var myCancelableFuture = CancelableOperation.fromFuture(
  Future<T> inner, 
  { FutureOr onCancel()? }
)
?
// call the cancel() method to cancel the future
myCancelableFuture.cancel();

為了更清楚，請(qǐng)參閱下面的實(shí)際示例。

完整示例

應(yīng)用預(yù)覽

我們要構(gòu)建的應(yīng)用程序有一個(gè)浮動(dòng)按鈕。按下此按鈕時(shí)，將開(kāi)始異步操作（這需要 5 秒才能完成）。按鈕的背景從靛藍(lán)變?yōu)榧t色，其標(biāo)簽從“開(kāi)始”變?yōu)?ldquo;取消”，現(xiàn)在您可以使用它來(lái)取消Future。

如果您在Future完成前 5 秒內(nèi)點(diǎn)擊取消按鈕，屏幕將顯示“Future已被取消”。
如果您什么都不做，則 5 秒后屏幕將顯示“Future completed”。

一個(gè)演示價(jià)值超過(guò)一千字：

代碼

1.通過(guò)執(zhí)行以下操作安裝異步包：

flutter pub add async

然后運(yùn)行：

flutter pub get

2.main.dart 中的完整源代碼（附解釋）：

// main.dart
import 'package:flutter/material.dart';
import 'package:async/async.dart';
?
void main() {
  runApp(const MyApp());
}
?
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        // Remove the debug banner
        debugShowCheckedModeBanner: false,
        title: '大前端之旅',
        theme: ThemeData(
          primarySwatch: Colors.indigo,
        ),
        home: const HomePage());
  }
}
?
class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);
?
  @override
  _HomePageState createState() => _HomePageState();
}
?
class _HomePageState extends State<HomePage> {
  // this future will return some text once it completes
  Future<String?> _myFuture() async {
    await Future.delayed(const Duration(seconds: 5));
    return 'Future completed';
  }
?
  // keep a reference to CancelableOperation
  CancelableOperation? _myCancelableFuture;
?
  // This is the result returned by the future
  String? _text;
?
  // Help you know whether the app is "loading" or not
  bool _isLoading = false;
?
  // This function is called when the "start" button is pressed
  void _getData() async {
    setState(() {
      _isLoading = true;
    });
?
    _myCancelableFuture = CancelableOperation.fromFuture(
      _myFuture(),
      onCancel: () => 'Future has been canceld',
    );
    final value = await _myCancelableFuture?.value;
?
    // update the UI
    setState(() {
      _text = value;
      _isLoading = false;
    });
  }
?
  // this function is called when the "cancel" button is tapped
  void _cancelFuture() async {
    final result = await _myCancelableFuture?.cancel();
    setState(() {
      _text = result;
      _isLoading = false;
    });
  }
?
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('大前端之旅')),
      body: Center(
        child: _isLoading
            ? const CircularProgressIndicator()
            : Text(
                _text ?? 'Press Start Button',
                style: const TextStyle(fontSize: 28),
              ),
      ),
      // This button is used to trigger _getDate() and _cancelFuture() functions
      // the function is called depends on the _isLoading variable
      floatingActionButton: ElevatedButton(
        onPressed: () => _isLoading ? _cancelFuture() : _getData(),
        child: Text(_isLoading ? 'Cancel' : 'Start'),
        style: ElevatedButton.styleFrom(
            padding: const EdgeInsets.symmetric(vertical: 20, horizontal: 30),
            primary: _isLoading ? Colors.red : Colors.indigo),
      ),
    );
  }
}

使用 timeout() 方法

這種方法既快速又簡(jiǎn)單。但是，它不是很靈活。

使用timeout()方法，您可以限制Future的時(shí)間（例如 3 秒）。如果 future 及時(shí)完成，它的值將被返回。另一方面，如果Future超過(guò)限制時(shí)間，將執(zhí)行onTimeout函數(shù)：

Future<T> timeout(
   Duration timeLimit,
  {FutureOr<T> onTimeout()?}
)

快速示例

創(chuàng)建一個(gè)虛擬的Future：

Future<String?> _myFuture() async {
    await Future.delayed(const Duration(seconds: 10));
    return 'Future completed';
}

設(shè)置超時(shí) 3 秒：

_myFuture().timeout(
      const Duration(seconds: 3),
      onTimeout: () =>
          'The process took too much time to finish. Please try again later',
);

將Future轉(zhuǎn)換為流

您可以使用 Future 類的asStream()方法來(lái)創(chuàng)建一個(gè)包含原始Future結(jié)果的流?，F(xiàn)在您可以取消對(duì)該流的訂閱。

快速示例

// don't forget to import this
import 'dart:async';
?
// Create a demo future
Future<dynamic> _loadData() async {
    await Future.delayed(const Duration(seconds: 10));
    return 'Some Data';
}
?
// a reference to the stream subscription
// so that we can call _sub.cancel() later
StreamSubscription<dynamic>? _sub;
?
// convert the future to a stream
_sub = _loadData().asStream().listen((data) {
    // do something with "data"
    print(data);
 });
?
// cancel the stream subscription
_sub.cancel();

請(qǐng)注意，這個(gè)快速示例僅簡(jiǎn)要描述了事物的工作原理。您必須對(duì)其進(jìn)行修改以使其可在現(xiàn)有項(xiàng)目中運(yùn)行。