.child] field and is now in the process of
 /// transitioning.
 ///
 /// The internal representation includes fields that we don't want to expose to
 /// the public API (like the controllers).
 class _ChildEntry {
   /// Creates a [_ChildEntry].
   ///
   /// The [primaryController], [secondaryController], [transition] and
   /// [widgetChild] parameters must not be null.
   _ChildEntry({
     required this.primaryController,
     required this.secondaryController,
     required this.transition,
     required this.widgetChild,
   });

   /// The animation controller for the child's transition.
   final AnimationController primaryController;

   /// The (curved) animation being used to drive the transition.
   final AnimationController secondaryController;

   /// The currently built transition for this child.
   Widget transition;

   /// The widget's child at the time this entry was created or updated.
   /// Used to rebuild the transition if necessary.
   Widget widgetChild;

   /// Release the resources used by this object.
   ///
   /// The object is no longer usable after this method is called.
   void dispose() {
     primaryController.dispose();
     secondaryController.dispose();
   }

   @override
   String toString() {
     return 'PageTransitionSwitcherEntry#${shortHash(this)}($widgetChild)';
   }
 }

 /// Signature for builders used to generate custom layouts for
 /// [PageTransitionSwitcher].
 ///
 /// The builder should return a widget which contains the given children, laid
 /// out as desired. It must not return null. The builder should be able to
 /// handle an empty list of `entries`.
 typedef PageTransitionSwitcherLayoutBuilder = Widget Function(
   List<Widget> entries,
 );

 /// Signature for builders used to generate custom transitions for
 /// [PageTransitionSwitcher].
 ///
 /// The function should return a widget which wraps the given `child`.
 ///
 /// When a [PageTransitionSwitcher]'s `child` is replaced, the new child's
 /// `primaryAnimation` runs forward and the value of its `secondaryAnimation` is
 /// usually fixed at 0.0. At the same time, the old child's `secondaryAnimation`
 /// runs forward, and the value of its primaryAnimation is usually fixed at 1.0.
 ///
 /// The widget returned by the [PageTransitionSwitcherTransitionBuilder] can
 /// incorporate both animations. It will use the primary animation to define how
 /// its child appears, and the secondary animation to define how its child
 /// disappears.
 typedef PageTransitionSwitcherTransitionBuilder = Widget Function(
   Widget child,
   Animation<double> primaryAnimation,
   Animation<double> secondaryAnimation,
 );

 /// A widget that transitions from an old child to a new child whenever [child]
 /// changes using an animation specified by [transitionBuilder].
 ///
 /// This is a variation of an [AnimatedSwitcher], but instead of using the
 /// same transition for enter and exit, two separate transitions can be
 /// specified, similar to how the enter and exit transitions of a [PageRoute]
 /// are defined.
 ///
 /// When a new [child] is specified, the [transitionBuilder] is effectively
 /// applied twice, once to the old child and once to the new one. When
 /// [reverse] is false, the old child's `secondaryAnimation` runs forward, and
 /// the value of its `primaryAnimation` is usually fixed at 1.0. The new child's
 /// `primaryAnimation` runs forward and the value of its `secondaryAnimation` is
 /// usually fixed at 0.0. The widget returned by the [transitionBuilder] can
 /// incorporate both animations. It will use the primary animation to define how
 /// its child appears, and the secondary animation to define how its child
 /// disappears. This is similar to the transition associated with pushing a new
 /// [PageRoute] on top of another.
 ///
 /// When [reverse] is true, the old child's `primaryAnimation` runs in reverse
 /// and the value of its `secondaryAnimation` is usually fixed at 0.0. The new
 /// child's `secondaryAnimation` runs in reverse and the value of its
 /// `primaryAnimation` is usually fixed at 1.0. This is similar to popping a
 /// [PageRoute] to reveal another [PageRoute] underneath it.
 ///
 /// This process is the same as the one used by [PageRoute.buildTransitions].
 ///
 /// The following example shows a [transitionBuilder] that slides out the
 /// old child to the right (driven by the `secondaryAnimation`) while the new
 /// child fades in (driven by the `primaryAnimation`):
 ///
 /// ```dart
 /// transitionBuilder: (
 ///   Widget child,
 ///   Animation<double> primaryAnimation,
 ///   Animation<double> secondaryAnimation,
 /// ) {
 ///   return SlideTransition(
 ///     position: Tween<Offset>(
 ///       begin: Offset.zero,
 ///       end: const Offset(1.5, 0.0),
 ///     ).animate(secondaryAnimation),
 ///     child: FadeTransition(
 ///       opacity: Tween<double>(
 ///         begin: 0.0,
 ///         end: 1.0,
 ///       ).animate(primaryAnimation),
 ///       child: child,
 ///     ),
 ///   );
 /// },
 /// ```
 ///
 /// If the children are swapped fast enough (i.e. before [duration] elapses),
 /// more than one old child can exist and be transitioning out while the
 /// newest one is transitioning in.
 ///
 /// If the *new* child is the same widget type and key as the *old* child,
 /// but with different parameters, then [PageTransitionSwitcher] will *not* do a
 /// transition between them, since as far as the framework is concerned, they
 /// are the same widget and the existing widget can be updated with the new
 /// parameters. To force the transition to occur, set a [Key] on each child
 /// widget that you wish to be considered unique (typically a [ValueKey] on the
 /// widget data that distinguishes this child from the others). For example,
 /// changing the child from `SizedBox(width: 10)` to `SizedBox(width: 100)`
 /// would not trigger a transition but changing the child from
 /// `SizedBox(width: 10)` to `SizedBox(key: Key('foo'), width: 100)` would.
 /// Similarly, changing the child to `Container(width: 10)` would trigger a
 /// transition.
 ///
 /// The same key can be used for a new child as was used for an already-outgoing
 /// child; the two will not be considered related. For example, if a progress
 /// indicator with key A is first shown, then an image with key B, then another
 /// progress indicator with key A again, all in rapid succession, then the old
 /// progress indicator and the image will be fading out while a new progress
 /// indicator is fading in.
 ///
 /// PageTransitionSwitcher uses the [layoutBuilder] property to lay out the
 /// old and new child widgets. By default, [defaultLayoutBuilder] is used.
 /// See the documentation for [layoutBuilder] for suggestions on how to
 /// configure the layout of the incoming and outgoing child widgets if
 /// [defaultLayoutBuilder] is not your desired layout.
 class PageTransitionSwitcher extends StatefulWidget {
   /// Creates a [PageTransitionSwitcher].
   ///
   /// The [duration], [reverse], and [transitionBuilder] parameters
   /// must not be null.
   const PageTransitionSwitcher({
     Key? key,
     this.duration = const Duration(milliseconds: 300),
     this.reverse = false,
     required this.transitionBuilder,
     this.layoutBuilder = defaultLayoutBuilder,
     this.child,
   }) : super(key: key);

   /// The current child widget to display.
   ///
   /// If there was an old child, it will be transitioned out using the
   /// secondary animation of the [transitionBuilder], while the new child
   /// transitions in using the primary animation of the [transitionBuilder].
   ///
   /// If there was no old child, then this child will transition in using
   /// the primary animation of the [transitionBuilder].
   ///
   /// The child is considered to be "new" if it has a different type or [Key]
   /// (see [Widget.canUpdate]).
   final Widget? child;

   /// The duration of the transition from the old [child] value to the new one.
   ///
   /// This duration is applied to the given [child] when that property is set to
   /// a new child. Changing [duration] will not affect the durations of
   /// transitions already in progress.
   final Duration duration;

   /// Indicates whether the new [child] will visually appear on top of or
   /// underneath the old child.
   ///
   /// When this is false, the new child will transition in on top of the
   /// old child while its primary animation and the secondary
   /// animation of the old child are running forward. This is similar to
   /// the transition associated with pushing a new [PageRoute] on top of
   /// another.
   ///
   /// When this is true, the new child will transition in below the
   /// old child while its secondary animation and the primary
   /// animation of the old child are running in reverse. This is similar to
   /// the transition associated with popping a [PageRoute] to reveal a new
   /// [PageRoute] below it.
   final bool reverse;

   /// A function that wraps a new [child] with a primary and secondary animation
   /// set define how the child appears and disappears.
   ///
   /// This is only called when a new [child] is set (not for each build), or
   /// when a new [transitionBuilder] is set. If a new [transitionBuilder] is
   /// set, then the transition is rebuilt for the current child and all old
   /// children using the new [transitionBuilder]. The function must not return
   /// null.
   ///
   /// The child provided to the transitionBuilder may be null.
   final PageTransitionSwitcherTransitionBuilder transitionBuilder;

   /// A function that wraps all of the children that are transitioning out, and
   /// the [child] that's transitioning in, with a widget that lays all of them
   /// out. This is called every time this widget is built. The function must not
   /// return null.
   ///
   /// The default [PageTransitionSwitcherLayoutBuilder] used is
   /// [defaultLayoutBuilder].
   ///
   /// The following example shows a [layoutBuilder] that places all entries in a
   /// [Stack] that sizes itself to match the largest of the active entries.
   /// All children are aligned on the top left corner of the [Stack].
   ///
   /// ```dart
   /// PageTransitionSwitcher(
   ///   duration: const Duration(milliseconds: 100),
   ///   child: Container(color: Colors.red),
   ///   layoutBuilder: (
   ///     List<Widget> entries,
   ///   ) {
   ///     return Stack(
   ///       children: entries,
   ///       alignment: Alignment.topLeft,
   ///     );
   ///   },
   /// ),
   /// ```
   /// See [PageTransitionSwitcherLayoutBuilder] for more information about
   /// how a layout builder should function.
   final PageTransitionSwitcherLayoutBuilder layoutBuilder;

   /// The default layout builder for [PageTransitionSwitcher].
   ///
   /// This function is the default way for how the new and old child widgets are placed
   /// during the transition between the two widgets. All children are placed in a
   /// [Stack] that sizes itself to match the largest of the child or a previous child.
   /// The children are centered on each other.
   ///
   /// See [PageTransitionSwitcherTransitionBuilder] for more information on the function
   /// signature.
   static Widget defaultLayoutBuilder(List<Widget> entries) {
     return Stack(
       children: entries,
       alignment: Alignment.center,
     );
   }

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

 class _PageTransitionSwitcherState extends State<PageTransitionSwitcher>
     with TickerProviderStateMixin {
   final List<_ChildEntry> _activeEntries = <_ChildEntry>[];
   _ChildEntry? _currentEntry;
   int _childNumber = 0;

   @override
   void initState() {
     super.initState();
     _addEntryForNewChild(shouldAnimate: false);
   }

   @override
   void didUpdateWidget(PageTransitionSwitcher oldWidget) {
     super.didUpdateWidget(oldWidget);

     // If the transition builder changed, then update all of the old
     // transitions.
     if (widget.transitionBuilder != oldWidget.transitionBuilder) {
       _activeEntries.forEach(_updateTransitionForEntry);
     }

     final bool hasNewChild = widget.child != null;
     final bool hasOldChild = _currentEntry != null;
     if (hasNewChild != hasOldChild ||
         hasNewChild &&
             !Widget.canUpdate(widget.child!, _currentEntry!.widgetChild)) {
       // Child has changed, fade current entry out and add new entry.
       _childNumber += 1;
       _addEntryForNewChild(shouldAnimate: true);
     } else if (_currentEntry != null) {
       assert(hasOldChild && hasNewChild);
       assert(Widget.canUpdate(widget.child!, _currentEntry!.widgetChild));
       // Child has been updated. Make sure we update the child widget and
       // transition in _currentEntry even though we're not going to start a new
       // animation, but keep the key from the old transition so that we
       // update the transition instead of replacing it.
       _currentEntry!.widgetChild = widget.child!;
       _updateTransitionForEntry(_currentEntry!); // uses entry.widgetChild
     }
   }

   void _addEntryForNewChild({required bool shouldAnimate}) {
     assert(shouldAnimate || _currentEntry == null);
     if (_currentEntry != null) {
       assert(shouldAnimate);
       if (widget.reverse) {
         _currentEntry!.primaryController.reverse();
       } else {
         _currentEntry!.secondaryController.forward();
       }
       _currentEntry = null;
     }
     if (widget.child == null) {
       return;
     }
     final AnimationController primaryController = AnimationController(
       duration: widget.duration,
       vsync: this,
     );
     final AnimationController secondaryController = AnimationController(
       duration: widget.duration,
       vsync: this,
     );
     if (shouldAnimate) {
       if (widget.reverse) {
         primaryController.value = 1.0;
         secondaryController.value = 1.0;
         secondaryController.reverse();
       } else {
         primaryController.forward();
       }
     } else {
       assert(_activeEntries.isEmpty);
       primaryController.value = 1.0;
     }
     _currentEntry = _newEntry(
       child: widget.child!,
       primaryController: primaryController,
       secondaryController: secondaryController,
       builder: widget.transitionBuilder,
     );
     if (widget.reverse && _activeEntries.isNotEmpty) {
       // Add below old child.
       _activeEntries.insert(_activeEntries.length - 1, _currentEntry!);
     } else {
       // Add on top of old child.
       _activeEntries.add(_currentEntry!);
     }
   }

   _ChildEntry _newEntry({
     required Widget child,
     required PageTransitionSwitcherTransitionBuilder builder,
     required AnimationController primaryController,
     required AnimationController secondaryController,
   }) {
     final Widget transition = builder(
       child,
       primaryController,
       secondaryController,
     );
     final _ChildEntry entry = _ChildEntry(
       widgetChild: child,
       transition: KeyedSubtree.wrap(
         transition,
         _childNumber,
       ),
       primaryController: primaryController,
       secondaryController: secondaryController,
     );
     secondaryController.addStatusListener((AnimationStatus status) {
       if (status == AnimationStatus.completed) {
         assert(mounted);
         assert(_activeEntries.contains(entry));
         setState(() {
           _activeEntries.remove(entry);
           entry.dispose();
         });
       }
     });
     primaryController.addStatusListener((AnimationStatus status) {
       if (status == AnimationStatus.dismissed) {
         assert(mounted);
         assert(_activeEntries.contains(entry));
         setState(() {
           _activeEntries.remove(entry);
           entry.dispose();
         });
       }
     });
     return entry;
   }

   void _updateTransitionForEntry(_ChildEntry entry) {
     final Widget transition = widget.transitionBuilder(
       entry.widgetChild,
       entry.primaryController,
       entry.secondaryController,
     );
     entry.transition = KeyedSubtree(
       key: entry.transition.key,
       child: transition,
     );
   }

   @override
   void dispose() {
     for (final _ChildEntry entry in _activeEntries) {
       entry.dispose();
     }
     super.dispose();
   }

   @override
   Widget build(BuildContext context) {
     return widget.layoutBuilder(_activeEntries
         .map<Widget>((_ChildEntry entry) => entry.transition)
         .toList());
   }
 }
	// Copyright 2013 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 'package:flutter/foundation.dart';
	import 'package:flutter/widgets.dart';

	/// An internal representation of a child widget subtree that, now or in the past,
	/// was set on the [PageTransitionSwitcher.child] field and is now in the process of
	/// transitioning.
	///
	/// The internal representation includes fields that we don't want to expose to
	/// the public API (like the controllers).
	class _ChildEntry {
	/// Creates a [_ChildEntry].
	///
	/// The [primaryController], [secondaryController], [transition] and
	/// [widgetChild] parameters must not be null.
	_ChildEntry({
	required this.primaryController,
	required this.secondaryController,
	required this.transition,
	required this.widgetChild,
	});

	/// The animation controller for the child's transition.
	final AnimationController primaryController;

	/// The (curved) animation being used to drive the transition.
	final AnimationController secondaryController;

	/// The currently built transition for this child.
	Widget transition;

	/// The widget's child at the time this entry was created or updated.
	/// Used to rebuild the transition if necessary.
	Widget widgetChild;

	/// Release the resources used by this object.
	///
	/// The object is no longer usable after this method is called.
	void dispose() {
	primaryController.dispose();
	secondaryController.dispose();
	}

	@override
	String toString() {
	return 'PageTransitionSwitcherEntry#${shortHash(this)}($widgetChild)';
	}
	}

	/// Signature for builders used to generate custom layouts for
	/// [PageTransitionSwitcher].
	///
	/// The builder should return a widget which contains the given children, laid
	/// out as desired. It must not return null. The builder should be able to
	/// handle an empty list of `entries`.
	typedef PageTransitionSwitcherLayoutBuilder = Widget Function(
	List<Widget> entries,
	);

	/// Signature for builders used to generate custom transitions for
	/// [PageTransitionSwitcher].
	///
	/// The function should return a widget which wraps the given `child`.
	///
	/// When a [PageTransitionSwitcher]'s `child` is replaced, the new child's
	/// `primaryAnimation` runs forward and the value of its `secondaryAnimation` is
	/// usually fixed at 0.0. At the same time, the old child's `secondaryAnimation`
	/// runs forward, and the value of its primaryAnimation is usually fixed at 1.0.
	///
	/// The widget returned by the [PageTransitionSwitcherTransitionBuilder] can
	/// incorporate both animations. It will use the primary animation to define how
	/// its child appears, and the secondary animation to define how its child
	/// disappears.
	typedef PageTransitionSwitcherTransitionBuilder = Widget Function(
	Widget child,
	Animation<double> primaryAnimation,
	Animation<double> secondaryAnimation,
	);

	/// A widget that transitions from an old child to a new child whenever [child]
	/// changes using an animation specified by [transitionBuilder].
	///
	/// This is a variation of an [AnimatedSwitcher], but instead of using the
	/// same transition for enter and exit, two separate transitions can be
	/// specified, similar to how the enter and exit transitions of a [PageRoute]
	/// are defined.
	///
	/// When a new [child] is specified, the [transitionBuilder] is effectively
	/// applied twice, once to the old child and once to the new one. When
	/// [reverse] is false, the old child's `secondaryAnimation` runs forward, and
	/// the value of its `primaryAnimation` is usually fixed at 1.0. The new child's
	/// `primaryAnimation` runs forward and the value of its `secondaryAnimation` is
	/// usually fixed at 0.0. The widget returned by the [transitionBuilder] can
	/// incorporate both animations. It will use the primary animation to define how
	/// its child appears, and the secondary animation to define how its child
	/// disappears. This is similar to the transition associated with pushing a new
	/// [PageRoute] on top of another.
	///
	/// When [reverse] is true, the old child's `primaryAnimation` runs in reverse
	/// and the value of its `secondaryAnimation` is usually fixed at 0.0. The new
	/// child's `secondaryAnimation` runs in reverse and the value of its
	/// `primaryAnimation` is usually fixed at 1.0. This is similar to popping a
	/// [PageRoute] to reveal another [PageRoute] underneath it.
	///
	/// This process is the same as the one used by [PageRoute.buildTransitions].
	///
	/// The following example shows a [transitionBuilder] that slides out the
	/// old child to the right (driven by the `secondaryAnimation`) while the new
	/// child fades in (driven by the `primaryAnimation`):
	///
	/// ```dart
	/// transitionBuilder: (
	/// Widget child,
	/// Animation<double> primaryAnimation,
	/// Animation<double> secondaryAnimation,
	/// ) {
	/// return SlideTransition(
	/// position: Tween<Offset>(
	/// begin: Offset.zero,
	/// end: const Offset(1.5, 0.0),
	/// ).animate(secondaryAnimation),
	/// child: FadeTransition(
	/// opacity: Tween<double>(
	/// begin: 0.0,
	/// end: 1.0,
	/// ).animate(primaryAnimation),
	/// child: child,
	/// ),
	/// );
	/// },
	/// ```
	///
	/// If the children are swapped fast enough (i.e. before [duration] elapses),
	/// more than one old child can exist and be transitioning out while the
	/// newest one is transitioning in.
	///
	/// If the new child is the same widget type and key as the old child,
	/// but with different parameters, then [PageTransitionSwitcher] will not do a
	/// transition between them, since as far as the framework is concerned, they
	/// are the same widget and the existing widget can be updated with the new
	/// parameters. To force the transition to occur, set a [Key] on each child
	/// widget that you wish to be considered unique (typically a [ValueKey] on the
	/// widget data that distinguishes this child from the others). For example,
	/// changing the child from `SizedBox(width: 10)` to `SizedBox(width: 100)`
	/// would not trigger a transition but changing the child from
	/// `SizedBox(width: 10)` to `SizedBox(key: Key('foo'), width: 100)` would.
	/// Similarly, changing the child to `Container(width: 10)` would trigger a
	/// transition.
	///
	/// The same key can be used for a new child as was used for an already-outgoing
	/// child; the two will not be considered related. For example, if a progress
	/// indicator with key A is first shown, then an image with key B, then another
	/// progress indicator with key A again, all in rapid succession, then the old
	/// progress indicator and the image will be fading out while a new progress
	/// indicator is fading in.
	///
	/// PageTransitionSwitcher uses the [layoutBuilder] property to lay out the
	/// old and new child widgets. By default, [defaultLayoutBuilder] is used.
	/// See the documentation for [layoutBuilder] for suggestions on how to
	/// configure the layout of the incoming and outgoing child widgets if
	/// [defaultLayoutBuilder] is not your desired layout.
	class PageTransitionSwitcher extends StatefulWidget {
	/// Creates a [PageTransitionSwitcher].
	///
	/// The [duration], [reverse], and [transitionBuilder] parameters
	/// must not be null.
	const PageTransitionSwitcher({
	Key? key,
	this.duration = const Duration(milliseconds: 300),
	this.reverse = false,
	required this.transitionBuilder,
	this.layoutBuilder = defaultLayoutBuilder,
	this.child,
	}) : super(key: key);

	/// The current child widget to display.
	///
	/// If there was an old child, it will be transitioned out using the
	/// secondary animation of the [transitionBuilder], while the new child
	/// transitions in using the primary animation of the [transitionBuilder].
	///
	/// If there was no old child, then this child will transition in using
	/// the primary animation of the [transitionBuilder].
	///
	/// The child is considered to be "new" if it has a different type or [Key]
	/// (see [Widget.canUpdate]).
	final Widget? child;

	/// The duration of the transition from the old [child] value to the new one.
	///
	/// This duration is applied to the given [child] when that property is set to
	/// a new child. Changing [duration] will not affect the durations of
	/// transitions already in progress.
	final Duration duration;

	/// Indicates whether the new [child] will visually appear on top of or
	/// underneath the old child.
	///
	/// When this is false, the new child will transition in on top of the
	/// old child while its primary animation and the secondary
	/// animation of the old child are running forward. This is similar to
	/// the transition associated with pushing a new [PageRoute] on top of
	/// another.
	///
	/// When this is true, the new child will transition in below the
	/// old child while its secondary animation and the primary
	/// animation of the old child are running in reverse. This is similar to
	/// the transition associated with popping a [PageRoute] to reveal a new
	/// [PageRoute] below it.
	final bool reverse;

	/// A function that wraps a new [child] with a primary and secondary animation
	/// set define how the child appears and disappears.
	///
	/// This is only called when a new [child] is set (not for each build), or
	/// when a new [transitionBuilder] is set. If a new [transitionBuilder] is
	/// set, then the transition is rebuilt for the current child and all old
	/// children using the new [transitionBuilder]. The function must not return
	/// null.
	///
	/// The child provided to the transitionBuilder may be null.
	final PageTransitionSwitcherTransitionBuilder transitionBuilder;

	/// A function that wraps all of the children that are transitioning out, and
	/// the [child] that's transitioning in, with a widget that lays all of them
	/// out. This is called every time this widget is built. The function must not
	/// return null.
	///
	/// The default [PageTransitionSwitcherLayoutBuilder] used is
	/// [defaultLayoutBuilder].
	///
	/// The following example shows a [layoutBuilder] that places all entries in a
	/// [Stack] that sizes itself to match the largest of the active entries.
	/// All children are aligned on the top left corner of the [Stack].
	///
	/// ```dart
	/// PageTransitionSwitcher(
	/// duration: const Duration(milliseconds: 100),
	/// child: Container(color: Colors.red),
	/// layoutBuilder: (
	/// List<Widget> entries,
	/// ) {
	/// return Stack(
	/// children: entries,
	/// alignment: Alignment.topLeft,
	/// );
	/// },
	/// ),
	/// ```
	/// See [PageTransitionSwitcherLayoutBuilder] for more information about
	/// how a layout builder should function.
	final PageTransitionSwitcherLayoutBuilder layoutBuilder;

	/// The default layout builder for [PageTransitionSwitcher].
	///
	/// This function is the default way for how the new and old child widgets are placed
	/// during the transition between the two widgets. All children are placed in a
	/// [Stack] that sizes itself to match the largest of the child or a previous child.
	/// The children are centered on each other.
	///
	/// See [PageTransitionSwitcherTransitionBuilder] for more information on the function
	/// signature.
	static Widget defaultLayoutBuilder(List<Widget> entries) {
	return Stack(
	children: entries,
	alignment: Alignment.center,
	);
	}

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

	class _PageTransitionSwitcherState extends State<PageTransitionSwitcher>
	with TickerProviderStateMixin {
	final List<_ChildEntry> _activeEntries = <_ChildEntry>[];
	_ChildEntry? _currentEntry;
	int _childNumber = 0;

	@override
	void initState() {
	super.initState();
	_addEntryForNewChild(shouldAnimate: false);
	}

	@override
	void didUpdateWidget(PageTransitionSwitcher oldWidget) {
	super.didUpdateWidget(oldWidget);

	// If the transition builder changed, then update all of the old
	// transitions.
	if (widget.transitionBuilder != oldWidget.transitionBuilder) {
	_activeEntries.forEach(_updateTransitionForEntry);
	}

	final bool hasNewChild = widget.child != null;
	final bool hasOldChild = _currentEntry != null;
	if (hasNewChild != hasOldChild \|\|
	hasNewChild &&
	!Widget.canUpdate(widget.child!, _currentEntry!.widgetChild)) {
	// Child has changed, fade current entry out and add new entry.
	_childNumber += 1;
	_addEntryForNewChild(shouldAnimate: true);
	} else if (_currentEntry != null) {
	assert(hasOldChild && hasNewChild);
	assert(Widget.canUpdate(widget.child!, _currentEntry!.widgetChild));
	// Child has been updated. Make sure we update the child widget and
	// transition in _currentEntry even though we're not going to start a new
	// animation, but keep the key from the old transition so that we
	// update the transition instead of replacing it.
	_currentEntry!.widgetChild = widget.child!;
	_updateTransitionForEntry(_currentEntry!); // uses entry.widgetChild
	}
	}

	void _addEntryForNewChild({required bool shouldAnimate}) {
	assert(shouldAnimate \|\| _currentEntry == null);
	if (_currentEntry != null) {
	assert(shouldAnimate);
	if (widget.reverse) {
	_currentEntry!.primaryController.reverse();
	} else {
	_currentEntry!.secondaryController.forward();
	}
	_currentEntry = null;
	}
	if (widget.child == null) {
	return;
	}
	final AnimationController primaryController = AnimationController(
	duration: widget.duration,
	vsync: this,
	);
	final AnimationController secondaryController = AnimationController(
	duration: widget.duration,
	vsync: this,
	);
	if (shouldAnimate) {
	if (widget.reverse) {
	primaryController.value = 1.0;
	secondaryController.value = 1.0;
	secondaryController.reverse();
	} else {
	primaryController.forward();
	}
	} else {
	assert(_activeEntries.isEmpty);
	primaryController.value = 1.0;
	}
	_currentEntry = _newEntry(
	child: widget.child!,
	primaryController: primaryController,
	secondaryController: secondaryController,
	builder: widget.transitionBuilder,
	);
	if (widget.reverse && _activeEntries.isNotEmpty) {
	// Add below old child.
	_activeEntries.insert(_activeEntries.length - 1, _currentEntry!);
	} else {
	// Add on top of old child.
	_activeEntries.add(_currentEntry!);
	}
	}

	_ChildEntry _newEntry({
	required Widget child,
	required PageTransitionSwitcherTransitionBuilder builder,
	required AnimationController primaryController,
	required AnimationController secondaryController,
	}) {
	final Widget transition = builder(
	child,
	primaryController,
	secondaryController,
	);
	final _ChildEntry entry = _ChildEntry(
	widgetChild: child,
	transition: KeyedSubtree.wrap(
	transition,
	_childNumber,
	),
	primaryController: primaryController,
	secondaryController: secondaryController,
	);
	secondaryController.addStatusListener((AnimationStatus status) {
	if (status == AnimationStatus.completed) {
	assert(mounted);
	assert(_activeEntries.contains(entry));
	setState(() {
	_activeEntries.remove(entry);
	entry.dispose();
	});
	}
	});
	primaryController.addStatusListener((AnimationStatus status) {
	if (status == AnimationStatus.dismissed) {
	assert(mounted);
	assert(_activeEntries.contains(entry));
	setState(() {
	_activeEntries.remove(entry);
	entry.dispose();
	});
	}
	});
	return entry;
	}

	void _updateTransitionForEntry(_ChildEntry entry) {
	final Widget transition = widget.transitionBuilder(
	entry.widgetChild,
	entry.primaryController,
	entry.secondaryController,
	);
	entry.transition = KeyedSubtree(
	key: entry.transition.key,
	child: transition,
	);
	}

	@override
	void dispose() {
	for (final _ChildEntry entry in _activeEntries) {
	entry.dispose();
	}
	super.dispose();
	}

	@override
	Widget build(BuildContext context) {
	return widget.layoutBuilder(_activeEntries
	.map<Widget>((_ChildEntry entry) => entry.transition)
	.toList());
	}
	}