page.dart

// 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 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;
  }
}

/// A [BoxPainter] used to draw the page transition shadow using gradients.
class _CupertinoEdgeShadowPainter extends BoxPainter {
  _CupertinoEdgeShadowPainter(
    @required 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);
  }
}

/// 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,
  }) : super(navigator) {
    assert(controller != null);
  }

  /// 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();
  }
}