Dart Cancellation Token

A Dart utility package for easy async task cancellation.

Features

  • Cancel futures and clean-up resources (e.g. closing an HttpClient) when a widget is disposed in Flutter
  • Reuse a single CancellationToken for multiple tasks, and cancel them all with a single call to .cancel()
  • Cancel isolates with cancellableCompute
  • Create your own cancellables that use CancellationTokens with the Cancellable mixin

Cancellation Tokens

CancellationToken

The standard CancellationToken for manually cancelling tasks. When .cancel() is called, all cancellables using the token will be cancelled. By default, async tasks cancelled with a CancellationToken will throw a CancelledException. You can pass a custom exception using .cancel(CustomException()) to change this.

TimeoutCancellationToken

To cancel tasks after a certain amount of time, you can use a TimeoutCancellationToken. By default, async tasks will be cancelled with a TimeoutException when the timeout duration ends. You can pass a custom exception by using the timeoutException parameter.

When a TimeoutCancellationToken is created, the timer will begin immediately. To only start the timer when the token is attached to a task, set the lazyStart parameter to true.

MergedCancellationToken

To combine multiple tokens together, you can use a MergedCancellationToken. To create one, use the MergedCancellationToken() constructor, or use the .merge() shortcut method on an existing token.

Caveats

When using a MergedCancellationToken, the cancellation exception thrown isn't guaranteed to be the exception of the token that was cancelled first. If no cancellable operations were running when the tokens were cancelled, the exception from the first token in the list will be used. When using the .merge() shortcut, this the token on which you called .merge().

Usage

Cancellable Future

The CancellableFuture class provides cancellable versions for many of Dart's Future constructors, including:

  • Future() ➡️ CancellableFuture.from()
  • Future.microtask() ➡️ CancellableFuture.microtask()
  • Future.sync() ➡️ CancellableFuture.sync()
  • Future.value() ➡️ CancellableFuture.value()
  • Future.delayed() ➡️ CancellableFuture.delayed()

To make existing futures cancellable, you can also use the .asCancellable() extension.

CancellationToken cancellationToken = CancellationToken();

@override
void initState() {
  super.initState();
  loadData();
}

@override
void dispose() {
  // All futures using this token will be cancelled when this widget is disposed
  cancellationToken.cancel();
  super.dispose();
}

Future<void> loadData() async {
  try {
    // The CancellationToken can be used for multiple tasks
    someDataA = await getDataA().asCancellable(cancellationToken);
    someDataB = await getDataB().asCancellable(cancellationToken);
    setState(() {
      // ...
    });
  } on CancelledException {
    // Ignore cancellations
  } catch (e, stackTrace) {
    setState(() => error = true);
  }
}

Cancellable Completer

The CancellableCompleter class can be used in place of a standard Completer to make it cancellable. This completer implements the base Completer class, so it works as a drop-in replacement.

CancellableCompleter<String> completer = CancellableCompleter<String>(
  cancellationtoken, 
  onCancel: () {
    // The optional onCancel callback can be used to clean up resources when the 
    // token is cancelled
  }
);

// Complete with either the result or an error as usual, these will only have an 
// effect if the completer hasn't already been cancelled
completer.complete(result);
complete.completeError(e, stackTrace);

// This future will complete with the result or the cancellation exception, 
// whichever is first
return completer.future;

Cancellable Isolate

If you need to run an intensive synchronous task, like parsing a large JSON API response, you can use an isolate to avoid blocking the UI thread. With CancellableIsolate.run(), you can run a computation in an isolate and kill the isolate early using a CancellationToken. This function is based on Dart's Isolate.run() method.

When cancelled, the isolate will be killed immediately to free up resources. If your callback function performs I/O operations such as file writes, these may not complete.

When building for web, this uses CancellableFuture.from() as a fallback due to isolates not being supported.

final ChunkyApiResponse response = await CancellableIsolate.run(
  () {
    final Map<String, dynamic> decodedJson = jsonDecode(json);
    return ChunkyApiResponse.fromJson(decodedJson);
  },
  cancellationToken,
);

Cancellable HTTP

For HTTP requests with cancellation support, check out the Cancellation Token HTTP package, a fork of the Dart HTTP package with the request cancellation powered by this package. If HTTP request cancellation is all you need, the package can be used standalone, but it's most powerful when paired when other cancellables like cancellableCompute.

import 'package:cancellation_token_http/http.dart' as http;

CancellationToken? cancellationToken;

Future<ChunkyApiResponse> makeRequest() async {
  // Cancel the request if it's already in progress
  cancellationToken?.cancel();
  // Create a CancellationToken for the new request
  cancellationToken = CancellationToken();
  // Make the cancellable request and parse the JSON in a cancellable isolate
  final http.Response response = await http.get(
    Uri.parse('https://example.com/bigjson'),
    cancellationToken: token,
  );
  return await cancellableCompute(parseJson, response.body, cancellationToken);
}

Custom Cancellables

The Cancellable mixin can be used to make your own cancellables. This might be useful for custom I/O libraries, like a custom HTTP library.

  • DO detach from the CancellationToken when your async task completes.
  • DON'T attach to a CancellationToken that has already been cancelled, instead use the maybeAttach method to check if it's already been cancelled and only start your async task if it returns `false.
  • DON'T cancel the CancellationToken within a Cancellable, as it may be used for other tasks.
class MyCancellable with Cancellable {
  MyCancellable(this.cancellationToken) {
    // Call `maybeAttach()` to only attach if the cancellation token hasn't 
    // already been cancelled
    if (maybeAttach(this.cancellationToken)) {
      // Start your async task here
    }
  }

  final CancellationToken cancellationToken;

  /// Override `onCancel()` to clean up resources after cancellation.
  @override
  void onCancel(Exception cancelException) {
    // Clean up resources here, like closing an HttpClient
  }
  
  void complete() {
    // If your async task completes before the token is cancelled, 
    // detatch from the token
    _cancellationToken.detach(this);
  }
}

Libraries

cancellation_token