async_call_queue

AsyncCallQueue is a Dart class which provides a queuing mechanism to prevent concurrent access to asynchronous code.

Getting Started

Add this to your app’s pubspec.yaml file:

dependencies:
  async_call_queue: ^1.0.0

Usage

Then you have to import the package with:

import 'package:async_call_queue/async_call_queue.dart';

And use AsyncCallQueue where appropriate.

Some examples:

// This is the control. The delayedWrite1234To function writes each
// number in the array [1, 2, 3, 4] to the string buffer with a
// millisecond delay before each write. Calling it two times in a row
// should result in '11223344'.
var buff = StringBuffer();
var f1 = delayedWrite1234To(buff);
var f2 = delayedWrite1234To(buff);
await Future.wait<void>([f1, f2]);
expect(buff.toString(), '11223344');

// This verifies that `queueCall` synchronizes the calls to
// delayedWrite1234To so that the first call finishes before the
// second call is executed, resulting in '12341234'.
var acq = AsyncCallQueue();
buff = StringBuffer();
f1 = acq.queueCall<void>((acq, callId) => delayedWrite1234To(buff));
f2 = acq.queueCall<void>((acq, callId) => delayedWrite1234To(buff));
await Future.wait<void>([f1, f2]);
acq.dispose();
expect(buff.toString(), '12341234');

// This verifies that the first call can cancel some of its work when
// the next call starts waiting. We delay the second call by two
// milliseconds, so the result should be '121234'
acq = AsyncCallQueue();
buff = StringBuffer();
buff = StringBuffer();
f1 = acq.queueCall<void>((acq, callId) async {
    for (final value in [1, 2, 3, 4]) {
      await _delayedWriteTo(buff, value);
      if (acq.hasCallsWaitingAfter(callId)) return;
    }
});
await Future<void>.delayed(twoMilliseconds);
f2 = acq.queueCall<void>((acq, callId) => delayedWrite1234To(buff));
await Future.wait<void>([f1, f2]);
acq.dispose();
expect(buff.toString(), '121234');

// In this example, only the last call should complete because
// we're making a new call every millisecond and the delay is
// 5 milliseconds.
final buff = StringBuffer();
final acq = AsyncCallQueue();
final completer = Completer<void>();
for (var c = 0; c < 10; c++) {
    await Future<void>.delayed(oneMillisecond);
    acq.delayCall((acq, callId) {
    buff.write(callId);
    completer.complete();
    }, delay: fiveMilliseconds);
}
await completer.future;
acq.dispose();
expect(buff.toString(), '10');

/// Writes the [value] to the [buff] after the [delay].
Future _delayedWriteTo(
  StringBuffer buff,
  int value, {
  Duration delay = oneMillisecond,
}) async {
  await Future<void>.delayed(delay);
  buff.write(value);
}

/// Writes the values 1, 2, 3, 4 to the [buff] with a [delay] before each
/// number is written.
Future delayedWrite1234To(
  StringBuffer buff, {
  Duration delay = oneMillisecond,
}) async {
  for (final value in [1, 2, 3, 4]) {
    await _delayedWriteTo(buff, value, delay: delay);
  }
}