Let StreamSubscription.cancel return a Future.

sdk/lib/async/stream.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.
 
 part of dart.async;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 318 matching lines @@
   Future<T> reduce(T combine(T previous, T element)) {
     _Future<T> result = new _Future<T>();
     bool seenFirst = false;
     T value;
     StreamSubscription subscription;
     subscription = this.listen(
       (T element) {
         if (seenFirst) {
           _runUserCode(() => combine(value, element),
                        (T newValue) { value = newValue; },
-                       _cancelAndError(subscription, result));
+                       _cancelAndErrorClosure(subscription, result));
         } else {
           value = element;
           seenFirst = true;
         }
       },
       onError: result._completeError,
       onDone: () {
         if (!seenFirst) {
           result._completeError(new StateError("No elements"));
         } else {
           result._complete(value);
         }
       },
       cancelOnError: true
     );
     return result;
   }
 
   /** Reduces a sequence of values by repeatedly applying [combine]. */
   Future fold(var initialValue, combine(var previous, T element)) {
     _Future result = new _Future();
     var value = initialValue;
     StreamSubscription subscription;
     subscription = this.listen(
       (T element) {
         _runUserCode(
           () => combine(value, element),
           (newValue) { value = newValue; },
-          _cancelAndError(subscription, result)
+          _cancelAndErrorClosure(subscription, result)
         );
       },
       onError: (e, st) {
         result._completeError(e, st);
       },
       onDone: () {
         result._complete(value);
       },
       cancelOnError: true);
     return result;
@@ skipping 16 matching lines @@
     bool first = true;
     subscription = this.listen(
       (T element) {
         if (!first) {
           buffer.write(separator);
         }
         first = false;
         try {
           buffer.write(element);
         } catch (e, s) {
-          subscription.cancel();
-          result._completeError(_asyncError(e, s));
+          _cancelAndError(subscription, result, _asyncError(e, s), s);
         }
       },
       onError: (e) {
         result._completeError(e);
       },
       onDone: () {
         result._complete(buffer.toString());
       },
       cancelOnError: true);
     return result;
   }
 
   /**
    * Checks whether [needle] occurs in the elements provided by this stream.
    *
    * Completes the [Future] when the answer is known.
    * If this stream reports an error, the [Future] will report that error.
    */
   Future<bool> contains(Object needle) {
     _Future<bool> future = new _Future<bool>();
     StreamSubscription subscription;
     subscription = this.listen(
         (T element) {
           _runUserCode(
             () => (element == needle),
             (bool isMatch) {
               if (isMatch) {
-                subscription.cancel();
-                future._complete(true);
+                _cancelAndValue(subscription, future, true);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
         onDone: () {
           future._complete(false);
         },
         cancelOnError: true);
     return future;
   }
 
   /**
    * Executes [action] on each data event of the stream.
    *
    * Completes the returned [Future] when all events of the stream
    * have been processed. Completes the future with an error if the
    * stream has an error event, or if [action] throws.
    */
   Future forEach(void action(T element)) {
     _Future future = new _Future();
     StreamSubscription subscription;
     subscription = this.listen(
         (T element) {
           _runUserCode(
             () => action(element),
             (_) {},
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
         onDone: () {
           future._complete(null);
         },
         cancelOnError: true);
     return future;
   }
 
   /**
    * Checks whether [test] accepts all elements provided by this stream.
    *
    * Completes the [Future] when the answer is known.
    * If this stream reports an error, the [Future] will report that error.
    */
   Future<bool> every(bool test(T element)) {
     _Future<bool> future = new _Future<bool>();
     StreamSubscription subscription;
     subscription = this.listen(
         (T element) {
           _runUserCode(
             () => test(element),
             (bool isMatch) {
               if (!isMatch) {
-                subscription.cancel();
-                future._complete(false);
+                _cancelAndValue(subscription, future, false);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
         onDone: () {
           future._complete(true);
         },
         cancelOnError: true);
     return future;
   }
 
   /**
    * Checks whether [test] accepts any element provided by this stream.
    *
    * Completes the [Future] when the answer is known.
    * If this stream reports an error, the [Future] will report that error.
    */
   Future<bool> any(bool test(T element)) {
     _Future<bool> future = new _Future<bool>();
     StreamSubscription subscription;
     subscription = this.listen(
         (T element) {
           _runUserCode(
             () => test(element),
             (bool isMatch) {
               if (isMatch) {
-                subscription.cancel();
-                future._complete(true);
+                _cancelAndValue(subscription, future, true);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
         onDone: () {
           future._complete(false);
         },
         cancelOnError: true);
     return future;
   }
 
@@ skipping 11 matching lines @@
       cancelOnError: true);
     return future;
   }
 
   /** Reports whether this stream contains any elements. */
   Future<bool> get isEmpty {
     _Future<bool> future = new _Future<bool>();
     StreamSubscription subscription;
     subscription = this.listen(
       (_) {
-        subscription.cancel();
-        future._complete(false);
+        _cancelAndValue(subscription, future, false);
       },
       onError: future._completeError,
       onDone: () {
         future._complete(true);
       },
       cancelOnError: true);
     return future;
   }
 
   /** Collects the data of this stream in a [List]. */
@@ skipping 111 matching lines @@
    * the resulting future completes with a [StateError].
    *
    * Except for the type of the error, this method is equivalent to
    * [:this.elementAt(0):].
    */
   Future<T> get first {
     _Future<T> future = new _Future<T>();
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
-        subscription.cancel();
-        future._complete(value);
-        return;
+        _cancelAndValue(subscription, future, value);
       },
       onError: future._completeError,
       onDone: () {
         future._completeError(new StateError("No elements"));
       },
       cancelOnError: true);
     return future;
   }
 
   /**
@@ skipping 33 matching lines @@
    * If [this] is empty or has more than one element throws a [StateError].
    */
   Future<T> get single {
     _Future<T> future = new _Future<T>();
     T result = null;
     bool foundResult = false;
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
         if (foundResult) {
-          subscription.cancel();
           // This is the second element we get.
           Error error = new StateError("More than one element");
-          future._completeError(error);
+          _cancelAndError(subscription, future, error, null);
           return;
         }
         foundResult = true;
         result = value;
       },
       onError: future._completeError,
       onDone: () {
         if (foundResult) {
           future._complete(result);
           return;
@@ skipping 20 matching lines @@
    */
   Future<dynamic> firstWhere(bool test(T element), {Object defaultValue()}) {
     _Future<dynamic> future = new _Future();
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
         _runUserCode(
           () => test(value),
           (bool isMatch) {
             if (isMatch) {
-              subscription.cancel();
-              future._complete(value);
+              _cancelAndValue(subscription, future, value);
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
       onDone: () {
         if (defaultValue != null) {
           _runUserCode(defaultValue, future._complete, future._completeError);
           return;
         }
         future._completeError(new StateError("firstMatch ended without match"));
       },
@@ skipping 16 matching lines @@
     subscription = this.listen(
       (T value) {
         _runUserCode(
           () => true == test(value),
           (bool isMatch) {
             if (isMatch) {
               foundResult = true;
               result = value;
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
       onDone: () {
         if (foundResult) {
           future._complete(result);
           return;
         }
         if (defaultValue != null) {
           _runUserCode(defaultValue, future._complete, future._completeError);
@@ skipping 16 matching lines @@
     T result = null;
     bool foundResult = false;
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
         _runUserCode(
           () => true == test(value),
           (bool isMatch) {
             if (isMatch) {
               if (foundResult) {
-                subscription.cancel();
-                future._completeError(
-                    new StateError('Multiple matches for "single"'));
+                _cancelAndError(
+                    subscription,
+                    future,
+                    new StateError('Multiple matches for "single"'),
+                    null);
                 return;
               }
               foundResult = true;
               result = value;
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
       onDone: () {
         if (foundResult) {
           future._complete(result);
           return;
         }
         future._completeError(new StateError("single ended without match"));
       },
@@ skipping 12 matching lines @@
    * If a done event occurs before the value is found, the future completes
    * with a [RangeError].
    */
   Future<T> elementAt(int index) {
     if (index is! int || index < 0) throw new ArgumentError(index);
     _Future<T> future = new _Future<T>();
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
         if (index == 0) {
-          subscription.cancel();
-          future._complete(value);
+          _cancelAndValue(subscription, future, value);
           return;
         }
         index -= 1;
       },
       onError: future._completeError,
       onDone: () {
         future._completeError(new RangeError.value(index));
       },
       cancelOnError: true);
     return future;
   }
 }
 
 /**
  * A control object for the subscription on a [Stream].
  *
  * When you subscribe on a [Stream] using [Stream.listen],
  * a [StreamSubscription] object is returned. This object
  * is used to later unsubscribe again, or to temporarily pause
  * the stream's events.
  */
 abstract class StreamSubscription<T> {
   /**
    * Cancels this subscription. It will no longer receive events.
    *
    * If an event is currently firing, this unsubscription will only
    * take effect after all subscribers have received the current event.
+   *
+   * In case the cancel is asynchronous, the returned [Future] can be used to
+   * wait for the operation to complete. If in doubt, wait for it. Note that

[floitsch, 2013/10/16 14:43:44]

Start sentence with "Returns".
===
Returns a future if the cancel-operation is not completed synchronously.
Otherwise returns `null`.
===

I don't think it buys to add "if in doubt...".

[Anders Johnsen, 2013/10/21 08:01:46]

Done.

+   * cancel can return `null`, if the operation was immediate.
    */
-  void cancel();
+  Future cancel();
 
   /** Set or override the data event handler of this subscription. */
   void onData(void handleData(T data));
 
   /**
    * Set or override the error event handler of this subscription.
    *
    * This method overrides the handler that has been set at the invocation of
    * [Stream.listen].
    */
@@ skipping 263 matching lines @@
    *
    * The stream iterator is automatically canceled if the [moveNext] future
    * completes with either `false` or an error.
    *
    * If a [moveNext] call has been made, it will complete with `false` as value,
    * as will all further calls to [moveNext].
    *
    * If you need to stop listening for values before the stream iterator is
    * automatically closed, you must call [cancel] to ensure that the stream
    * is properly closed.
+   *

[floitsch, 2013/10/16 14:43:44]

ditto.

[Anders Johnsen, 2013/10/21 08:01:46]

Done.

+   * In case the cancel is asynchronous, the returned [Future] can be used to
+   * wait for the operation to complete. If in doubt, wait for it. Note that
+   * cancel can return `null`, if the operation was immediate.
    */
-  void cancel();
+  Future cancel();
 }