structured_async 0.3.2

Structured asynchronous programming in Dart.

structured_async

Structured concurrency for the Dart Programming Language.

User Guide

CancellableFuture

The basic construct in structured_async is CancellableFuture. It looks like a normal Dart Future but with the following differences:

• it has a cancel method.
• if any unhandled error occurs within it:
  • all asynchronous computations started within it are stopped.
  • the error is propagated to the caller even if the Future it comes from was not await-ed.
• when it completes, anything¹ it started but not waited for is cancelled.

This example shows the basic difference:

_runForever() async {
  while (true) {
    print('Tick');
    await Future.delayed(Duration(milliseconds: 500));
  }
}

Future<void> futureNeverStops() async {
  await Future(() {
    _runForever(); // no await here
    return Future.delayed(Duration(milliseconds: 1200));
  });
  print('Stopped');
}

Future<void> cancellableFutureStopsWhenItReturns() async {
  await CancellableFuture(() { // <--- this is the only difference!
    _runForever(); // no await here
    return Future.delayed(Duration(milliseconds: 1200));
  });
  print('Stopped');
}

If you run futureNeverStops, you'll see this:

Tick
Tick
Tick
Stopped
Tick
Tick
Tick
...

The program never ends because Dart's Futures that are not await'ed for don't stop after their _parent Future completes.

However, running cancellableFutureStopsWhenItReturns, you should see:

Tick
Tick
Tick
Tick
Stopped

And the program dies. When a CancellableFuture completes, most asynchronous computation within it are terminated (see also Limitations). Any pending Futures and Timers are completed immediately, and attempting to create any new Future and Timer within the CancellableFuture computation will fail from that point on.

To make it clearer what is going on, run example 2 (shown above) with the time option, which will print the time since the program started (in ms) when each print is called:

dart example/readme_examples.dart 2 time

Result:

Time  | Message
------+--------
26    | Tick
542   | Tick
1046  | Tick
1243  | Tick
1249  | Stopped

Notice how the Tick messages are initially printed every 500ms, as the code intended, but once the CancellableFuture completes, at t=1200 approximately, the delayed Future in _runForever is awakened early, the loop continues by again printing Tick (hence the last Tick message), and when it tries to await again on a new delayed Future, its computation is aborted with a FutureCancelled Exception (because the CancellableFuture ended, any pending computation is thus automatically cancelled) which in this example happens to be silently ignored.

You can register a callback to receive uncaught errors when you create a CancellableFuture (notice that uncaught errors may be received just after the CancellableFuture returns, but are otherwise unobservable):

await CancellableFuture(() {
  ...
}, uncaughtErrorHandler: (e, st) {
  print('Error: $e\n$st');
});

Running the example again, the result would be:

Time  | Message
------+--------
35    | Tick
550   | Tick
1051  | Tick
1252  | Tick
1257  | Error: FutureCancelled
#0      StructuredAsyncZoneState.remember (package:structured_async/src/_state.dart:47:7)
#1      _createZoneSpec.<anonymous closure> (package:structured_async/src/core.dart:164:20)
#2      _CustomZone.createTimer (dart:async/zone.dart:1388:19)
#3      new Timer (dart:async/timer.dart:54:10)
#4      new Future.delayed (dart:async/future.dart:388:9)
#5      _runForever (file:///projects/structured_async/example/readme_examples.dart:59:18)
<asynchronous suspension>

1258  | Stopped

Cancelling computations

To cancel a CancellableFuture, you've guessed it: call the cancel() method.

From within the CancellableFuture computation itself, throwing an error has a similar effect as being cancelled from the outside, i.e. stop everything and complete with an error. However, to be more explicit, you can either build the Future with CancellableFuture.ctx, then call cancel() on the provided context object (after which no more async computations may succeed within the same computation), or more simply, use throw const FutureCancelled().

Example:

Future<void> explicitCancel() async {
  Future<void> printForever(String message) async {
    while(true) {
      await Future.delayed(Duration(seconds: 1), () => print(message));
    }
  }
  final future = CancellableFuture(() async {
    printForever('Tic'); // no await
    await Future.delayed(Duration(milliseconds: 500));
    await printForever('Tac');
  });

  // cancel after 3 seconds
  Future.delayed(Duration(seconds: 3), future.cancel);

  try {
    await future;
  } on FutureCancelled {
    print('Future was cancelled');
  }
}

Result:

Tic
Tac
Tic
Tac
Tic
Future was cancelled
Tac

Error Propagation

With CancellableFuture, any error that occurs within its computation, even on non-awaited Futures it has started, propagate to the caller as long as the CancellableFuture has not completed yet.

To illustrate the difference with Future, let's look at what happens when we run this:

_throw() async {
  throw 'not great';
}

Future<void> futureWillNotPropagateThisError() async {
  try {
    await Future(() {
      _throw();
      return Future.delayed(Duration(milliseconds: 100));
    });
  } catch (e) {
    print('ERROR: $e');
  } finally {
    print('Stopped');
  }
}

Result:

Unhandled exception:
not great
#0      _throw
...

The program crashes without running the catch block.

Replacing Future with CancellableFuture, this is the result:

ERROR: not great
Stopped

The error is handled correctly and the program terminates successfully.

If you want to explicitly allow a computation to fail, use Dart's runZoneGuarded.

Periodic Timers

Any periodic timers started within a CancellableFuture will be cancelled when the CancellableFuture itself completes, successfully or not, including when it gets cancelled.

This example shows how that works:

Future<void> periodicTimerIsCancelledOnCompletion() async {
  final task = CancellableFuture(() async {
    // fire and forget a periodic timer
    Timer.periodic(Duration(milliseconds: 500), (_) => print('Tick'));
    await Future.delayed(Duration(milliseconds: 1200));
    return 10;
  });
  print(await task);
}

We fire and forget a periodic timer, wait a second or so, then finish the CancellableFuture with the value 10.

Outside the CancellableFuture, we await its completion and print its result.

Result:

Tick
Tick
10

As you can see, the periodic timer is immediately stopped when the CancellableFuture that created it completes.

CancellableFuture.group() and stream()

CancellableFuture.group() makes it easier to run multiple asynchronous computations within the same CancellableFuture and waiting for all their results.

Example:

Future<void> groupExample() async {
  var result = 0;
  final group = CancellableFuture.group([
            () async => 10,
            () async => 20,
  ], receiver: (int item) => result += item);
  await group;
  print('Result: $result');
}

Result:

Result: 30

As with any CancellableFuture, if some error happens in any of the computations within a group, all other computations are stopped and the error propagates to the await-er.

For convenience, there's also a stream factory method that returns a Stream<T> instead of CancellableFuture<void>, but which has the exact same semantics as a group:

Future<void> streamExample() async {
  final group = CancellableFuture.stream([
    () async => 10,
    () async => 20,
  ]);
  print('Result: ${await group.toList()}');
}

Result:

Result: [10, 20]

Limitations

Not everything can be stopped immediately when a CancellableFuture is cancelled.

Known issues are listed below.

stopped Timers and Futures.

When CancellableFuture completes or is cancelled explicitly, any pending Future and Timer within it will be immediately awakened so the synchronous code that follows them will be executed.

For example, this simple code probably won't work as you think it should:

Future<void> scheduledFutureWillRun() async {
  final task = CancellableFuture(() =>
          Future.delayed(Duration(seconds: 2), () => print('2 seconds later')));
  await Future.delayed(Duration(seconds: 1), task.cancel);
  await task;
}

This will actually print 2 seconds later, but after only 1 second, and the program will terminate successfully because no more asynchronous calls were made within the Future.

If you ever run into this problem, you can try to insert a few explicit checks to see if your task has been cancelled before doing anything.

That's what the isComputationCancelled() function is for, as this example demonstrates:

Future<void> explicitCheckForCancellation() async {
  final task = CancellableFuture.ctx((ctx) =>
          Future.delayed(Duration(seconds: 2), () {
            if (ctx.isComputationCancelled()) return 'Cancelled';
            return '2 seconds later';
          }));
  await Future.delayed(Duration(seconds: 1), task.cancel);
  print(await task);
}

Result:

Cancelled

Running with the time option proves that the CancellableFuture indeed returned after around 1 second:

Time  | Message
------+--------
1032  | Cancelled

Notice that calling any async method or creating a Future from within a task would have caused the above CancellableFuture to abort with a FutureCancelled Exception.

As shown above, the CancellableFuture.ctx constructor must be used to get access to the context object which exposes isComputationCancelled(), amongst other helper functions.

If passing the context object into where it's needed gets cumbersome, you can use the top-level function CancellableContext? currentCancellableContext(), which returns null when it's not executed from within a CancellableFuture computation.

Isolates.

Dart Isolates started within a CancellableFuture may continue running even after the CancellableFuture completes.

To work around this problem, use the context's scheduleOnCompletion function and the following general pattern to ensure the Isolates don't survive after a CancellableFuture it was created from returns:

Future<void> stoppingIsolates() async {
  final task = CancellableFuture.ctx((ctx) async {
    final responsePort = ReceivePort()..listen(print);

    final iso = await Isolate.spawn((message) async {
      for (var i = 0; i < 10; i++) {
        await Future.delayed(Duration(milliseconds: 500),
                        () => message.send('Isolate says: hello'));
      }
      message.send('Isolate finished');
    }, responsePort.sendPort);

    Zone zone = Zone.current;
    // this runs in the root Zone
    ctx.scheduleOnCompletion(() {
      // ensure Isolate is terminated on completion
      zone.print('Killing ISO');
      responsePort.close();
      iso.kill();
    });

    // let this Future continue to run for a few seconds by
    // pretending to do some work
    for (var i = 0; i < 20; i++) {
      try {
        await Future.delayed(Duration(milliseconds: 200));
      } on FutureCancelled {
        break;
      }
    }
    // no more async computations, so it completes normally
    print('CancellableFuture finished');
  });

  Future.delayed(Duration(seconds: 2), () async {
    task.cancel();
    print('XXX Task was cancelled now! XXX');
  });

  await task;

  print('Done');
}

Result:

Isolate says: hello
Isolate says: hello
Isolate says: hello
XXX Task was cancelled now! XXX
CancellableFuture finished
Killing ISO
Done

Examples

All examples on this page, and more, can be found in the example directory.