flutter:第一条返回语句后的取消函数

import 'package:flutter/material.dart';
import 'package:async/async.dart'; // 引入async库

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Cancel Example')),
        body: CancelExample(),
      ),
    );
  }
}

class CancelExample extends StatefulWidget {
  @override
  _CancelExampleState createState() => _CancelExampleState();
}

class _CancelExampleState extends State<CancelExample> {
  late final CancelableOperation<void> _operation;

  @override
  void initState() {
    super.initState();
    _operation = CancelableOperation.fromFuture(
      fetchData(),
      onCancel: () {
        print('Operation was canceled');
      },
    );
  }

  Future<void> fetchData() async {
    try {
      await Future.delayed(Duration(seconds: 5)); // 模拟长时间运行的任务
      print('Data fetched');
    } catch (e) {
      if (_operation.isCanceled) {
        print('Fetch data was canceled');
      } else {
        print('Error: $e');
      }
    }
  }

  void _cancelOperation() {
    _operation.cancel();
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          ElevatedButton(
            onPressed: _cancelOperation,
            child: Text('Cancel Operation'),
          ),
        ],
      ),
    );
  }

  @override
  void dispose() {
    _operation.cancel();
    super.dispose();
  }
}