Example CancellationToken[Source] a la .net. Provides a way to signal that an operation has been canceled

cancellation_token.dart

import 'package:w_common/func.dart';

/// Provides a way to signal that an operation has been canceled.
///
/// Typically, the consumer creates a [CancellationTokenSource] and then passes
/// the [token] to another operation. The consumer can then call [cancel],
/// while the operation can only check if it has been canceled.
///
/// FUTURE: If we want a stream to listen for when a cancellation has happened,
/// use CancelableCompleter from the async package instead.
class CancellationTokenSource {
  CancellationToken _token;
  bool _isCancellationRequested = false;

  CancellationTokenSource() {
    _token = new CancellationToken(() {
      return _isCancellationRequested;
    });
  }

  CancellationToken get token => _token;

  /// Signal to the [token] bearer that they should cancel
  void cancel() {
    _isCancellationRequested = true;
  }
}

/// Provides a way to check if an operation should be canceled.
class CancellationToken {
  final Func<bool> isCancellationRequested;

  CancellationToken(this.isCancellationRequested);
}