Let StreamSubscription.cancel return a Future.

tests/lib/async/stream_subscription_cancel_test.dart

+// Copyright (c) 2013, 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.
+
+// Test the StreamSubscription.cancel return Future.
+library stream_subscription_cancel;
+
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+
+void main() {
+  test('subscription.cancel', () {
+    var completer = new Completer();
+    StreamController controller = new StreamController(
+        onCancel: () => completer.future);
+
+    bool done = false;
+    expect(controller.stream.listen(null).cancel().then((_) => done),
+           completion(equals(true)));
+
+    new Timer(const Duration(milliseconds: 10), () {
+      done = true;
+      completer.complete();
+    });
+  });
+
+  test('subscription.cancel after close', () {
+    var completer = new Completer();
+    StreamController controller = new StreamController(
+        onCancel: completer.complete);
+
+    controller.close();
+
+    var sub;
+    void onDone() {
+      expect(sub.cancel(), isNull);
+    }
+
+    sub = controller.stream.listen(null, onDone: onDone);
+    expect(completer.future, completes);
+  });
+
+  test('subscription.cancel after error', () {
+    var completer = new Completer();
+    StreamController controller = new StreamController(
+        onCancel: () => completer.future);
+
+    controller.addError("error");
+
+    bool done = false;
+
+    var subscription;
+    var doneCompleter = new Completer();
+    void onError(e) {
+      subscription.cancel().then((_) => doneCompleter.complete(done));
+      done = true;
+      completer.complete();
+    }
+    subscription = controller.stream.listen(null, onError: onError);
+    expect(doneCompleter.future, completion(equals(true)));
+  });
+
+  test('subscription.cancel after error (cancelOnError)', () {
+    bool called = false;
+    StreamController controller = new StreamController(
+        onCancel: () {
+          called = true;
+        });
+
+    controller.addError("error");
+
+    var doneCompleter = new Completer();
+    void onError(e) {
+      expect(called, equals(true));
+      doneCompleter.complete(true);
+    }
+    controller.stream.listen(null, onError: onError, cancelOnError: true);
+    expect(doneCompleter.future, completion(equals(true)));
+  });
+
+  test('subscription.cancel before error (cancelOnError)', () {
+    var doneCompleter = new Completer();
+    StreamController controller = new StreamController(
+        onCancel: () {
+          doneCompleter.complete(true);
+        });
+
+    controller.addError("error");
+
+    void onError(e) {
+      fail("onError is unexpected");
+    }
+    controller.stream
+      .listen(null, onError: onError, cancelOnError: true)
+      .cancel();
+    expect(doneCompleter.future, completion(equals(true)));
+  });
+
+  test('subscription.cancel on error (cancelOnError)', () {
+    bool called = false;
+    StreamController controller = new StreamController(
+        onCancel: () {
+          expect(called, isFalse);
+          called = true;
+        });
+
+    controller.addError("error");
+
+    var doneCompleter = new Completer();
+    var sub;
+    void onError(e) {
+      expect(called, equals(true));
+      sub.cancel();
+      doneCompleter.complete(true);
+    }
+    sub = controller.stream
+      .listen(null, onError: onError, cancelOnError: true);
+    expect(doneCompleter.future, completion(equals(true)));
+  });
+
+  test('subscription.cancel before done', () {
+    var doneCompleter = new Completer();
+    StreamController controller = new StreamController(
+        onCancel: () {
+          doneCompleter.complete(true);
+        });
+
+    controller.close();
+
+    void onDone() {
+      fail("onDone is unexpected");
+    }
+    controller.stream
+      .listen(null, onDone: onDone)
+      .cancel();
+    expect(doneCompleter.future, completion(equals(true)));
+  });
+}