Flutter18.Animation

ex:

import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(
  home: HomePage(),
));

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage>
    with SingleTickerProviderStateMixin {
  Animation<double> _doubleAnim;//Animation是抽象类
  AnimationController _animationController;
  String _animValue;

  //四种状态
  //1 dismissed 初始状态
  //2 forward 从头到尾播放状态
  //3 reverse 从尾到头播放状态
  //4 completed 完成状态

  //初始化AnimationController
  //vsync是TickerProvider
  @override
  void initState() {
    super.initState();
    _animationController =
        AnimationController(vsync: this, duration: const Duration(seconds: 3));
    _doubleAnim = Tween(begin: 0.0, end: 3.0).animate(_animationController)//Animation用Tween来操作
      ..addListener(() {//监听
        print(_doubleAnim.value.toString());
        setState(() {
          _animValue = _doubleAnim.value.toString();
        });
      })
      ..addStatusListener((status) {//监听
        print('$status');
      });
  }

  //生命周期方法，
  @override
  void dispose() {
    super.dispose();
    _animationController.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text(_animValue ??= '0.0'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _animationController.forward(from: 0.0);
        },
        child: Icon(Icons.play_arrow),
      ),
    );
  }
}

输出：