FadeTransition 透明变换

可容纳一个子组件，并使其进行透明度渐变动画，需要提供动画器opacity。

相关组件

Opacity AnimatedOpacity

FadeTransition基本使用

   <br />【child】 : 孩子组件   【Widget】<br />【opacity】 : 动画   【Animation<double>】<br />![193.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589508542272-9c7844a9-4a00-46b1-9803-5878d23c5c80.gif#align=left&display=inline&height=120&margin=%5Bobject%20Object%5D&name=193.gif&originHeight=120&originWidth=397&size=71216&status=done&style=none&width=397)

import 'package:flutter/material.dart';
class CustomFadeTransition extends StatefulWidget {
  @override
  _CustomFadeTransitionState createState() => _CustomFadeTransitionState();
}

class _CustomFadeTransitionState extends State<CustomFadeTransition>
    with SingleTickerProviderStateMixin {
  AnimationController _ctrl;

  @override
  void initState() {
    _ctrl = AnimationController(vsync: this, duration: Duration(seconds: 2));
    _ctrl.forward();
    super.initState();
  }

  @override
  void dispose() {
    _ctrl.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        setState(() {
          _ctrl.reset();
          _ctrl.forward();
        });
      },
      child: Container(
        color: Colors.grey.withAlpha(22),
        width: 100,
        height: 100,
        child: FadeTransition(
          opacity: CurvedAnimation(parent: _ctrl, curve: Curves.linear),
          child: Icon(Icons.android, color: Colors.green, size: 60),
        ),
      ),
    );
  }
}