Add a CancelableFuture class.

lib/src/cancelable_operation.dart

Index: lib/src/cancelable_operation.dart
diff --git a/lib/src/cancelable_operation.dart b/lib/src/cancelable_operation.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a7fe6a87368c630ef1e42c9422af901353853443
--- /dev/null
+++ b/lib/src/cancelable_operation.dart
@@ -0,0 +1,148 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library async.cancelable_operation;
+
+import 'dart:async';
+
+import 'package:async/async.dart';
+
+/// An asynchronuos operation that can be cancelled.
+///
+/// The value of this operation is exposed as [value]. When this operation is
+/// cancelled, [value] won't complete either successfully or with an error. If
+/// [value] has already completed, cancelling the operation does nothing.
+class CancelableOperation<T> {
+  /// The completer that produced this operation.
+  ///
+  /// This is canceled when [cancel] is called.
+  final CancelableCompleter<T> _completer;
+
+  CancelableOperation._(this._completer);
+
+  /// Creates a [CancelableOperation] wrapping [inner].
+  ///
+  /// When this operation is canceled, [onCancel] will be called and any value
+  /// or error produced by [inner] will be discarded. The callback may return a
+  /// Future to indicate that asynchronous work has to be done to cancel the
+  /// future; this Future will be returned by [cancel].
+  ///
+  /// [onCancel] will be called synchronously when the operation is canceled.
+  /// It's guaranteed to only be called once.
+  factory CancelableOperation.fromFuture(Future<T> inner, {onCancel()}) {
+    var completer = new CancelableCompleter<T>(onCancel: onCancel);
+    completer.complete(inner);
+    return completer.operation;
+  }
+
+  /// The value returned by the operation.
+  Future<T> get value => _completer._inner.future;
+
+  /// Creates a [Stream] containing the result of this operation.
+  ///
+  /// This is like `value.asStream()`, but if a subscription to the stream is
+  /// canceled, this is as well.
+  Stream<T> asStream() {
+    var controller = new StreamController<T>(
+        sync: true, onCancel: _completer._cancel);
+
+    value.then((value) {
+      controller.add(value);
+      controller.close();
+    }, onError: (error, stackTrace) {
+      controller.addError(error, stackTrace);
+      controller.close();
+    });
+    return controller.stream;
+  }
+
+  /// Cancels this operation.
+  ///
+  /// This returns the [Future] returned by the [CancelableCompleter]'s
+  /// `onCancel` callback. Unlike [Stream.cancel], it never returns `null`.
+  Future cancel() => _completer._cancel();
+}
+
+/// A completer for a [CancelableOperation].
+class CancelableCompleter<T> {
+  /// The completer for the wrapped future.
+  final Completer<T> _inner;
+
+  /// The callback to call if the future is canceled.
+  final ZoneCallback _onCancel;
+
+  /// Creates a new completer for a [CancelableOperation].
+  ///
+  /// When the future operation canceled, as long as the completer hasn't yet
+  /// completed, [onCancel] is called. The callback may return a [Future]; if
+  /// so, that [Future] is returned by [CancelableOperation.cancel].
+  ///
+  /// [onCancel] will be called synchronously when the operation is canceled.
+  /// It's guaranteed to only be called once.
+  CancelableCompleter({onCancel()})
+      : _onCancel = onCancel,
+        _inner = new Completer<T>() {
+    _operation = new CancelableOperation<T>._(this);
+  }
+
+  /// The operation controlled by this completer.
+  CancelableOperation<T> get operation => _operation;
+  CancelableOperation<T> _operation;
+
+  /// Whether the completer has completed.
+  bool get isCompleted => _isCompleted;
+  bool _isCompleted = false;
+
+  /// Whether the completer was canceled before being completed.
+  bool get isCanceled => _isCanceled;
+  bool _isCanceled = false;
+
+  /// The memoizer for [_cancel].
+  final _cancelMemo = new AsyncMemoizer();
+
+  /// Completes [operation] to [value].
+  ///
+  /// If [value] is a [Future], this will complete to the result of that
+  /// [Future] once it completes.
+  void complete([value]) {
+    if (_isCompleted) throw new StateError("Operation already completed");
+    _isCompleted = true;
+
+    if (value is! Future) {
+      if (_isCanceled) return;
+      _inner.complete(value);
+      return;
+    }
+
+    if (_isCanceled) {
+      // Make sure errors from [value] aren't top-leveled.
+      value.catchError((_) {});
+      return;
+    }
+
+    value.then((result) {
+      if (_isCanceled) return;
+      _inner.complete(result);
+    }, onError: (error, stackTrace) {
+      if (_isCanceled) return;
+      _inner.completeError(error, stackTrace);
+    });
+  }
+
+  /// Completes [operation] to [error].
+  void completeError(Object error, [StackTrace stackTrace]) {
+    if (_isCompleted) throw new StateError("Operation already completed");
+    _isCompleted = true;
+
+    if (_isCanceled) return;
+    _inner.completeError(error, stackTrace);
+  }
+
+  /// Cancel the completer.
+  Future _cancel() => _cancelMemo.runOnce(() {
+    if (_inner.isCompleted) return null;
+    _isCanceled = true;
+    if (_onCancel != null) return _onCancel();
+  });
+}