RefreshIndicator 刷新指示器

内部嵌套可滑动区域，下滑时会显示刷新图标，松手后可以执行指定的异步方法。可指定颜色、到顶端距离等属性。

RefreshIndicator基本使用

   <br />【child】 : 孩子(可滑动)   【Widget】<br />【displacement】 : 指示器悬浮高度   【double】<br />【color】 : 指示器颜色   【Color】,<br />![93.gif](https://cdn.nlark.com/yuque/0/2020/gif/326147/1589446240668-1c606eee-f143-4142-8d58-59ccdcd28279.gif#align=left&display=inline&height=394&margin=%5Bobject%20Object%5D&name=93.gif&originHeight=394&originWidth=404&size=166714&status=done&style=none&width=404)

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

class _CustomRefreshIndicatorState extends State<CustomRefreshIndicator> {
  int _count = 0;

  @override
  Widget build(BuildContext context) {

    return Container(
      height: 200,
      width: 200,
      child: RefreshIndicator(
        onRefresh: _increment,
        displacement: 20,
        color: Colors.orange,
        backgroundColor: Colors.white,
        child: SingleChildScrollView(
          child: Container(
            alignment: Alignment.center,
            width: 200,
            height: 300,
            color: Colors.blue,
            child: Text('$_count',style: TextStyle(color: Colors.white,fontSize: 40)),
          ),
        ),
      ),
    );
  }

  Future<void> _increment() async {
    await Future.delayed(Duration(seconds: 2));
    setState(() {
      _count++;
    });
  }
}