final RefreshCallback onRefresh;

typedef RefreshCallback = Future<void> Function();

/// The signature for a function that's called when the user has dragged a
/// [RefreshIndicator] far enough to demonstrate that they want the app to
/// refresh. The returned [Future] must complete when the refresh operation is
/// finished.
///
/// Used by [RefreshIndicator.onRefresh].
typedef RefreshCallback = Future<void> Function();
class RefreshIndicator extends StatefulWidget {
  const RefreshIndicator({
    Key? key,
    required this.child,
    this.displacement = 40.0,
    this.edgeOffset = 0.0,
    required this.onRefresh,
    this.color,
    this.backgroundColor,
    this.notificationPredicate = defaultScrollNotificationPredicate,
    this.semanticsLabel,
    this.semanticsValue,
    this.strokeWidth = 2.0,
    this.triggerMode = RefreshIndicatorTriggerMode.onEdge,
  })
  /// A function that's called when the user has dragged the refresh indicator
  /// far enough to demonstrate that they want the app to refresh. The returned
  /// [Future] must complete when the refresh operation is finished.
  final RefreshCallback onRefresh;
}

import 'package:flutter/material.dart';
var NAMES = [ '宋江', '卢俊义', '吴用', '公孙胜', '关胜',
  '林冲', '秦明', '呼延灼', '花荣', '柴进',
  '李应', '朱仝', '鲁智深', '武松', '董平',
  '张清', '杨志', '徐宁', '索超', '岱宗',
  '刘唐', '李逵', '史进', '穆弘' '雷横' ];
/// ListView 垂直列表 , RefreshIndicator  下拉刷新
void main() {
  runApp(MyApp());
}
class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  _MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    /// 材料设计主题
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          /// 标题组件
          title: Text("ListView 示例"),
        ),
        /// 列表组件
        body: RefreshIndicator(
          onRefresh: _onRefresh,
          child: ListView(
            children: _buildList(),
          ),
        ),
      ),
    );
  }
  /// 下拉刷新回调方法
  Future<Null> _onRefresh() async {
    /// 强制休眠 1 秒
    await Future.delayed(Duration(seconds: 1));
    /// 更新状态
    setState(() {
      /// 将 List 元素翻转
      NAMES = NAMES.reversed.toList();
    });
    return null;
  }
  /// 创建列表
  List<Widget> _buildList(){
    /// 遍历 NAMES 数组
    /// 调用 map 方法遍历数组元素
    return NAMES.map((name) => _generateWidget(name)).toList();
  }
  Widget _generateWidget(name){
    return Container(
      height: 80,
      margin: EdgeInsets.only(bottom: 5),
      alignment: Alignment.center,
      decoration: BoxDecoration(color: Colors.black),
      child: Text(
        name,
        style: TextStyle(
            color: Colors.yellowAccent,
            fontSize: 20
        ),
      ),
    );
  }
}