is there any way to cancel a dart Future?

问题

In a Dart UI, I have a button [submit] to launch a long async request. The [submit] handler returns a Future. Next, the button [submit] is replaced by a button [cancel] to allow the cancellation of the whole operation. In the [cancel] handler, I would like to cancel the long operation. How can I cancel the Future returned by the submit handler? I found no method to do that.

回答1:

As far as I know, there isn't a way to cancel a Future. But there is a way to cancel a Stream subscription, and maybe that can help you.

Calling onSubmit on a button returns a StreamSubscription object. You can explicitly store that object and then call cancel() on it to cancel the stream subscription:

StreamSubscription subscription = someDOMElement.onSubmit.listen((data) {

   // you code here

   if (someCondition == true) {
     subscription.cancel();
   }
});

Later, as a response to some user action, perhaps, you can cancel the subscription:

回答2:

You can use CancelableOperation or CancelableCompleter to cancel a future. See below the 2 versions:

Solution 1: CancelableOperation (included in a test so you can try it yourself):

• cancel a future

test("CancelableOperation with future", () async {

  var cancellableOperation = CancelableOperation.fromFuture(
    Future.value('future result'),
    onCancel: () => {debugPrint('onCancel')},
  );

// cancellableOperation.cancel();  // uncomment this to test cancellation

  cancellableOperation.value.then((value) => {
    debugPrint('then: $value'),
  });
  cancellableOperation.value.whenComplete(() => {
    debugPrint('onDone'),
  });
});

• cancel a stream

test("CancelableOperation with stream", () async {

  var cancellableOperation = CancelableOperation.fromFuture(
    Future.value('future result'),
    onCancel: () => {debugPrint('onCancel')},
  );

  //  cancellableOperation.cancel();  // uncomment this to test cancellation

  cancellableOperation.asStream().listen(
        (value) => { debugPrint('value: $value') },
    onDone: () => { debugPrint('onDone') },
  );
});

Both above tests will output:

then: future result
onDone

Now if we uncomment the cancellableOperation.cancel(); then both above tests will output:

onCancel

Solution 2: CancelableCompleter (if you need more control)

test("CancelableCompleter is cancelled", () async {

  CancelableCompleter completer = CancelableCompleter(onCancel: () {
    print('onCancel');
  });

  // completer.operation.cancel();  // uncomment this to test cancellation

  completer.complete(Future.value('future result'));
  print('isCanceled: ${completer.isCanceled}');
  print('isCompleted: ${completer.isCompleted}');
  completer.operation.value.then((value) => {
    print('then: $value'),
  });
  completer.operation.value.whenComplete(() => {
    print('onDone'),
  });
});

Output:

isCanceled: false
isCompleted: true
then: future result
onDone

Now if we uncomment the cancellableOperation.cancel(); we get output:

onCancel
isCanceled: true
isCompleted: true

Be aware that if you use await cancellableOperation.value or await completer.operation then the future will never return a result and it will await indefinitely if the operation was cancelled. This is because await cancellableOperation.value is the same as writing cancellableOperation.value.then(...) but then() will never be called if the operation was cancelled.

Remember to add async Dart package.

Code gist

回答3:

One way I accomplished to 'cancel' a scheduled execution was using a Timer. In this case I was actually postponing it. :)

Timer _runJustOnceAtTheEnd;

void runMultipleTimes() {
  _runJustOnceAtTheEnd?.cancel();
  _runJustOnceAtTheEnd = null;

  // do your processing

  _runJustOnceAtTheEnd = Timer(Duration(seconds: 1), onceAtTheEndOfTheBatch);
}

void onceAtTheEndOfTheBatch() {
  print("just once at the end of a batch!");
}


runMultipleTimes();
runMultipleTimes();
runMultipleTimes();
runMultipleTimes();

// will print 'just once at the end of a batch' one second after last execution

The runMultipleTimes() method will be called multiple times in sequence, but only after 1 second of a batch the onceAtTheEndOfTheBatch will be executed.

回答4:

For those, who are trying to achieve this in Flutter, here is the simple example for the same.

class MyPage extends StatelessWidget {
  final CancelableCompleter<bool> _completer = CancelableCompleter(onCancel: () => false);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Future")),
      body: Column(
        children: <Widget>[
          RaisedButton(
            child: Text("Submit"),
            onPressed: () async {
              // it is true only if the future got completed
              bool _isFutureCompleted = await _submit();
            },
          ),
          RaisedButton(child: Text("Cancel"), onPressed: _cancel),
        ],
      ),
    );
  }

  Future<bool> _submit() async {
    _completer.complete(Future.value(_solve()));
    return _completer.operation.value;
  }

  // This is just a simple method that will finish the future in 5 seconds
  Future<bool> _solve() async {
    return await Future.delayed(Duration(seconds: 5), () => true);
  }

  void _cancel() async {
    var value = await _completer.operation.cancel();
    // if we stopped the future, we get false
    assert(value == false);
  }
}

回答5:

my 2 cents worth...

class CancelableFuture {
  bool cancelled = false;
  CancelableFuture(Duration duration, void Function() callback) {
    Future<void>.delayed(duration, () {
      if (!cancelled) {
        callback();
      }
    });
  }

  void cancel() {
    cancelled = true;
  }
}

回答6:

Change the future's task from 'do something' to 'do something unless it has been cancelled'. An obvious way to implement this would be to set a boolean flag and check it in the future's closure before embarking on processing, and perhaps at several points during the processing.

Also, this seems to be a bit of a hack, but setting the future's timeout to zero would appear to effectively cancel the future.

来源：https://stackoverflow.com/questions/17552757/is-there-any-way-to-cancel-a-dart-future