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Flutter Dart取消Future

大前端之旅 人气:0

使用异步包(推荐)

async包由 Dart 编程语言的作者开发和发布。它提供了dart:async风格的实用程序来增强异步计算。可以帮助我们取消Future的是CancelableOperation类:

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

为了更清楚,请参阅下面的实际示例。

完整示例

应用预览

我们要构建的应用程序有一个浮动按钮。按下此按钮时,将开始异步操作(这需要 5 秒才能完成)。按钮的背景从靛蓝变为红色,其标签从“开始”变为“取消”,现在您可以使用它来取消Future。

一个演示价值超过一千字:

代码

1.通过执行以下操作安装异步包:

flutter pub add async

然后运行:

flutter pub get

2.main.dart 中的完整源代码(附解释):

// 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

这种方法既快速又简单。但是,它不是很灵活。

使用timeout()方法,您可以限制Future的时间(例如 3 秒)。如果 future 及时完成,它的值将被返回。另一方面,如果Future超过限制时间,将执行onTimeout函数:

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

快速示例

创建一个虚拟的Future:

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

设置超时 3 秒:

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

将Future转换为流

您可以使用 Future 类的asStream()方法来创建一个包含原始Future结果的流。现在您可以取消对该流的订阅。

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();

请注意,这个快速示例仅简要描述了事物的工作原理。您必须对其进行修改以使其可在现有项目中运行。

结论

你已经学会了不止一种方法来取消 Flutter 中的Future。从其中选择一个以在您的应用程序中实现,以使其在处理异步任务时更加健壮和吸引人。

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