c/c++语言开发共享详解Flutter和Dart取消Future的三种方法

var mycancelablefuture = cancelableoperation.fromfuture(    future<t> inner,     { futureor oncancel()? }  )  ​  // call the cancel() method to cancel the future  mycancelablefuture.cancel();

flutter pub add async

flutter pub get

// 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),        ),      );    }  }

future<t> timeout(     duration timelimit,    {futureor<t> ontimeout()?}  )

future<string?> _myfuture() async {      await future.delayed(const duration(seconds: 10));      return 'future completed';  }

_myfuture().timeout(        const duration(seconds: 3),        ontimeout: () =>            'the process took too much time to finish. please try again later',  );

// 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();