Dart Cancellation Token

A Dart utility package for easy async task cancellation.


  • 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


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.


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.


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();

void initState() {

void dispose() {
  // All futures using this token will be cancelled when this widget is disposed

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>(
  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
complete.completeError(e, stackTrace);

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

Cancellable Isolates

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 the cancellableCompute() function, you can run a static or top level function in an isolate and kill the isolate early using a CancellationToken. This function is based on Flutter's compute() method.

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

Isolates aren't supported when building for web. As a fallback, cancellableCompute will return a future that completes with either the cancellation exception or the result of the callback, depending on whether or not the CancellationToken was already cancelled when it was called.

final ChunkyApiResponse response = await cancellableCompute(

static ChunkyApiResponse _readAndParseJson(String json) {
  final Map<String, dynamic> decodedJson = jsonDecode(json);
  return ChunkyApiResponse.fromJson(decodedJson);

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
  // 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(
    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.
  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