Opacity 透明化

可容纳一个子组件，将其透明度变为opacity值, opacity在0~1之间。

相关组件

FadeTransition AnimatedOpacity

Opacity基本使用

   <br />【child】 : 孩子组件   【Widget】<br />【opacity】 : 透明度0~1   【double】<br />![187.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589507779204-a854a641-5f3e-424a-90fe-a937a2d9f8cc.gif#align=left&display=inline&height=217&margin=%5Bobject%20Object%5D&name=187.gif&originHeight=217&originWidth=397&size=401945&status=done&style=none&width=397)

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

class _CustomOpacityState extends State<CustomOpacity> {
  var _opacity = 0.2;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[_buildSlider(), _buildOpacity()],
    );
  }
  // 创建Opacity
  Widget _buildOpacity() => Opacity(
        opacity: _opacity,
        child: Image.asset(// 图片
          'assets/images/icon_head.png',
          width: 100,
        ),
      );
  Widget _buildSlider() => Slider(
      divisions: 20,
      label: _opacity.toString(),
      value: _opacity,
      onChanged: (v) => setState(() => _opacity = v));
}