Flutter widget动画效果之CurvedAnimati

在Android中，我们可以用XML来指定动画样式，或者调用View的animate()方法。在Flutter中，widget的动画效果利用animated动画化组件的动画库来实现。
Flutter中，使用AnimationController来控制动画暂停、调整进度、停止和倒退。AnimationController继承自Animation<double>。
在vsync信号发出时，需要一个触发器来通知它，从而在每帧中产生一个0到1的线性差值。
一个Controller可以与多个动画进行关联。

当整个屏幕刷新完毕，即一个垂直刷新周期完成，会有短暂的空白期，此时发出 VSync 信号。

动画样式示例 - CurvedAnimation与FadeTransition

用CurvedAnimation实现一个动画效果。
给widget指定动画效果，用Controller来控制动画的播放。
下面这个例子是关于FadeTransition的。用一个FloatingActionButton来控制动画播放。

import 'package:flutter/material.dart';

void main() => runApp(new AnimationApp());

class AnimationApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Animation',
      theme: new ThemeData(primarySwatch: Colors.blue),
      home: new HomePage(
        title: '动画示例',
      ),
    );
  }
}

class HomePage extends StatefulWidget {
  final String title;

  HomePage({Key key, this.title}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new _HomePageState();
}

// 需要Ticker
class _HomePageState extends State<HomePage> with TickerProviderStateMixin {
  AnimationController controller;
  CurvedAnimation curveEaseIn, bounceIn, linear, decelerate;
  bool _animationFlag = true;

  @override
  void initState() {
    super.initState();
    controller = new AnimationController(
        duration: new Duration(milliseconds: 2000), vsync: this);
    curveEaseIn = new CurvedAnimation(parent: controller, curve: Curves.easeIn);
    bounceIn = new CurvedAnimation(parent: controller, curve: Curves.bounceIn);
    linear = new CurvedAnimation(parent: controller, curve: Curves.linear);
    decelerate =
        new CurvedAnimation(parent: controller, curve: Curves.decelerate);
  }

  @override
  Widget build(BuildContext context) {
    buildItemWidget(
        CurvedAnimation animation, MaterialColor color, String itemText) {
      return new Column(
        children: <Widget>[
          new FadeTransition(
            opacity: animation,
            child: new FlutterLogo(
              size: 100.0,
              colors: color,
            ),
          ),
          new Text(itemText)
        ],
      );
    }

    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          if (_animationFlag) {
            controller.forward();
          } else {
            controller.reverse();
          }
          _animationFlag = !_animationFlag;
        },
        child: new Icon(Icons.star),
      ),
      body: new Center(
        child: new GridView.extent(
          maxCrossAxisExtent: 170.0,
          padding: const EdgeInsets.all(12.0),
          children: <Widget>[
            buildItemWidget(curveEaseIn, Colors.blue, 'Curves.easeIn'),
            buildItemWidget(bounceIn, Colors.amber, 'Curves.bounceIn'),
            buildItemWidget(linear, Colors.red, 'Curves.linear'),
            buildItemWidget(decelerate, Colors.indigo, 'Curves.decelerate'),
            buildItemWidget(
                new CurvedAnimation(
                    parent: controller, curve: Curves.elasticIn),
                Colors.pink,
                'Curves.elasticIn'),
            buildItemWidget(
                new CurvedAnimation(parent: controller, curve: Curves.ease),
                Colors.purple,
                'Curves.ease'),
          ],
        ),
      ),
    );
  }
}