add carousel, card_swipe, and focus_image samples (#119)

* add carousel, card_swipe, and focus_image samples

* fix image assets

* fix more asset images

* add repeating animation

* fix import

* add copyright headers

* remove Center widget

* imageAssetName

* use ClipRect, refactor _SwipeableCardState

* use offset.zero

* add comments

* remove reference to coverflow package

* change spread to toList()

* refactor coverflow sample

don't set width and height
use const
data -> images

* return widget directly, fix formatting

* inline transitionsBuilder

* image -> imageAssetName

* _rectTween() => _createTween()

* _expandToPageRoute -> _createRoute

* move non-updating widgets out of AnimatedBuilder

* code review updates to animations demos

animations/lib/src/misc/card_swipe.dart

@@ -0,0 +1,188 @@
+// 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 'dart:ui';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/physics.dart';
+
+class CardSwipeDemo extends StatefulWidget {
+  static String routeName = '/misc/card_swipe';
+
+  @override
+  _CardSwipeDemoState createState() => _CardSwipeDemoState();
+}
+
+class _CardSwipeDemoState extends State<CardSwipeDemo> {
+  List<String> fileNames;
+
+  void initState() {
+    super.initState();
+    _resetCards();
+  }
+
+  void _resetCards() {
+    fileNames = [
+      'assets/eat_cape_town_sm.jpg',
+      'assets/eat_new_orleans_sm.jpg',
+      'assets/eat_sydney_sm.jpg',
+    ];
+  }
+
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Card Swipe'),
+      ),
+      body: Padding(
+        padding: const EdgeInsets.all(12.0),
+        child: Center(
+          child: Column(
+            children: <Widget>[
+              Expanded(
+                child: ClipRect(
+                  child: Stack(
+                    overflow: Overflow.clip,
+                    children: <Widget>[
+                      for (final fileName in fileNames)
+                        SwipeableCard(
+                          imageAssetName: fileName,
+                          onSwiped: () {
+                            setState(() {
+                              fileNames.remove(fileName);
+                            });
+                          },
+                        ),
+                    ],
+                  ),
+                ),
+              ),
+              RaisedButton(
+                child: const Text('Refill'),
+                onPressed: () {
+                  setState(() {
+                    _resetCards();
+                  });
+                },
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class Card extends StatelessWidget {
+  final String imageAssetName;
+
+  Card(this.imageAssetName);
+
+  Widget build(BuildContext context) {
+    return AspectRatio(
+      aspectRatio: 3 / 5,
+      child: DecoratedBox(
+        decoration: BoxDecoration(
+          borderRadius: BorderRadius.circular(20.0),
+          image: DecorationImage(
+            image: AssetImage(imageAssetName),
+            fit: BoxFit.cover,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class SwipeableCard extends StatefulWidget {
+  final String imageAssetName;
+  final VoidCallback onSwiped;
+
+  SwipeableCard({
+    this.onSwiped,
+    this.imageAssetName,
+  });
+
+  _SwipeableCardState createState() => _SwipeableCardState();
+}
+
+class _SwipeableCardState extends State<SwipeableCard>
+    with SingleTickerProviderStateMixin {
+  AnimationController _controller;
+  Animation<Offset> _animation;
+  double _dragStartX;
+  bool _isSwipingLeft = false;
+
+  void initState() {
+    super.initState();
+    _controller = AnimationController.unbounded(vsync: this);
+    _animation = _controller.drive(Tween<Offset>(
+      begin: Offset.zero,
+      end: Offset(1, 0),
+    ));
+  }
+
+  Widget build(BuildContext context) {
+    return SlideTransition(
+      position: _animation,
+      child: GestureDetector(
+        onHorizontalDragStart: _dragStart,
+        onHorizontalDragUpdate: _dragUpdate,
+        onHorizontalDragEnd: _dragEnd,
+        child: Card(widget.imageAssetName),
+      ),
+    );
+  }
+
+  /// Sets the starting position the user dragged from.
+  void _dragStart(DragStartDetails details) {
+    _dragStartX = details.localPosition.dx;
+  }
+
+  /// Changes the animation to animate to the left or right depending on the
+  /// swipe, and sets the AnimationController's value to the swiped amount.
+  void _dragUpdate(DragUpdateDetails details) {
+    var isSwipingLeft = (details.localPosition.dx - _dragStartX) < 0;
+    if (isSwipingLeft != _isSwipingLeft) {
+      _isSwipingLeft = isSwipingLeft;
+      _updateAnimation(details.localPosition.dx);
+    }
+
+    setState(() {
+      // Calculate the amount dragged in unit coordinates (between 0 and 1)
+      // using this widgets width.
+      _controller.value =
+          (details.localPosition.dx - _dragStartX).abs() / context.size.width;
+    });
+  }
+
+  /// Runs the fling / spring animation using the final velocity of the drag
+  /// gesture.
+  void _dragEnd(DragEndDetails details) {
+    var velocity =
+        (details.velocity.pixelsPerSecond.dx / context.size.width).abs();
+    _animate(velocity: velocity);
+  }
+
+  void _updateAnimation(double dragPosition) {
+    _animation = _controller.drive(Tween<Offset>(
+      begin: Offset.zero,
+      end: _isSwipingLeft ? Offset(-1, 0) : Offset(1, 0),
+    ));
+  }
+
+  void _animate({double velocity = 0}) {
+    var description = SpringDescription(mass: 50, stiffness: 1, damping: 1);
+    var simulation =
+        SpringSimulation(description, _controller.value, 1, velocity);
+    _controller.animateWith(simulation).then((_) {
+      widget.onSwiped();
+    });
+  }
+
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+}