packages/flutter/lib/src/cupertino/page.dart - mirrors/flutter - Git at Google

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

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

 const double _kMinFlingVelocity = 1.0;  // screen width per second.

 // Fractional offset from offscreen to the right to fully on screen.
 final FractionalOffsetTween _kRightMiddleTween = new FractionalOffsetTween(
   begin: FractionalOffset.topRight,
   end: FractionalOffset.topLeft,
 );

 // Fractional offset from fully on screen to 1/3 offscreen to the left.
 final FractionalOffsetTween _kMiddleLeftTween = new FractionalOffsetTween(
   begin: FractionalOffset.topLeft,
   end: const FractionalOffset(-1.0/3.0, 0.0),
 );

 // Fractional offset from offscreen below to fully on screen.
 final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
   begin: FractionalOffset.bottomLeft,
   end: FractionalOffset.topLeft,
 );

 // Custom decoration from no shadow to page shadow mimicking iOS page
 // transitions using gradients.
 final DecorationTween _kGradientShadowTween = new DecorationTween(
   begin: _CupertinoEdgeShadowDecoration.none, // No decoration initially.
   end: const _CupertinoEdgeShadowDecoration(
     edgeGradient: const LinearGradient(
       // Spans 5% of the page.
       begin: const FractionalOffset(0.95, 0.0),
       end: FractionalOffset.topRight,
       // Eyeballed gradient used to mimic a drop shadow on the left side only.
       colors: const <Color>[
         const Color(0x00000000),
         const Color(0x04000000),
         const Color(0x12000000),
         const Color(0x38000000)
       ],
       stops: const <double>[0.0, 0.3, 0.6, 1.0],
     ),
   ),
 );

 /// A modal route that replaces the entire screen with an iOS transition.
 ///
 /// The page slides in from the right and exits in reverse.
 /// The page also shifts to the left in parallax when another page enters to cover it.
 ///
 /// The page slides in from the bottom and exits in reverse with no parallax effect
 /// for fullscreen dialogs.
 ///
 /// By default, when a modal route is replaced by another, the previous route
 /// remains in memory. To free all the resources when this is not necessary, set
 /// [maintainState] to false.
 ///
 /// See also:
 ///
 ///  * [MaterialPageRoute] for an adaptive [PageRoute] that uses a platform appropriate transition.
 class CupertinoPageRoute<T> extends PageRoute<T> {
   /// Creates a page route for use in an iOS designed app.
   CupertinoPageRoute({
     @required this.builder,
     RouteSettings settings: const RouteSettings(),
     this.maintainState: true,
     bool fullscreenDialog: false,
   }) : assert(builder != null),
        assert(opaque),
        super(settings: settings, fullscreenDialog: fullscreenDialog);

   /// Builds the primary contents of the route.
   final WidgetBuilder builder;

   @override
   final bool maintainState;

   @override
   Duration get transitionDuration => const Duration(milliseconds: 350);

   @override
   Color get barrierColor => null;

   @override
   bool canTransitionFrom(TransitionRoute<dynamic> nextRoute) {
     return nextRoute is CupertinoPageRoute<dynamic>;
   }

   @override
   bool canTransitionTo(TransitionRoute<dynamic> nextRoute) {
     // Don't perform outgoing animation if the next route is a fullscreen dialog.
     return nextRoute is CupertinoPageRoute && !nextRoute.fullscreenDialog;
   }

   @override
   void dispose() {
     _backGestureController?.dispose();
     // If the route is never installed (i.e. pushed into a Navigator) such as the
     // case when [MaterialPageRoute] delegates transition building to [CupertinoPageRoute],
     // don't dispose super.
     if (overlayEntries.isNotEmpty)
       super.dispose();
   }

   CupertinoBackGestureController _backGestureController;

   /// Support for dismissing this route with a horizontal swipe.
   ///
   /// Swiping will be disabled if the page is a fullscreen dialog or if
   /// dismissals can be overriden because a [WillPopCallback] was
   /// defined for the route.
   ///
   /// See also:
   ///
   ///  * [hasScopedWillPopCallback], which is true if a `willPop` callback
   ///    is defined for this route.
   @override
   NavigationGestureController startPopGesture() {
     return startPopGestureForRoute(this);
   }

   /// Create a CupertinoBackGestureController using a specific PageRoute.
   ///
   /// Used when [MaterialPageRoute] delegates the back gesture to [CupertinoPageRoute]
   /// since the [CupertinoPageRoute] is not actually inserted into the Navigator.
   NavigationGestureController startPopGestureForRoute(PageRoute<T> hostRoute) {
     // If attempts to dismiss this route might be vetoed such as in a page
     // with forms, then do not allow the user to dismiss the route with a swipe.
     if (hostRoute.hasScopedWillPopCallback)
       return null;
     // Fullscreen dialogs aren't dismissable by back swipe.
     if (fullscreenDialog)
       return null;
     if (hostRoute.controller.status != AnimationStatus.completed)
       return null;
     assert(_backGestureController == null);
     _backGestureController = new CupertinoBackGestureController(
       navigator: hostRoute.navigator,
       controller: hostRoute.controller,
     );

     Function handleBackGestureEnded;
     handleBackGestureEnded = (AnimationStatus status) {
       if (status == AnimationStatus.completed) {
         _backGestureController?.dispose();
         _backGestureController = null;
         hostRoute.controller.removeStatusListener(handleBackGestureEnded);
       }
     };

     hostRoute.controller.addStatusListener(handleBackGestureEnded);
     return _backGestureController;
   }

   @override
   Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
     final Widget result = builder(context);
     assert(() {
       if (result == null) {
         throw new FlutterError(
           'The builder for route "${settings.name}" returned null.\n'
           'Route builders must never return null.'
         );
       }
       return true;
     });
     return result;
   }

   @override
   Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
     if (fullscreenDialog)
       return new CupertinoFullscreenDialogTransition(
         animation: animation,
         child: child,
       );
     else
       return new CupertinoPageTransition(
         primaryRouteAnimation: animation,
         secondaryRouteAnimation: secondaryAnimation,
         child: child,
         // In the middle of a back gesture drag, let the transition be linear to match finger
         // motions.
         linearTransition: _backGestureController != null,
       );
   }

   @override
   String get debugLabel => '${super.debugLabel}(${settings.name})';
 }

 /// Provides an iOS-style page transition animation.
 ///
 /// The page slides in from the right and exits in reverse. It also shifts to the left in
 /// a parallax motion when another page enters to cover it.
 class CupertinoPageTransition extends StatelessWidget {
   /// Creates an iOS-style page transition.
   ///
   ///  * `primaryRouteAnimation` is a linear route animation from 0.0 to 1.0
   ///    when this screen is being pushed.
   ///  * `secondaryRouteAnimation` is a linear route animation from 0.0 to 1.0
   ///    when another screen is being pushed on top of this one.
   ///  * `linearTransition` is whether to perform primary transition linearly.
   ///    Used to precisely track back gesture drags.
   CupertinoPageTransition({
     Key key,
     @required Animation<double> primaryRouteAnimation,
     @required Animation<double> secondaryRouteAnimation,
     @required this.child,
     bool linearTransition,
   }) :
       _primaryPositionAnimation = linearTransition
         ? _kRightMiddleTween.animate(primaryRouteAnimation)
         : _kRightMiddleTween.animate(
             new CurvedAnimation(
               parent: primaryRouteAnimation,
               curve: Curves.easeOut,
               reverseCurve: Curves.easeIn,
             )
           ),
       _secondaryPositionAnimation = _kMiddleLeftTween.animate(
         new CurvedAnimation(
           parent: secondaryRouteAnimation,
           curve: Curves.easeOut,
           reverseCurve: Curves.easeIn,
         )
       ),
       _primaryShadowAnimation = _kGradientShadowTween.animate(
         new CurvedAnimation(
           parent: primaryRouteAnimation,
           curve: Curves.easeOut,
         )
       ),
       super(key: key);

   // When this page is coming in to cover another page.
   final Animation<FractionalOffset> _primaryPositionAnimation;
   // When this page is becoming covered by another page.
   final Animation<FractionalOffset> _secondaryPositionAnimation;
   final Animation<Decoration> _primaryShadowAnimation;

   /// The widget below this widget in the tree.
   final Widget child;

   @override
   Widget build(BuildContext context) {
     // TODO(ianh): tell the transform to be un-transformed for hit testing
     // but not while being controlled by a gesture.
     return new SlideTransition(
       position: _secondaryPositionAnimation,
       child: new SlideTransition(
         position: _primaryPositionAnimation,
         child: new DecoratedBoxTransition(
           decoration: _primaryShadowAnimation,
           child: child,
         ),
       ),
     );
   }
 }

 /// An iOS-style transition used for summoning fullscreen dialogs.
 ///
 /// For example, used when creating a new calendar event by bringing in the next
 /// screen from the bottom.
 class CupertinoFullscreenDialogTransition extends StatelessWidget {
   /// Creates an iOS-style transition used for summoning fullscreen dialogs.
   CupertinoFullscreenDialogTransition({
     Key key,
     @required Animation<double> animation,
     @required this.child,
   }) : _positionAnimation = _kBottomUpTween.animate(
          new CurvedAnimation(
            parent: animation,
            curve: Curves.easeInOut,
          )
        ),
        super(key: key);

   final Animation<FractionalOffset> _positionAnimation;

   /// The widget below this widget in the tree.
   final Widget child;

   @override
   Widget build(BuildContext context) {
     return new SlideTransition(
       position: _positionAnimation,
       child: child,
     );
   }
 }

 /// A controller for an iOS-style back gesture.
 ///
 /// Uses a drag gesture to control the route's transition animation progress.
 class CupertinoBackGestureController extends NavigationGestureController {
   /// Creates a controller for an iOS-style back gesture.
   ///
   /// The [navigator] and [controller] arguments must not be null.
   CupertinoBackGestureController({
     @required NavigatorState navigator,
     @required this.controller,
   }) : assert(controller != null),
        super(navigator);

   /// The animation controller that the route uses to drive its transition
   /// animation.
   final AnimationController controller;

   @override
   void dispose() {
     controller.removeStatusListener(_handleStatusChanged);
     super.dispose();
   }

   @override
   void dragUpdate(double delta) {
     // This assert can be triggered the Scaffold is reparented out of the route
     // associated with this gesture controller and continues to feed it events.
     // TODO(abarth): Change the ownership of the gesture controller so that the
     // object feeding it these events (e.g., the Scaffold) is responsible for
     // calling dispose on it as well.
     assert(controller != null);
     controller.value -= delta;
   }

   @override
   bool dragEnd(double velocity) {
     // This assert can be triggered the Scaffold is reparented out of the route
     // associated with this gesture controller and continues to feed it events.
     // TODO(abarth): Change the ownership of the gesture controller so that the
     // object feeding it these events (e.g., the Scaffold) is responsible for
     // calling dispose on it as well.
     assert(controller != null);

     if (velocity.abs() >= _kMinFlingVelocity) {
       controller.fling(velocity: -velocity);
     } else if (controller.value <= 0.5) {
       controller.fling(velocity: -1.0);
     } else {
       controller.fling(velocity: 1.0);
     }

     // Don't end the gesture until the transition completes.
     final AnimationStatus status = controller.status;
     _handleStatusChanged(status);
     controller?.addStatusListener(_handleStatusChanged);

     return (status == AnimationStatus.reverse || status == AnimationStatus.dismissed);
   }

   void _handleStatusChanged(AnimationStatus status) {
     if (status == AnimationStatus.dismissed)
       navigator.pop();
   }
 }

 /// A custom [Decoration] used to paint an extra shadow on the left edge of the
 /// box it's decorating. It's like a [BoxDecoration] with only a gradient except
 /// it paints to the left of the box instead of behind the box.
 class _CupertinoEdgeShadowDecoration extends Decoration {
   const _CupertinoEdgeShadowDecoration({ this.edgeGradient });

   /// A Decoration with no decorating properties.
   static const _CupertinoEdgeShadowDecoration none =
       const _CupertinoEdgeShadowDecoration();

   /// A gradient to draw to the left of the box being decorated.
   /// FractionalOffsets are relative to the original box translated one box
   /// width to the left.
   final LinearGradient edgeGradient;

   /// Linearly interpolate between two edge shadow decorations decorations.
   ///
   /// See also [Decoration.lerp].
   static _CupertinoEdgeShadowDecoration lerp(
     _CupertinoEdgeShadowDecoration a,
     _CupertinoEdgeShadowDecoration b,
     double t
   ) {
     if (a == null && b == null)
       return null;
     return new _CupertinoEdgeShadowDecoration(
       edgeGradient: LinearGradient.lerp(a?.edgeGradient, b?.edgeGradient, t),
     );
   }

   @override
   _CupertinoEdgeShadowDecoration lerpFrom(Decoration a, double t) {
     if (a is! _CupertinoEdgeShadowDecoration)
       return _CupertinoEdgeShadowDecoration.lerp(null, this, t);
     return _CupertinoEdgeShadowDecoration.lerp(a, this, t);
   }

   @override
   _CupertinoEdgeShadowDecoration lerpTo(Decoration b, double t) {
     if (b is! _CupertinoEdgeShadowDecoration)
       return _CupertinoEdgeShadowDecoration.lerp(this, null, t);
     return _CupertinoEdgeShadowDecoration.lerp(this, b, t);
   }

   @override
   _CupertinoEdgeShadowPainter createBoxPainter([VoidCallback onChanged]) {
     return new _CupertinoEdgeShadowPainter(this, onChanged);
   }

   @override
   bool operator ==(dynamic other) {
     if (identical(this, other))
       return true;
     if (other.runtimeType != _CupertinoEdgeShadowDecoration)
       return false;
     final _CupertinoEdgeShadowDecoration typedOther = other;
     return edgeGradient == typedOther.edgeGradient;
   }

   @override
   int get hashCode {
     return edgeGradient.hashCode;
   }

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(new DiagnosticsProperty<LinearGradient>('edgeGradient', edgeGradient));
   }
 }

 /// A [BoxPainter] used to draw the page transition shadow using gradients.
 class _CupertinoEdgeShadowPainter extends BoxPainter {
   _CupertinoEdgeShadowPainter(
     this._decoration,
     VoidCallback onChange
   ) : assert(_decoration != null),
       super(onChange);

   final _CupertinoEdgeShadowDecoration _decoration;

   @override
   void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
     final LinearGradient gradient = _decoration.edgeGradient;
     if (gradient == null)
       return;
     // The drawable space for the gradient is a rect with the same size as
     // its parent box one box width to the left of the box.
     final Rect rect =
         (offset & configuration.size).translate(-configuration.size.width, 0.0);
     final Paint paint = new Paint()
       ..shader = gradient.createShader(rect);

     canvas.drawRect(rect, paint);
   }
 }
	// Copyright 2017 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

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

	const double _kMinFlingVelocity = 1.0; // screen width per second.

	// Fractional offset from offscreen to the right to fully on screen.
	final FractionalOffsetTween _kRightMiddleTween = new FractionalOffsetTween(
	begin: FractionalOffset.topRight,
	end: FractionalOffset.topLeft,
	);

	// Fractional offset from fully on screen to 1/3 offscreen to the left.
	final FractionalOffsetTween _kMiddleLeftTween = new FractionalOffsetTween(
	begin: FractionalOffset.topLeft,
	end: const FractionalOffset(-1.0/3.0, 0.0),
	);

	// Fractional offset from offscreen below to fully on screen.
	final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
	begin: FractionalOffset.bottomLeft,
	end: FractionalOffset.topLeft,
	);

	// Custom decoration from no shadow to page shadow mimicking iOS page
	// transitions using gradients.
	final DecorationTween _kGradientShadowTween = new DecorationTween(
	begin: _CupertinoEdgeShadowDecoration.none, // No decoration initially.
	end: const _CupertinoEdgeShadowDecoration(
	edgeGradient: const LinearGradient(
	// Spans 5% of the page.
	begin: const FractionalOffset(0.95, 0.0),
	end: FractionalOffset.topRight,
	// Eyeballed gradient used to mimic a drop shadow on the left side only.
	colors: const <Color>[
	const Color(0x00000000),
	const Color(0x04000000),
	const Color(0x12000000),
	const Color(0x38000000)
	],
	stops: const <double>[0.0, 0.3, 0.6, 1.0],
	),
	),
	);

	/// A modal route that replaces the entire screen with an iOS transition.
	///
	/// The page slides in from the right and exits in reverse.
	/// The page also shifts to the left in parallax when another page enters to cover it.
	///
	/// The page slides in from the bottom and exits in reverse with no parallax effect
	/// for fullscreen dialogs.
	///
	/// By default, when a modal route is replaced by another, the previous route
	/// remains in memory. To free all the resources when this is not necessary, set
	/// [maintainState] to false.
	///
	/// See also:
	///
	/// * [MaterialPageRoute] for an adaptive [PageRoute] that uses a platform appropriate transition.
	class CupertinoPageRoute<T> extends PageRoute<T> {
	/// Creates a page route for use in an iOS designed app.
	CupertinoPageRoute({
	@required this.builder,
	RouteSettings settings: const RouteSettings(),
	this.maintainState: true,
	bool fullscreenDialog: false,
	}) : assert(builder != null),
	assert(opaque),
	super(settings: settings, fullscreenDialog: fullscreenDialog);

	/// Builds the primary contents of the route.
	final WidgetBuilder builder;

	@override
	final bool maintainState;

	@override
	Duration get transitionDuration => const Duration(milliseconds: 350);

	@override
	Color get barrierColor => null;

	@override
	bool canTransitionFrom(TransitionRoute<dynamic> nextRoute) {
	return nextRoute is CupertinoPageRoute<dynamic>;
	}

	@override
	bool canTransitionTo(TransitionRoute<dynamic> nextRoute) {
	// Don't perform outgoing animation if the next route is a fullscreen dialog.
	return nextRoute is CupertinoPageRoute && !nextRoute.fullscreenDialog;
	}

	@override
	void dispose() {
	_backGestureController?.dispose();
	// If the route is never installed (i.e. pushed into a Navigator) such as the
	// case when [MaterialPageRoute] delegates transition building to [CupertinoPageRoute],
	// don't dispose super.
	if (overlayEntries.isNotEmpty)
	super.dispose();
	}

	CupertinoBackGestureController _backGestureController;

	/// Support for dismissing this route with a horizontal swipe.
	///
	/// Swiping will be disabled if the page is a fullscreen dialog or if
	/// dismissals can be overriden because a [WillPopCallback] was
	/// defined for the route.
	///
	/// See also:
	///
	/// * [hasScopedWillPopCallback], which is true if a `willPop` callback
	/// is defined for this route.
	@override
	NavigationGestureController startPopGesture() {
	return startPopGestureForRoute(this);
	}

	/// Create a CupertinoBackGestureController using a specific PageRoute.
	///
	/// Used when [MaterialPageRoute] delegates the back gesture to [CupertinoPageRoute]
	/// since the [CupertinoPageRoute] is not actually inserted into the Navigator.
	NavigationGestureController startPopGestureForRoute(PageRoute<T> hostRoute) {
	// If attempts to dismiss this route might be vetoed such as in a page
	// with forms, then do not allow the user to dismiss the route with a swipe.
	if (hostRoute.hasScopedWillPopCallback)
	return null;
	// Fullscreen dialogs aren't dismissable by back swipe.
	if (fullscreenDialog)
	return null;
	if (hostRoute.controller.status != AnimationStatus.completed)
	return null;
	assert(_backGestureController == null);
	_backGestureController = new CupertinoBackGestureController(
	navigator: hostRoute.navigator,
	controller: hostRoute.controller,
	);

	Function handleBackGestureEnded;
	handleBackGestureEnded = (AnimationStatus status) {
	if (status == AnimationStatus.completed) {
	_backGestureController?.dispose();
	_backGestureController = null;
	hostRoute.controller.removeStatusListener(handleBackGestureEnded);
	}
	};

	hostRoute.controller.addStatusListener(handleBackGestureEnded);
	return _backGestureController;
	}

	@override
	Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
	final Widget result = builder(context);
	assert(() {
	if (result == null) {
	throw new FlutterError(
	'The builder for route "${settings.name}" returned null.\n'
	'Route builders must never return null.'
	);
	}
	return true;
	});
	return result;
	}

	@override
	Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
	if (fullscreenDialog)
	return new CupertinoFullscreenDialogTransition(
	animation: animation,
	child: child,
	);
	else
	return new CupertinoPageTransition(
	primaryRouteAnimation: animation,
	secondaryRouteAnimation: secondaryAnimation,
	child: child,
	// In the middle of a back gesture drag, let the transition be linear to match finger
	// motions.
	linearTransition: _backGestureController != null,
	);
	}

	@override
	String get debugLabel => '${super.debugLabel}(${settings.name})';
	}

	/// Provides an iOS-style page transition animation.
	///
	/// The page slides in from the right and exits in reverse. It also shifts to the left in
	/// a parallax motion when another page enters to cover it.
	class CupertinoPageTransition extends StatelessWidget {
	/// Creates an iOS-style page transition.
	///
	/// * `primaryRouteAnimation` is a linear route animation from 0.0 to 1.0
	/// when this screen is being pushed.
	/// * `secondaryRouteAnimation` is a linear route animation from 0.0 to 1.0
	/// when another screen is being pushed on top of this one.
	/// * `linearTransition` is whether to perform primary transition linearly.
	/// Used to precisely track back gesture drags.
	CupertinoPageTransition({
	Key key,
	@required Animation<double> primaryRouteAnimation,
	@required Animation<double> secondaryRouteAnimation,
	@required this.child,
	bool linearTransition,
	}) :
	_primaryPositionAnimation = linearTransition
	? _kRightMiddleTween.animate(primaryRouteAnimation)
	: _kRightMiddleTween.animate(
	new CurvedAnimation(
	parent: primaryRouteAnimation,
	curve: Curves.easeOut,
	reverseCurve: Curves.easeIn,
	)
	),
	_secondaryPositionAnimation = _kMiddleLeftTween.animate(
	new CurvedAnimation(
	parent: secondaryRouteAnimation,
	curve: Curves.easeOut,
	reverseCurve: Curves.easeIn,
	)
	),
	_primaryShadowAnimation = _kGradientShadowTween.animate(
	new CurvedAnimation(
	parent: primaryRouteAnimation,
	curve: Curves.easeOut,
	)
	),
	super(key: key);

	// When this page is coming in to cover another page.
	final Animation<FractionalOffset> _primaryPositionAnimation;
	// When this page is becoming covered by another page.
	final Animation<FractionalOffset> _secondaryPositionAnimation;
	final Animation<Decoration> _primaryShadowAnimation;

	/// The widget below this widget in the tree.
	final Widget child;

	@override
	Widget build(BuildContext context) {
	// TODO(ianh): tell the transform to be un-transformed for hit testing
	// but not while being controlled by a gesture.
	return new SlideTransition(
	position: _secondaryPositionAnimation,
	child: new SlideTransition(
	position: _primaryPositionAnimation,
	child: new DecoratedBoxTransition(
	decoration: _primaryShadowAnimation,
	child: child,
	),
	),
	);
	}
	}

	/// An iOS-style transition used for summoning fullscreen dialogs.
	///
	/// For example, used when creating a new calendar event by bringing in the next
	/// screen from the bottom.
	class CupertinoFullscreenDialogTransition extends StatelessWidget {
	/// Creates an iOS-style transition used for summoning fullscreen dialogs.
	CupertinoFullscreenDialogTransition({
	Key key,
	@required Animation<double> animation,
	@required this.child,
	}) : _positionAnimation = _kBottomUpTween.animate(
	new CurvedAnimation(
	parent: animation,
	curve: Curves.easeInOut,
	)
	),
	super(key: key);

	final Animation<FractionalOffset> _positionAnimation;

	/// The widget below this widget in the tree.
	final Widget child;

	@override
	Widget build(BuildContext context) {
	return new SlideTransition(
	position: _positionAnimation,
	child: child,
	);
	}
	}

	/// A controller for an iOS-style back gesture.
	///
	/// Uses a drag gesture to control the route's transition animation progress.
	class CupertinoBackGestureController extends NavigationGestureController {
	/// Creates a controller for an iOS-style back gesture.
	///
	/// The [navigator] and [controller] arguments must not be null.
	CupertinoBackGestureController({
	@required NavigatorState navigator,
	@required this.controller,
	}) : assert(controller != null),
	super(navigator);

	/// The animation controller that the route uses to drive its transition
	/// animation.
	final AnimationController controller;

	@override
	void dispose() {
	controller.removeStatusListener(_handleStatusChanged);
	super.dispose();
	}

	@override
	void dragUpdate(double delta) {
	// This assert can be triggered the Scaffold is reparented out of the route
	// associated with this gesture controller and continues to feed it events.
	// TODO(abarth): Change the ownership of the gesture controller so that the
	// object feeding it these events (e.g., the Scaffold) is responsible for
	// calling dispose on it as well.
	assert(controller != null);
	controller.value -= delta;
	}

	@override
	bool dragEnd(double velocity) {
	// This assert can be triggered the Scaffold is reparented out of the route
	// associated with this gesture controller and continues to feed it events.
	// TODO(abarth): Change the ownership of the gesture controller so that the
	// object feeding it these events (e.g., the Scaffold) is responsible for
	// calling dispose on it as well.
	assert(controller != null);

	if (velocity.abs() >= _kMinFlingVelocity) {
	controller.fling(velocity: -velocity);
	} else if (controller.value <= 0.5) {
	controller.fling(velocity: -1.0);
	} else {
	controller.fling(velocity: 1.0);
	}

	// Don't end the gesture until the transition completes.
	final AnimationStatus status = controller.status;
	_handleStatusChanged(status);
	controller?.addStatusListener(_handleStatusChanged);

	return (status == AnimationStatus.reverse \|\| status == AnimationStatus.dismissed);
	}

	void _handleStatusChanged(AnimationStatus status) {
	if (status == AnimationStatus.dismissed)
	navigator.pop();
	}
	}

	/// A custom [Decoration] used to paint an extra shadow on the left edge of the
	/// box it's decorating. It's like a [BoxDecoration] with only a gradient except
	/// it paints to the left of the box instead of behind the box.
	class _CupertinoEdgeShadowDecoration extends Decoration {
	const _CupertinoEdgeShadowDecoration({ this.edgeGradient });

	/// A Decoration with no decorating properties.
	static const _CupertinoEdgeShadowDecoration none =
	const _CupertinoEdgeShadowDecoration();

	/// A gradient to draw to the left of the box being decorated.
	/// FractionalOffsets are relative to the original box translated one box
	/// width to the left.
	final LinearGradient edgeGradient;

	/// Linearly interpolate between two edge shadow decorations decorations.
	///
	/// See also [Decoration.lerp].
	static _CupertinoEdgeShadowDecoration lerp(
	_CupertinoEdgeShadowDecoration a,
	_CupertinoEdgeShadowDecoration b,
	double t
	) {
	if (a == null && b == null)
	return null;
	return new _CupertinoEdgeShadowDecoration(
	edgeGradient: LinearGradient.lerp(a?.edgeGradient, b?.edgeGradient, t),
	);
	}

	@override
	_CupertinoEdgeShadowDecoration lerpFrom(Decoration a, double t) {
	if (a is! _CupertinoEdgeShadowDecoration)
	return _CupertinoEdgeShadowDecoration.lerp(null, this, t);
	return _CupertinoEdgeShadowDecoration.lerp(a, this, t);
	}

	@override
	_CupertinoEdgeShadowDecoration lerpTo(Decoration b, double t) {
	if (b is! _CupertinoEdgeShadowDecoration)
	return _CupertinoEdgeShadowDecoration.lerp(this, null, t);
	return _CupertinoEdgeShadowDecoration.lerp(this, b, t);
	}

	@override
	_CupertinoEdgeShadowPainter createBoxPainter([VoidCallback onChanged]) {
	return new _CupertinoEdgeShadowPainter(this, onChanged);
	}

	@override
	bool operator ==(dynamic other) {
	if (identical(this, other))
	return true;
	if (other.runtimeType != _CupertinoEdgeShadowDecoration)
	return false;
	final _CupertinoEdgeShadowDecoration typedOther = other;
	return edgeGradient == typedOther.edgeGradient;
	}

	@override
	int get hashCode {
	return edgeGradient.hashCode;
	}

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(new DiagnosticsProperty<LinearGradient>('edgeGradient', edgeGradient));
	}
	}

	/// A [BoxPainter] used to draw the page transition shadow using gradients.
	class _CupertinoEdgeShadowPainter extends BoxPainter {
	_CupertinoEdgeShadowPainter(
	this._decoration,
	VoidCallback onChange
	) : assert(_decoration != null),
	super(onChange);

	final _CupertinoEdgeShadowDecoration _decoration;

	@override
	void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
	final LinearGradient gradient = _decoration.edgeGradient;
	if (gradient == null)
	return;
	// The drawable space for the gradient is a rect with the same size as
	// its parent box one box width to the left of the box.
	final Rect rect =
	(offset & configuration.size).translate(-configuration.size.width, 0.0);
	final Paint paint = new Paint()
	..shader = gradient.createShader(rect);

	canvas.drawRect(rect, paint);
	}
	}