Add scroll physics demo (#123)

* add spring physics card demo

* fix button colors in animations project

* code review updates

* refactor method into runAnimation

* combine updateAnimation and runAnimation

* update comment

animations/lib/main.dart

@@ -14,6 +14,7 @@ import 'src/misc/card_swipe.dart';
 import 'src/misc/carousel.dart';
 import 'src/misc/expand_card.dart';
 import 'src/misc/focus_image.dart';
+import 'src/misc/physics_card_drag.dart';
 import 'src/misc/repeating_animation.dart';
 
 void main() => runApp(AnimationSamples());
@@ -48,6 +49,8 @@ final miscDemos = [
   Demo('Card Swipe', CardSwipeDemo.routeName, (context) => CardSwipeDemo()),
   Demo('Repeating Animation', RepeatingAnimationDemo.routeName,
       (context) => RepeatingAnimationDemo()),
+  Demo('Spring Physics', PhysicsCardDragDemo.routeName,
+      (context) => PhysicsCardDragDemo()),
 ];
 
 final basicDemoRoutes =

animations/lib/src/basics/02_page_route_builder.dart

@@ -9,7 +9,6 @@ class PageRouteBuilderDemo extends StatelessWidget {
       body: Center(
         child: RaisedButton(
           child: Text('Go!'),
-          color: Theme.of(context).primaryColor,
           onPressed: () {
             Navigator.of(context).push(_createRoute());
           },

animations/lib/src/basics/05_custom_tween.dart

@@ -43,12 +43,13 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
     return Scaffold(
       appBar: AppBar(
         actions: [
-          RaisedButton(
+          MaterialButton(
             child: Text(
               controller.status == AnimationStatus.completed
                   ? 'Delete Essay'
                   : 'Write Essay',
             ),
+            textColor: Colors.white,
             onPressed: () {
               if (controller.status == AnimationStatus.completed) {
                 controller.reverse();

animations/lib/src/misc/physics_card_drag.dart

@@ -0,0 +1,117 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
+import 'package:flutter/physics.dart';
+
+class PhysicsCardDragDemo extends StatelessWidget {
+  static const String routeName = '/misc/physics_card';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: DraggableCard(
+        child: FlutterLogo(
+          size: 128,
+        ),
+      ),
+    );
+  }
+}
+
+/// A draggable card that moves back to [Alignment.center] when it's
+/// released.
+class DraggableCard extends StatefulWidget {
+  final Widget child;
+  DraggableCard({this.child});
+
+  @override
+  _DraggableCardState createState() => _DraggableCardState();
+}
+
+class _DraggableCardState extends State<DraggableCard>
+    with SingleTickerProviderStateMixin {
+  AnimationController _controller;
+
+  /// The alignment of the card as it is dragged or being animated.
+  ///
+  /// While the card is being dragged, this value is set to the values computed
+  /// in the GestureDetector onPanUpdate callback. If the animation is running,
+  /// this value is set to the value of the [_animation].
+  Alignment _dragAlignment = Alignment.center;
+
+  Animation<Alignment> _animation;
+
+  /// Calculates and runs a [SpringSimulation]
+  void _runAnimation(Offset pixelsPerSecond, Size size) {
+    _animation = _controller.drive(
+      AlignmentTween(
+        begin: _dragAlignment,
+        end: Alignment.center,
+      ),
+    );
+    // Calculate the velocity relative to the unit interval, [0,1],
+    // used by the animation controller.
+    final unitsPerSecondX = pixelsPerSecond.dx / size.width;
+    final unitsPerSecondY = pixelsPerSecond.dy / size.height;
+    final unitsPerSecond = Offset(unitsPerSecondX, unitsPerSecondY);
+    final unitVelocity = unitsPerSecond.distance;
+
+    const spring = SpringDescription(
+      mass: 30,
+      stiffness: 1,
+      damping: 1,
+    );
+
+    final simulation = SpringSimulation(spring, 0, 1, -unitVelocity);
+
+    _controller.animateWith(simulation);
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = AnimationController(vsync: this);
+
+    _controller.addListener(() {
+      setState(() {
+        _dragAlignment = _animation.value;
+      });
+    });
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final size = MediaQuery.of(context).size;
+    return GestureDetector(
+      onPanDown: (details) {
+        _controller.stop();
+      },
+      onPanUpdate: (details) {
+        setState(() {
+          _dragAlignment += Alignment(
+            details.delta.dx / (size.width / 2),
+            details.delta.dy / (size.height / 2),
+          );
+        });
+      },
+      onPanEnd: (details) {
+        _runAnimation(details.velocity.pixelsPerSecond, size);
+      },
+      child: Align(
+        alignment: _dragAlignment,
+        child: Card(
+          child: widget.child,
+        ),
+      ),
+    );
+  }
+}