Make StreamSubscription be the active part of a stream.

sdk/lib/async/stream_impl.dart

 // Copyright (c) 2012, 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;
 
-// States shared by single/multi stream implementations.
-
-// Completion state of the stream.
-/// Initial and default state where the stream can receive and send events.
-const int _STREAM_OPEN = 0;
-/// The stream has received a request to complete, but hasn't done so yet.
-/// No further events can be added to the stream.
-const int _STREAM_CLOSED = 1;
-/// The stream has completed and will no longer receive or send events.
-/// Also counts as closed. The stream must not be paused when it's completed.
-/// Always used in conjunction with [_STREAM_CLOSED].
-const int _STREAM_COMPLETE = 2;
-
-/// Bit that alternates between events, and listeners are updated to the
-/// current value when they are notified of the event.
-const int _STREAM_EVENT_ID = 4;
-const int _STREAM_EVENT_ID_SHIFT = 2;
-
-// The activity state of the stream: What is it currently doing.
-/// Bit set while firing and clear while not.
-const int _STREAM_FIRING = 8;
-/// Bit set while calling a pause-state or subscription-state change callback.
-const int _STREAM_CALLBACK = 16;
-
-// The pause state of the stream.
-/// Bit set when resuming with pending events. Cleared after all pending events
-/// have been transmitted. Means that the controller still considers the
-/// stream paused, even if the listener doesn't.
-const int _STREAM_PENDING_RESUME = 32;
-/// The count of times a stream has paused is stored in the
-/// state, shifted by this amount.
-const int _STREAM_PAUSE_COUNT_SHIFT = 6;
-
-// States for listeners.
-
-/// The listener is currently not subscribed to its source stream.
-const int _LISTENER_UNSUBSCRIBED = 0;
-/// The listener is actively subscribed to its source stream.
-const int _LISTENER_SUBSCRIBED = 1;
-/// The listener is subscribed until it has been notified of the current event.
-/// This flag bit is always used in conjuction with [_LISTENER_SUBSCRIBED].
-const int _LISTENER_PENDING_UNSUBSCRIBE = 2;
-
-/// Bit that contains the last sent event's "id bit".
-const int _LISTENER_EVENT_ID = 4;
-const int _LISTENER_EVENT_ID_SHIFT = 2;
-
-/// The count of times a listener has paused is stored in the
-/// state, shifted by this amount.
-const int _LISTENER_PAUSE_COUNT_SHIFT = 3;
-
 /** Throws the given error in the next cycle. */
 _throwDelayed(var error, [Object stackTrace]) {
   // We are going to reach the top-level here, but there might be a global
   // exception handler. This means that we shouldn't print the stack trace.
   // TODO(floitsch): Find better solution that doesn't print the stack trace
   // if there is a global exception handler.
   runAsync(() {
     if (stackTrace != null) print(stackTrace);
     var trace = getAttachedStackTrace(error);
     if (trace != null && trace != stackTrace) print(trace);
     throw error;
   });
 }
 
+/** Abstract and private interface for a place to put events. */
+abstract class _EventSink<T> {
+  void _add(T data);
+  void _addError(Object error);
+  void _close();
+}
+
+/**
+ * Abstract and private interface for a place to send events.
+ *
+ * Used by event buffering to finally dispatch the pending event, where
+ * [_EventSink] is where the event first enters the stream subscription,
+ * and may yet be buffered.
+ */
+abstract class _EventDispatch<T> {
+  void _sendData(T data);
+  void _sendError(Object error);
+  void _sendDone();
+}
+
+/**
+ * Default implementation of stream subscription of buffering events.
+ *
+ * The only public methods are those of [StreamSubscription], so instances of
+ * [_BufferingStreamSubscription] can be returned directly as a
+ * [StreamSubscription] without exposing internal functionality.
+ *
+ * The [StreamController] is a public facing version of [Stream] and this class,
+ * with some methods made public.
+ *
+ * The user interface of [_BufferingStreamSubscription] are the following
+ * methods:
+ * * [_add]: Add a data event to the stream.
+ * * [_addError]: Add an error event to the stream.
+ * * [_close]: Request to close the stream.
+ * * [_onCancel]: Called when the subscription will provide no more events,
+ *     either due to being actively canceled, or after sending a done event.
+ * * [_onPause]: Called when the subscription wants the event source to pause.
+ * * [_onResume]: Called when allowing new events after a pause.
+ * The user should not add new events when the subscription requests a paused,
+ * but if it happens anyway, the subscription will enqueue the events just as
+ * when new events arrive while still firing an old event.
+ */
+class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
+                                                 _EventSink<T>,
+                                                 _EventDispatch<T> {
+  /** The `cancelOnError` flag from the `listen` call. */
+  static const int _STATE_CANCEL_ON_ERROR = 1;
+  /**
+   * Whether the "done" event has been received.
+   * No further events are accepted after this.
+   */
+  static const int _STATE_CLOSED = 2;
+  /**
+   * Set if the input has been asked not to send events.
+   *
+   * This is not the same as being paused, since the input will remain paused
+   * after a call to [resume] if there are pending events.
+   */
+  static const int _STATE_INPUT_PAUSED = 4;
+  /**
+   * Whether the subscription has been canceled.
+   *
+   * Set by calling [cancel], or by handling a "done" event, or an "error" event
+   * when `cancelOnError` is true.
+   */
+  static const int _STATE_CANCELED = 8;
+  static const int _STATE_IN_CALLBACK = 16;
+  static const int _STATE_HAS_PENDING = 32;
+  static const int _STATE_PAUSE_COUNT = 64;
+  static const int _STATE_PAUSE_COUNT_SHIFT = 6;
+
+  /* Event handlers provided in constructor. */
+  /* TODO(7733): Fix Function->_DataHandler<T> when dart2js understands
+   * parameterized function types. */
+  Function _onData;
+  _ErrorHandler _onError;
+  _DoneHandler _onDone;
+
+  /** Bit vector based on state-constants above. */
+  int _state;
+
+  /**
+   * Queue of pending events.
+   *
+   * Is created when necessary, or set in constructor for preconfigured events.
+   */
+  _PendingEvents _pending;
+
+  _BufferingStreamSubscription(this._onData,
+                               this._onError,
+                               this._onDone,
+                               bool cancelOnError)
+      : _state = (cancelOnError ? _STATE_CANCEL_ON_ERROR : 0) {
+    assert(_onData != null);
+    assert(_onError != null);
+    assert(_onDone != null);
+  }
+
+  /**
+   * Sets the subscription's pending events object.
+   *
+   * This can only be done once. The pending events object is used for the
+   * rest of the subscription's life cycle.
+   */
+  void _setPendingEvents(_PendingEvents pendingEvents) {
+    assert(_pending == null);
+    if (pendingEvents == null) return;
+    _pending = pendingEvents;
+    if (!pendingEvents.isEmpty) {
+      _state |= _STATE_HAS_PENDING;
+      _pending.schedule(this);
+    }
+  }
+
+  /**
+   * Extracts the pending events from a canceled stream.
+   *
+   * This can only be done during the [_onCancel] method call. After that,
+   * any remaining pending events will be cleared.
+   */
+  _PendingEvents _extractPending() {
+    assert(_isCanceled);
+    _PendingEvents events = _pending;
+    _pending = null;
+    return events;
+  }
+
+  // StreamSubscription interface.
+
+  void onData(void handleData(T event)) {
+    if (handleData == null) handleData = _nullDataHandler;
+    _onData = handleData;
+  }
+
+  void onError(void handleError(error)) {
+    if (handleError == null) handleError = _nullErrorHandler;
+    _onError = handleError;
+  }
+
+  void onDone(void handleDone()) {
+    if (handleDone == null) handleDone = _nullDoneHandler;
+    _onDone = handleDone;
+  }
+
+  void pause([Future resumeSignal]) {
+    if (_isCanceled) return;
+    bool wasPaused = _isPaused;
+    bool wasInputPaused = _isInputPaused;
+    // Increment pause count and mark input paused (if it isn't already).
+    _state = (_state + _STATE_PAUSE_COUNT) | _STATE_INPUT_PAUSED;
+    if (resumeSignal != null) resumeSignal.whenComplete(resume);
+    if (!wasPaused && _pending != null) _pending.cancelSchedule();
+    if (!wasInputPaused && !_inCallback) _guardCallback(_onPause);
+  }
+
+  void resume() {
+    if (_isCanceled) return;
+    if (_isPaused) {
+      _decrementPauseCount();
+      if (!_isPaused) {
+        if (_hasPending && !_pending.isEmpty) {
+          // Input is still paused.
+          _pending.schedule(this);
+        } else {
+          assert(_mayResumeInput);
+          _state &= ~_STATE_INPUT_PAUSED;
+          if (!_inCallback) _guardCallback(_onResume);
+        }
+      }
+    }
+  }
+
+  void cancel() {
+    if (_isCanceled) return;
+    _cancel();
+    if (!_inCallback) {
+      // otherwise checkState will be called after firing or callback completes.
+      _state |= _STATE_IN_CALLBACK;
+      _onCancel();
+      _pending = null;
+      _state &= ~_STATE_IN_CALLBACK;
+    }
+  }
+
+  Future asFuture([var futureValue]) {
+    _FutureImpl<T> result = new _FutureImpl<T>();
+
+    // Overwrite the onDone and onError handlers.
+    _onDone = () { result._setValue(futureValue); };
+    _onError = (error) {
+      cancel();
+      result._setError(error);
+    };
+
+    return result;
+  }
+
+  // State management.
+
+  bool get _isInputPaused => (_state & _STATE_INPUT_PAUSED) != 0;
+  bool get _isClosed => (_state & _STATE_CLOSED) != 0;
+  bool get _isCanceled => (_state & _STATE_CANCELED) != 0;
+  bool get _inCallback => (_state & _STATE_IN_CALLBACK) != 0;
+  bool get _hasPending => (_state & _STATE_HAS_PENDING) != 0;
+  bool get _isPaused => _state >= _STATE_PAUSE_COUNT;
+  bool get _canFire => _state < _STATE_IN_CALLBACK;
+  bool get _mayResumeInput =>
+      !_isPaused && (_pending == null || _pending.isEmpty);
+  bool get _cancelOnError => (_state & _STATE_CANCEL_ON_ERROR) != 0;
+
+  bool get isPaused => _isPaused;
+
+  void _cancel() {
+    _state |= _STATE_CANCELED;
+    if (_hasPending) {
+      _pending.cancelSchedule();
+    }
+  }
+
+  /**
+   * Increment the pause count.
+   *
+   * Also marks input as paused.
+   */
+  void _incrementPauseCount() {
+    _state = (_state + _STATE_PAUSE_COUNT) | _STATE_INPUT_PAUSED;
+  }
+
+  /**
+   * Decrements the pause count.
+   *
+   * Does not automatically unpause the input (call [_onResume]) when
+   * the pause count reaches zero. This is handled elsewhere, and only
+   * if there are no pending events buffered.
+   */
+  void _decrementPauseCount() {
+    assert(_isPaused);
+    _state -= _STATE_PAUSE_COUNT;
+  }
+
+  // _EventSink interface.
+
+  void _add(T data) {
+    assert(!_isClosed);
+    if (_isCanceled) return;
+    if (_canFire) {
+      _sendData(data);
+    } else {
+      _addPending(new _DelayedData(data));
+    }
+  }
+
+  void _addError(Object error) {
+    if (_isCanceled) return;
+    if (_canFire) {
+      _sendError(error);  // Reports cancel after sending.
+    } else {
+      _addPending(new _DelayedError(error));
+    }
+  }
+
+  void _close() {
+    assert(!_isClosed);
+    if (_isCanceled) return;
+    _state |= _STATE_CLOSED;
+    if (_canFire) {
+      _sendDone();
+    } else {
+      _addPending(const _DelayedDone());
+    }
+  }
+
+  // Hooks called when the input is paused, unpaused or canceled.
+  // These must not throw. If overwritten to call user code, include suitable
+  // try/catch wrapping and send any errors to [_throwDelayed].
+  void _onPause() {
+    assert(_isInputPaused);
+  }
+
+  void _onResume() {
+    assert(!_isInputPaused);
+  }
+
+  void _onCancel() {
+    assert(_isCanceled);
+  }
+
+  // Handle pending events.
+
+  /**
+   * Add a pending event.
+   *
+   * If the subscription is not paused, this also schedules a firing
+   * of pending events later (if necessary).
+   */
+  void _addPending(_DelayedEvent event) {
+    _StreamImplEvents pending = _pending;
+    if (_pending == null) pending = _pending = new _StreamImplEvents();
+    pending.add(event);
+    if (!_hasPending) {
+      _state |= _STATE_HAS_PENDING;
+      if (!_isPaused) {
+        _pending.schedule(this);
+      }
+    }
+  }
+
+  /* _EventDispatch interface. */
+
+  void _sendData(T data) {
+    assert(!_isCanceled);
+    assert(!_isPaused);
+    assert(!_inCallback);
+    bool wasInputPaused = _isInputPaused;
+    _state |= _STATE_IN_CALLBACK;
+    try {
+      _onData(data);
+    } catch (e, s) {
+      _throwDelayed(e, s);
+    }
+    _state &= ~_STATE_IN_CALLBACK;
+    _checkState(wasInputPaused);
+  }
+
+  void _sendError(var error) {
+    assert(!_isCanceled);
+    assert(!_isPaused);
+    assert(!_inCallback);
+    bool wasInputPaused = _isInputPaused;
+    _state |= _STATE_IN_CALLBACK;
+    try {
+      _onError(error);
+    } catch (e, s) {
+      _throwDelayed(e, s);
+    }
+    _state &= ~_STATE_IN_CALLBACK;
+    if (_cancelOnError) {
+      _cancel();
+    }
+    _checkState(wasInputPaused);
+  }
+
+  void _sendDone() {
+    assert(!_isCanceled);
+    assert(!_isPaused);
+    assert(!_inCallback);
+    _state |= (_STATE_CANCELED | _STATE_CLOSED | _STATE_IN_CALLBACK);
+    try {
+      _onDone();
+    } catch (e, s) {
+      _throwDelayed(e, s);
+    }
+    _onCancel();  // No checkState after cancel, it is always the last event.
+    _state &= ~_STATE_IN_CALLBACK;
+  }
+
+  /**
+   * Call a hook function.
+   *
+   * The call is properly wrapped in code to avoid other callbacks
+   * during the call, and it checks for state changes after the call
+   * that should cause further callbacks.
+   */
+  void _guardCallback(callback) {
+    assert(!_inCallback);
+    bool wasInputPaused = _isInputPaused;
+    _state |= _STATE_IN_CALLBACK;
+    callback();
+    _state &= ~_STATE_IN_CALLBACK;
+    _checkState(wasInputPaused);
+  }
+
+  /**
+   * Check if the input needs to be informed of state changes.
+   *
+   * State changes are pausing, resuming and canceling.
+   *
+   * After canceling, no further callbacks will happen.
+   *
+   * The cancel callback is called after a user cancel, or after
+   * the final done event is sent.
+   */
+  void _checkState(bool wasInputPaused) {
+    assert(!_inCallback);
+    if (_hasPending && _pending.isEmpty) {
+      _state &= ~_STATE_HAS_PENDING;
+      if (_isInputPaused && _mayResumeInput) {
+        _state &= ~_STATE_INPUT_PAUSED;
+      }
+    }
+    // If the state changes during a callback, we immediately
+    // make a new state-change callback. Loop until the state didn't change.
+    while (true) {
+      if (_isCanceled) {
+        _onCancel();
+        _pending = null;
+        return;
+      }
+      bool isInputPaused = _isInputPaused;
+      if (wasInputPaused == isInputPaused) break;
+      _state ^= _STATE_IN_CALLBACK;
+      if (isInputPaused) {
+        _onPause();
+      } else {
+        _onResume();
+      }
+      _state &= ~_STATE_IN_CALLBACK;
+      wasInputPaused = isInputPaused;
+    }
+    if (_hasPending && !_isPaused) {
+      _pending.schedule(this);
+    }
+  }
+}
 
 // -------------------------------------------------------------------
 // Common base class for single and multi-subscription streams.
 // -------------------------------------------------------------------
 abstract class _StreamImpl<T> extends Stream<T> {
-  /** Current state of the stream. */
-  int _state = _STREAM_OPEN;
-
-  /**
-   * List of pending events.
-   *
-   * If events are added to the stream (using [_add], [_addError] or [_done])
-   * while the stream is paused, or while another event is firing, events will
-   * stored here.
-   * Also supports scheduling the events for later execution.
-   */
-  _PendingEvents _pendingEvents;
-
   // ------------------------------------------------------------------
   // Stream interface.
 
   StreamSubscription<T> listen(void onData(T data),
                                { void onError(error),
                                  void onDone(),
                                  bool cancelOnError }) {
-    if (_isComplete) {
-      return new _DoneSubscription(onDone);
-    }
     if (onData == null) onData = _nullDataHandler;
     if (onError == null) onError = _nullErrorHandler;
     if (onDone == null) onDone = _nullDoneHandler;
     cancelOnError = identical(true, cancelOnError);
-    _StreamSubscriptionImpl subscription =
+    StreamSubscription subscription =
         _createSubscription(onData, onError, onDone, cancelOnError);
-    _addListener(subscription);
+    _onListen(subscription);
     return subscription;
   }
 
-  // ------------------------------------------------------------------
-  // EventSink interface-like methods for sending events into the stream.
-  // It's the responsibility of the caller to ensure that the stream is not
-  // paused when adding events. If the stream is paused, the events will be
-  // queued, but it's better to not send events at all.
-
-  /**
-   * Send or queue a data event.
-   */
-  void _add(T value) {
-    if (_isClosed) throw new StateError("Sending on closed stream");
-    if (!_mayFireState) {
-      // Not the time to send events.
-      _addPendingEvent(new _DelayedData<T>(value));
-      return;
-    }
-    if (_hasPendingEvent) {
-      _addPendingEvent(new _DelayedData<T>(value));
-    } else {
-      _sendData(value);
-    }
-    _handlePendingEvents();
-  }
-
-  /**
-   * Send or enqueue an error event.
-   *
-   * If a subscription has requested to be unsubscribed on errors,
-   * it will be unsubscribed after receiving this event.
-   */
-  void _addError(error) {
-    if (_isClosed) throw new StateError("Sending on closed stream");
-    if (!_mayFireState) {
-      // Not the time to send events.
-      _addPendingEvent(new _DelayedError(error));
-      return;
-    }
-    if (_hasPendingEvent) {
-      _addPendingEvent(new _DelayedError(error));
-    } else {
-      _sendError(error);
-    }
-    _handlePendingEvents();
-  }
-
-  /**
-   * Send or enqueue a "done" message.
-   *
-   * The "done" message should be sent at most once by a stream, and it
-   * should be the last message sent.
-   */
-  void _close() {
-    if (_isClosed) return;
-    _state |= _STREAM_CLOSED;
-    if (!_mayFireState) {
-      // Not the time to send events.
-      _addPendingEvent(const _DelayedDone());
-      return;
-    }
-    if (_hasPendingEvent) {
-      _addPendingEvent(new _DelayedDone());
-      _handlePendingEvents();
-    } else {
-      _sendDone();
-      assert(_isComplete);
-      assert(!_hasPendingEvent);
-    }
-  }
-
   // -------------------------------------------------------------------
-  // Internal implementation.
-
-  // State predicates.
-
-  // Lifecycle state.
-  /** Whether the stream is in the default, open, state for events. */
-  bool get _isOpen => (_state & (_STREAM_CLOSED | _STREAM_COMPLETE)) == 0;
-
-  /** Whether the stream has been closed (a done event requested). */
-  bool get _isClosed => (_state & _STREAM_CLOSED) != 0;
-
-  /** Whether the stream is completed. */
-  bool get _isComplete => (_state & _STREAM_COMPLETE) != 0;
-
-  // Pause state.
-
-  /** Whether one or more active subscribers have requested a pause. */
-  bool get _isPaused => _state >= (1 << _STREAM_PAUSE_COUNT_SHIFT);
-
-  /** How many times the stream has been paused. */
-  int get _pauseCount => _state >> _STREAM_PAUSE_COUNT_SHIFT;
-
-  /**
-   * Whether a controller thinks the stream is paused.
-   *
-   * When this changes, a pause-state change callback is performed.
-   *
-   * It may differ from [_isPaused] if there are pending events
-   * in the queue when the listeners resume. The controller won't
-   * be informed until all queued events have been fired.
-   */
-  bool get _isInputPaused => _state >= (_STREAM_PENDING_RESUME);
-
-  /** Whether we have a pending resume scheduled. */
-  bool get _hasPendingResume => (_state & _STREAM_PENDING_RESUME) != 0;
-
-
-  // Action state. If the stream makes a call-out to external code,
-  // this state tracks it and avoids reentrancy problems.
-
-  /** Whether the stream is not currently firing or calling a callback. */
-  bool get _isInactive => (_state & (_STREAM_CALLBACK | _STREAM_FIRING)) == 0;
-
-  /** Whether we are currently executing a state-chance callback. */
-  bool get _isInCallback => (_state & _STREAM_CALLBACK) != 0;
-
-  /** Whether we are currently firing an event. */
-  bool get _isFiring => (_state & _STREAM_FIRING) != 0;
-
-  /** Check whether the pending event queue is non-empty */
-  bool get _hasPendingEvent =>
-      _pendingEvents != null && !_pendingEvents.isEmpty;
-
-  /**
-   * The bit representing the current or last event fired.
-   *
-   * This bit matches a bit on listeners that have received the corresponding
-   * event. It is toggled for each new event being fired.
-   */
-  int get _currentEventIdBit =>
-      (_state & _STREAM_EVENT_ID ) >> _STREAM_EVENT_ID_SHIFT;
-
-  /** Whether there is currently a subscriber on this [Stream]. */
-  bool get _hasListener;
-
-
-  /** Whether the state bits allow firing. */
-  bool get _mayFireState {
-    // The state allows firing unless:
-    // - it's currently firing
-    // - it's currently in a callback
-    // - it's paused
-    const int mask =
-        _STREAM_FIRING |
-        _STREAM_CALLBACK |
-        ~((1 << _STREAM_PAUSE_COUNT_SHIFT) - 1);
-    return (_state & mask) == 0;
-  }
-
-  // State modification.
-
-  /** Record an increases in the number of times the listener has paused. */
-  void _incrementPauseCount(_StreamListener<T> listener) {
-    listener._incrementPauseCount();
-    _state &= ~_STREAM_PENDING_RESUME;
-    _updatePauseCount(1);
-  }
-
-  /** Record a decrease in the number of times the listener has paused. */
-  void _decrementPauseCount(_StreamListener<T> listener) {
-    assert(_isPaused);
-    listener._decrementPauseCount();
-    _updatePauseCount(-1);
-  }
-
-  /** Update the stream's own pause count only. */
-  void _updatePauseCount(int by) {
-    int oldState = _state;
-    // We can't just _state += by << _STREAM_PAUSE_COUNT_SHIFT, since dart2js
-    // converts the result of the left-shift to a positive number.
-    if (by >= 0) {
-      _state = oldState + (by << _STREAM_PAUSE_COUNT_SHIFT);
-    } else {
-      _state = oldState - ((-by) << _STREAM_PAUSE_COUNT_SHIFT);
-    }
-    assert(_state >= 0);
-    assert((_state >> _STREAM_PAUSE_COUNT_SHIFT) ==
-        (oldState >> _STREAM_PAUSE_COUNT_SHIFT) + by);
-  }
-
-  void _setClosed() {
-    assert(!_isClosed);
-    _state |= _STREAM_CLOSED;
-  }
-
-  void _setComplete() {
-    assert(_isClosed);
-    _state = _state |_STREAM_COMPLETE;
-  }
-
-  void _startFiring() {
-    assert(!_isFiring);
-    assert(!_isInCallback);
-    assert(_hasListener);
-    assert(!_isPaused);
-    // This sets the _STREAM_FIRING bit and toggles the _STREAM_EVENT_ID
-    // bit. All current subscribers will now have a _LISTENER_EVENT_ID
-    // that doesn't match _STREAM_EVENT_ID, and they will receive the
-    // event being fired.
-    _state ^= _STREAM_FIRING | _STREAM_EVENT_ID;
-  }
-
-  void _endFiring(bool wasInputPaused) {
-    assert(_isFiring);
-    _state ^= _STREAM_FIRING;
-    // Had listeners, or we wouldn't have fired.
-    _checkCallbacks(true, wasInputPaused);
-  }
-
-  /**
-   * Record that a listener wants a pause from events.
-   *
-   * This methods is called from [_StreamListener.pause()].
-   * Subclasses can override this method, along with [isPaused] and
-   * [createSubscription], if they want to do a different handling of paused
-   * subscriptions, e.g., a filtering stream pausing its own source if all its
-   * subscribers are paused.
-   */
-  void _pause(_StreamListener<T> listener, Future resumeSignal) {
-    assert(identical(listener._source, this));
-    if (!listener._isSubscribed) {
-      throw new StateError("Subscription has been canceled.");
-    }
-    assert(!_isComplete);  // There can be no subscribers when complete.
-    bool wasInputPaused = _isInputPaused;
-    bool wasPaused = _isPaused;
-    _incrementPauseCount(listener);
-    if (resumeSignal != null) {
-      resumeSignal.whenComplete(() { this._resume(listener, true); });
-    }
-    if (!wasPaused && _hasPendingEvent && _pendingEvents.isScheduled) {
-      _pendingEvents.cancelSchedule();
-    }
-    if (_isInactive && !wasInputPaused) {
-      _checkCallbacks(true, false);
-      if (!_isPaused && _hasPendingEvent) {
-        _schedulePendingEvents();
-      }
-    }
-  }
-
-  /** Stops pausing due to one request from the given listener. */
-  void _resume(_StreamListener<T> listener, bool fromEvent) {
-    if (!listener.isPaused) return;
-    assert(listener._isSubscribed);
-    assert(_isPaused);
-    _decrementPauseCount(listener);
-    if (!_isPaused) {
-      if (_hasPendingEvent) {
-        _state |= _STREAM_PENDING_RESUME;
-        // Controller's pause state hasn't changed.
-        // If we can fire events now, fire any pending events right away.
-        if (_isInactive) {
-          if (fromEvent) {
-            _handlePendingEvents();
-          } else {
-            _schedulePendingEvents();
-          }
-        }
-      } else if (_isInactive) {
-        _checkCallbacks(true, true);
-        if (!_isPaused && _hasPendingEvent) {
-          if (fromEvent) {
-            _handlePendingEvents();
-          } else {
-            _schedulePendingEvents();
-          }
-        }
-      }
-    }
-  }
-
-  /** Schedule pending events to be executed. */
-  void _schedulePendingEvents() {
-    assert(_hasPendingEvent);
-    _pendingEvents.schedule(this);
-  }
-
   /** Create a subscription object. Called by [subcribe]. */
-  _StreamSubscriptionImpl<T> _createSubscription(
-      void onData(T data),
-      void onError(error),
-      void onDone(),
-      bool cancelOnError);
-
-  /**
-   * Adds a listener to this stream.
-   */
-  void _addListener(_StreamSubscriptionImpl subscription);
-
-  /**
-   * Handle a cancel requested from a [_StreamSubscriptionImpl].
-   *
-   * This method is called from [_StreamSubscriptionImpl.cancel].
-   *
-   * If an event is currently firing, the cancel is delayed
-   * until after the subscribers have received the event.
-   */
-  void _cancel(_StreamSubscriptionImpl subscriber);
-
-  /**
-   * Iterate over all current subscribers and perform an action on each.
-   *
-   * Subscribers added during the iteration will not be visited.
-   * Subscribers unsubscribed during the iteration will only be removed
-   * after they have been acted on.
-   *
-   * Any change in the pause state is only reported after all subscribers have
-   * received the event.
-   *
-   * The [action] must not throw, or the controller will be left in an
-   * invalid state.
-   *
-   * This method must not be called while [isFiring] is true.
-   */
-  void _forEachSubscriber(void action(_StreamSubscriptionImpl<T> subscription));
-
-  /**
-   * Checks whether the subscription/pause state has changed.
-   *
-   * Calls the appropriate callback if the state has changed from the
-   * provided one. Repeats calling callbacks as long as the call changes
-   * the state.
-   */
-  void _checkCallbacks(bool hadListener, bool wasPaused) {
-    assert(!_isFiring);
-    // Will be handled after the current callback.
-    if (_isInCallback) return;
-    if (_hasPendingResume && !_hasPendingEvent) {
-      _state ^= _STREAM_PENDING_RESUME;
-    }
-    _state |= _STREAM_CALLBACK;
-    while (true) {
-      bool hasListener = _hasListener;
-      bool isPaused = _isInputPaused;
-      if (hadListener != hasListener) {
-        _onSubscriptionStateChange();
-      } else if (isPaused != wasPaused) {
-        _onPauseStateChange();
-      } else {
-        _state ^= _STREAM_CALLBACK;
-        return;
-      }
-      wasPaused = isPaused;
-      hadListener = hasListener;
-    }
-  }
-
-  /**
-   * Called when the first subscriber requests a pause or the last a resume.
-   *
-   * Read [isPaused] to see the new state.
-   */
-  void _onPauseStateChange() {}
-
-  /**
-   * Called when the first listener subscribes or the last unsubscribes.
-   *
-   * Read [hasListener] to see what the new state is.
-   */
-  void _onSubscriptionStateChange() {}
-
-  /**
-   * Add a pending event at the end of the pending event queue.
-   *
-   * Schedules events if currently not paused and inside a callback.
-   */
-  void _addPendingEvent(_DelayedEvent event) {
-    if (_pendingEvents == null) _pendingEvents = new _StreamImplEvents();
-    _StreamImplEvents events = _pendingEvents;
-    events.add(event);
-    if (_isPaused || _isFiring) return;
-    if (_isInCallback) {
-      _schedulePendingEvents();
-      return;
-    }
-  }
-
-  /** Fire any pending events until the pending event queue is empty. */
-  void _handlePendingEvents() {
-    assert(_isInactive);
-    if (!_hasPendingEvent) return;
-    _PendingEvents events = _pendingEvents;
-    do {
-      if (_isPaused) return;
-      if (events.isScheduled) events.cancelSchedule();
-      events.handleNext(this);
-    } while (!events.isEmpty);
-  }
-
-  /**
-   * Send a data event directly to each subscriber.
-   */
-  _sendData(T value) {
-    assert(!_isPaused);
-    assert(!_isComplete);
-    if (!_hasListener) return;
-    _forEachSubscriber((subscriber) {
-      try {
-        subscriber._sendData(value);
-      } catch (e, s) {
-        _throwDelayed(e, s);
-      }
-    });
-  }
-
-  /**
-   * Sends an error event directly to each subscriber.
-   */
-  void _sendError(error) {
-    assert(!_isPaused);
-    assert(!_isComplete);
-    if (!_hasListener) return;
-    _forEachSubscriber((subscriber) {
-      try {
-        subscriber._sendError(error);
-      } catch (e, s) {
-        _throwDelayed(e, s);
-      }
-    });
-  }
-
-  /**
-   * Sends the "done" message directly to each subscriber.
-   * This automatically stops further subscription and
-   * unsubscribes all subscribers.
-   */
-  void _sendDone() {
-    assert(!_isPaused);
-    assert(_isClosed);
-    _setComplete();
-    if (!_hasListener) return;
-    _forEachSubscriber((subscriber) {
-      _cancel(subscriber);
-      try {
-        subscriber._sendDone();
-      } catch (e, s) {
-        _throwDelayed(e, s);
-      }
-    });
-    assert(!_hasListener);
-  }
-}
-
-// -------------------------------------------------------------------
-// Default implementation of a stream with a single subscriber.
-// -------------------------------------------------------------------
-/**
- * Default implementation of stream capable of sending events to one subscriber.
- *
- * Any class needing to implement [Stream] can either directly extend this
- * class, or extend [Stream] and delegate the subscribe method to an instance
- * of this class.
- *
- * The only public methods are those of [Stream], so instances of
- * [_SingleStreamImpl] can be returned directly as a [Stream] without exposing
- * internal functionality.
- *
- * The [StreamController] is a public facing version of this class, with
- * some methods made public.
- *
- * The user interface of [_SingleStreamImpl] are the following methods:
- * * [_add]: Add a data event to the stream.
- * * [_addError]: Add an error event to the stream.
- * * [_close]: Request to close the stream.
- * * [_onSubscriberStateChange]: Called when receiving the first subscriber or
- *                               when losing the last subscriber.
- * * [_onPauseStateChange]: Called when entering or leaving paused mode.
- * * [_hasListener]: Test whether there are currently any subscribers.
- * * [_isInputPaused]: Test whether the stream is currently paused.
- * The user should not add new events while the stream is paused, but if it
- * happens anyway, the stream will enqueue the events just as when new events
- * arrive while still firing an old event.
- */
-class _SingleStreamImpl<T> extends _StreamImpl<T> {
-  _StreamListener _subscriber = null;
-
-  /** Whether there is currently a subscriber on this [Stream]. */
-  bool get _hasListener => _subscriber != null;
-
-  // -------------------------------------------------------------------
-  // Internal implementation.
-
-  _SingleStreamImpl() {
-    // Start out paused.
-    _updatePauseCount(1);
-  }
-
-  /**
-   * Create the new subscription object.
-   */
-  _StreamSubscriptionImpl<T> _createSubscription(
+  _BufferingStreamSubscription<T> _createSubscription(
       void onData(T data),
       void onError(error),
       void onDone(),
       bool cancelOnError) {
-    return new _StreamSubscriptionImpl<T>(
-        this, onData, onError, onDone, cancelOnError);
-  }
-
-  void _addListener(_StreamListener subscription) {
-    assert(!_isComplete);
-    if (_hasListener) {
-      throw new StateError("Stream already has subscriber.");
-    }
-    assert(_pauseCount == 1);
-    _updatePauseCount(-1);
-    _subscriber = subscription;
-    subscription._setSubscribed(0);
-    if (_isInactive) {
-      _checkCallbacks(false, true);
-      if (!_isPaused && _hasPendingEvent) {
-        _schedulePendingEvents();
-      }
-    }
-  }
-
-  /**
-   * Handle a cancel requested from a [_StreamSubscriptionImpl].
-   *
-   * This method is called from [_StreamSubscriptionImpl.cancel].
-   */
-  void _cancel(_StreamListener subscriber) {
-    assert(identical(subscriber._source, this));
-    // We allow unsubscribing the currently firing subscription during
-    // the event firing, because it is indistinguishable from delaying it since
-    // that event has already received the event.
-    if (!identical(_subscriber, subscriber)) {
-      // You may unsubscribe more than once, only the first one counts.
-      return;
-    }
-    _subscriber = null;
-    // Unsubscribing a paused subscription also cancels its pauses.
-    int resumeCount = subscriber._setUnsubscribed();
-    // Keep being paused while there is no subscriber and the stream is not
-    // complete.
-    _updatePauseCount(_isComplete ? -resumeCount : -resumeCount + 1);
-    if (_isInactive) {
-      _checkCallbacks(true, resumeCount > 0);
-      if (!_isPaused && _hasPendingEvent) {
-        _schedulePendingEvents();
-      }
-    }
-  }
-
-  void _forEachSubscriber(
-      void action(_StreamListener<T> subscription)) {
-    assert(!_isPaused);
-    bool wasInputPaused = _isInputPaused;
-    _StreamListener subscription = _subscriber;
-    assert(subscription != null);
-    _startFiring();
-    action(subscription);
-    _endFiring(wasInputPaused);
-  }
-}
-
-// -------------------------------------------------------------------
-// Default implementation of a stream with subscribers.
-// -------------------------------------------------------------------
-
-/**
- * Default implementation of stream capable of sending events to subscribers.
- *
- * Any class needing to implement [Stream] can either directly extend this
- * class, or extend [Stream] and delegate the subscribe method to an instance
- * of this class.
- *
- * The only public methods are those of [Stream], so instances of
- * [_MultiStreamImpl] can be returned directly as a [Stream] without exposing
- * internal functionality.
- *
- * The [StreamController] is a public facing version of this class, with
- * some methods made public.
- *
- * The user interface of [_MultiStreamImpl] are the following methods:
- * * [_add]: Add a data event to the stream.
- * * [_addError]: Add an error event to the stream.
- * * [_close]: Request to close the stream.
- * * [_onSubscriptionStateChange]: Called when receiving the first subscriber or
- *                                 when losing the last subscriber.
- * * [_onPauseStateChange]: Called when entering or leaving paused mode.
- * * [_hasListener]: Test whether there are currently any subscribers.
- * * [_isPaused]: Test whether the stream is currently paused.
- * The user should not add new events while the stream is paused, but if it
- * happens anyway, the stream will enqueue the events just as when new events
- * arrive while still firing an old event.
- */
-class _MultiStreamImpl<T> extends _StreamImpl<T>
-                          implements _InternalLinkList {
-  // Link list implementation (mixin when possible).
-  _InternalLink _nextLink;
-  _InternalLink _previousLink;
-
-  _MultiStreamImpl() {
-    _nextLink = _previousLink = this;
-  }
-
-  bool get isBroadcast => true;
-
-  Stream<T> asBroadcastStream() => this;
-
-  // ------------------------------------------------------------------
-  // Helper functions that can be overridden in subclasses.
-
-  /** Whether there are currently any subscribers on this [Stream]. */
-  bool get _hasListener => !_InternalLinkList.isEmpty(this);
-
-  /**
-   * Create the new subscription object.
-   */
-  _StreamListener<T> _createSubscription(
-      void onData(T data),
-      void onError(error),
-      void onDone(),
-      bool cancelOnError) {
-    return new _StreamSubscriptionImpl<T>(
-        this, onData, onError, onDone, cancelOnError);
-  }
-
-  // -------------------------------------------------------------------
-  // Internal implementation.
-
-  /**
-   * Iterate over all current subscribers and perform an action on each.
-   *
-   * The set of subscribers cannot be modified during this iteration.
-   * All attempts to add or unsubscribe subscribers will be delayed until
-   * after the iteration is complete.
-   *
-   * The [action] must not throw, or the controller will be left in an
-   * invalid state.
-   *
-   * This method must not be called while [isFiring] is true.
-   */
-  void _forEachSubscriber(
-      void action(_StreamListener<T> subscription)) {
-    assert(!_isFiring);
-    if (!_hasListener) return;
-    bool wasInputPaused = _isInputPaused;
-    _startFiring();
-    _InternalLink cursor = this._nextLink;
-    while (!identical(cursor, this)) {
-      _StreamListener<T> current = cursor;
-      if (current._needsEvent(_currentEventIdBit)) {
-        action(current);
-        // Marks as having received the event.
-        current._toggleEventReceived();
-      }
-      cursor = current._nextLink;
-      if (current._isPendingUnsubscribe) {
-        _removeListener(current);
-      }
-    }
-    _endFiring(wasInputPaused);
-  }
-
-  void _addListener(_StreamListener listener) {
-    listener._setSubscribed(_currentEventIdBit);
-    bool hadListener = _hasListener;
-    _InternalLinkList.add(this, listener);
-    if (!hadListener && _isInactive) {
-      _checkCallbacks(false, false);
-      if (!_isPaused && _hasPendingEvent) {
-        _schedulePendingEvents();
-      }
-    }
-  }
-
-  /**
-   * Handle a cancel requested from a [_StreamListener].
-   *
-   * This method is called from [_StreamListener.cancel].
-   *
-   * If an event is currently firing, the cancel is delayed
-   * until after the subscribers have received the event.
-   */
-  void _cancel(_StreamListener listener) {
-    assert(identical(listener._source, this));
-    if (_InternalLink.isUnlinked(listener)) {
-      // You may unsubscribe more than once, only the first one counts.
-      return;
-    }
-    if (_isFiring) {
-      if (listener._needsEvent(_currentEventIdBit)) {
-        assert(listener._isSubscribed);
-        listener._setPendingUnsubscribe(_currentEventIdBit);
-      } else {
-        // The listener has been notified of the event (or don't need to,
-        // if it's still pending subscription) so it's safe to remove it.
-        _removeListener(listener);
-      }
-      // Pause and subscription state changes are reported when we end
-      // firing.
-    } else {
-      bool wasInputPaused = _isInputPaused;
-      _removeListener(listener);
-      if (_isInactive) {
-        _checkCallbacks(true, wasInputPaused);
-        if (!_isPaused && _hasPendingEvent) {
-          _schedulePendingEvents();
-        }
-      }
-    }
-  }
-
-  /**
-   * Removes a listener from this stream and cancels its pauses.
-   *
-   * This is a low-level action that doesn't call [_onSubscriptionStateChange].
-   * or [_callOnPauseStateChange].
-   */
-  void _removeListener(_StreamListener listener) {
-    int pauseCount = listener._setUnsubscribed();
-    _InternalLinkList.remove(listener);
-    if (pauseCount > 0) {
-      _updatePauseCount(-pauseCount);
-      if (!_isPaused && _hasPendingEvent) {
-        _state |= _STREAM_PENDING_RESUME;
-      }
-    }
-  }
-}
-
+    return new _BufferingStreamSubscription<T>(
+        onData, onError, onDone, cancelOnError);
+  }
+
+  /** Hook called when the subscription has been created. */
+  void _onListen(StreamSubscription subscription) {}
+}
+
+typedef _PendingEvents _EventGenerator();
 
 /** Stream that generates its own events. */
-class _GeneratedSingleStreamImpl<T> extends _SingleStreamImpl<T> {
-  /**
-   * Initializes the stream to have only the events provided by [events].
-   *
-   * A [_PendingEvents] implementation provides events that are handled
-   * by calling [_PendingEvents.handleNext] with the [_StreamImpl].
-   */
-  _GeneratedSingleStreamImpl(_PendingEvents events) {
-    _pendingEvents = events;
-    _setClosed();  // Closed for input since all events are already pending.
-  }
-
-  void _add(T value) {
-    throw new UnsupportedError("Cannot inject events into generated stream");
-  }
-
-  void _addError(value) {
-    throw new UnsupportedError("Cannot inject events into generated stream");
-  }
-
-  void _close() {
-    throw new UnsupportedError("Cannot inject events into generated stream");
+class _GeneratedStreamImpl<T> extends _StreamImpl<T> {
+  final _EventGenerator _pending;
+  /**
+   * Initializes the stream to have only the events provided by a
+   * [_PendingEvents].
+   *
+   * A new [_PendingEvents] must be generated for each listen.
+   */
+  _GeneratedStreamImpl(this._pending);
+
+  StreamSubscription _createSubscription(void onData(T data),
+                                         void onError(Object error),
+                                         void onDone(),
+                                         bool cancelOnError) {
+    _BufferingStreamSubscription<T> subscription =
+         new _BufferingStreamSubscription(
+             onData, onError, onDone, cancelOnError);
+    subscription._setPendingEvents(_pending());
+    return subscription;
   }
 }
 
 
 /** Pending events object that gets its events from an [Iterable]. */
 class _IterablePendingEvents<T> extends _PendingEvents {
-  final Iterator<T> _iterator;
-  /**
-   * Whether there are no more events to be sent.
-   *
-   * This starts out as [:false:] since there is always at least
-   * a 'done' event to be sent.
-   */
-  bool _isDone = false;
+  // The iterator providing data for data events.
+  // Set to null when iteration has completed.
+  Iterator<T> _iterator;
 
   _IterablePendingEvents(Iterable<T> data) : _iterator = data.iterator;
 
-  bool get isEmpty => _isDone;
-
-  void handleNext(_StreamImpl<T> stream) {
-    if (_isDone) throw new StateError("No events pending.");
+  bool get isEmpty => _iterator == null;
+
+  void handleNext(_EventDispatch dispatch) {
+    if (_iterator == null) {
+      throw new StateError("No events pending.");
+    }
+    // Send one event per call to moveNext.
+    // If moveNext returns true, send the current element as data.
+    // If moveNext returns false, send a done event and clear the _iterator.
+    // If moveNext throws an error, send an error and clear the _iterator.
+    // After an error, no further events will be sent.
+    bool isDone;
     try {
-      _isDone = !_iterator.moveNext();
-      if (!_isDone) {
-        stream._sendData(_iterator.current);
-      } else {
-        stream._sendDone();
-      }
+      isDone = !_iterator.moveNext();
     } catch (e, s) {
-      stream._sendError(_asyncError(e, s));
-      stream._sendDone();
-      _isDone = true;
-    }
-  }
-}
-
-
-/**
- * The subscription class that the [StreamController] uses.
- *
- * The [_StreamImpl.createSubscription] method should
- * create an object of this type, or another subclass of [_StreamListener].
- * A subclass of [_StreamImpl] can specify which subclass
- * of [_StreamSubscriptionImpl] it uses by overriding
- * [_StreamImpl.createSubscription].
- *
- * The subscription is in one of three states:
- * * Subscribed.
- * * Paused-and-subscribed.
- * * Unsubscribed.
- * Unsubscribing also resumes any pauses started by the subscription.
- */
-class _StreamSubscriptionImpl<T> extends _StreamListener<T>
-                                 implements StreamSubscription<T> {
-  final bool _cancelOnError;
-  // TODO(ahe): Restore type when feature is implemented in dart2js
-  // checked mode. http://dartbug.com/7733
-  var /* _DataHandler<T> */ _onData;
-  _ErrorHandler _onError;
-  _DoneHandler _onDone;
-  _StreamSubscriptionImpl(_StreamImpl source,
-                          this._onData,
-                          this._onError,
-                          this._onDone,
-                          this._cancelOnError) : super(source);
-
-  void onData(void handleData(T event)) {
-    if (handleData == null) handleData = _nullDataHandler;
-    _onData = handleData;
-  }
-
-  void onError(void handleError(error)) {
-    if (handleError == null) handleError = _nullErrorHandler;
-    _onError = handleError;
-  }
-
-  void onDone(void handleDone()) {
-    if (handleDone == null) handleDone = _nullDoneHandler;
-    _onDone = handleDone;
-  }
-
-  void _sendData(T data) {
-    _onData(data);
-  }
-
-  void _sendError(error) {
-    _onError(error);
-    if (_cancelOnError) _source._cancel(this);
-  }
-
-  void _sendDone() {
-    _onDone();
-  }
-
-  void cancel() {
-    if (!_isSubscribed) return;
-    _source._cancel(this);
-  }
-
-  void pause([Future resumeSignal]) {
-    if (!_isSubscribed) return;
-    _source._pause(this, resumeSignal);
-  }
-
-  void resume() {
-    if (!_isSubscribed || !isPaused) return;
-    _source._resume(this, false);
-  }
-
-  Future asFuture([var futureValue]) {
-    _FutureImpl<T> result = new _FutureImpl<T>();
-
-    // Overwrite the onDone and onError handlers.
-    onDone(() { result._setValue(futureValue); });
-    onError((error) {
-      cancel();
-      result._setError(error);
-    });
-
-    return result;
-  }
-}
+      _iterator = null;
+      dispatch._sendError(_asyncError(e, s));
+      return;
+    }
+    if (!isDone) {
+      dispatch._sendData(_iterator.current);
+    } else {
+      _iterator = null;
+      dispatch._sendDone();
+    }
+  }
+
+  void clear() {
+    if (isScheduled) cancelSchedule();
+    _iterator = null;
+  }
+}
+
 
 // Internal helpers.
 
 // Types of the different handlers on a stream. Types used to type fields.
 typedef void _DataHandler<T>(T value);
 typedef void _ErrorHandler(error);
 typedef void _DoneHandler();
 
 
 /** Default data handler, does nothing. */
 void _nullDataHandler(var value) {}
 
 /** Default error handler, reports the error to the global handler. */
 void _nullErrorHandler(error) {
   _throwDelayed(error);
 }
 
 /** Default done handler, does nothing. */
 void _nullDoneHandler() {}
 
 
-/** A delayed event on a stream implementation. */
+/** A delayed event on a buffering stream subscription. */
 abstract class _DelayedEvent {
   /** Added as a linked list on the [StreamController]. */
   _DelayedEvent next;
   /** Execute the delayed event on the [StreamController]. */
-  void perform(_StreamImpl stream);
+  void perform(_EventDispatch dispatch);
 }
 
 /** A delayed data event. */
 class _DelayedData<T> extends _DelayedEvent{
   final T value;
   _DelayedData(this.value);
-  void perform(_StreamImpl<T> stream) {
-    stream._sendData(value);
+  void perform(_EventDispatch<T> dispatch) {
+    dispatch._sendData(value);
   }
 }
 
 /** A delayed error event. */
 class _DelayedError extends _DelayedEvent {
   final error;
   _DelayedError(this.error);
-  void perform(_StreamImpl stream) {
-    stream._sendError(error);
+  void perform(_EventDispatch dispatch) {
+    dispatch._sendError(error);
   }
 }
 
 /** A delayed done event. */
 class _DelayedDone implements _DelayedEvent {
   const _DelayedDone();
-  void perform(_StreamImpl stream) {
-    stream._sendDone();
+  void perform(_EventDispatch dispatch) {
+    dispatch._sendDone();
   }
 
   _DelayedEvent get next => null;
 
   void set next(_DelayedEvent _) {
     throw new StateError("No events after a done.");
   }
 }
 
 /**
@@ skipping 69 matching lines @@
     listLast._nextLink = otherNext;
     otherNext._previousLink = listLast;
     _InternalLink otherLast = other._previousLink;
     list._previousLink = otherLast;
     otherLast._nextLink = list;
     // Clean up [other].
     other._nextLink = other._previousLink = other;
   }
 }
 
-/** Abstract type for an internal interface for sending events. */
-abstract class _EventOutputSink<T> {
-  _sendData(T data);
-  _sendError(error);
-  _sendDone();
-}
-
-abstract class _StreamListener<T> extends _InternalLink
-                                  implements _EventOutputSink<T> {
-  final _StreamImpl _source;
-  int _state = _LISTENER_UNSUBSCRIBED;
-
-  _StreamListener(this._source);
-
-  bool get isPaused => _state >= (1 << _LISTENER_PAUSE_COUNT_SHIFT);
-
-  bool get _isPendingUnsubscribe =>
-      (_state & _LISTENER_PENDING_UNSUBSCRIBE) != 0;
-
-  bool get _isSubscribed => (_state & _LISTENER_SUBSCRIBED) != 0;
-
-  /**
-   * Whether the listener still needs to receive the currently firing event.
-   *
-   * The currently firing event is identified by a single bit, which alternates
-   * between events. The [_state] contains the previously sent event's bit in
-   * the [_LISTENER_EVENT_ID] bit. If the two don't match, this listener
-   * still need the current event.
-   */
-  bool _needsEvent(int currentEventIdBit) {
-    int lastEventIdBit =
-        (_state & _LISTENER_EVENT_ID) >> _LISTENER_EVENT_ID_SHIFT;
-    return lastEventIdBit != currentEventIdBit;
-  }
-
-  /// If a subscriber's "firing bit" doesn't match the stream's firing bit,
-  /// we are currently firing an event and the subscriber still need to receive
-  /// the event.
-  void _toggleEventReceived() {
-    _state ^= _LISTENER_EVENT_ID;
-  }
-
-  void _setSubscribed(int eventIdBit) {
-    assert(eventIdBit == 0 || eventIdBit == 1);
-    _state = _LISTENER_SUBSCRIBED | (eventIdBit << _LISTENER_EVENT_ID_SHIFT);
-  }
-
-  void _setPendingUnsubscribe(int currentEventIdBit) {
-    assert(_isSubscribed);
-    // Sets the pending unsubscribe, and ensures that the listener
-    // won't get the current event.
-    _state |= _LISTENER_PENDING_UNSUBSCRIBE | _LISTENER_EVENT_ID;
-    _state ^= (1 ^ currentEventIdBit) << _LISTENER_EVENT_ID_SHIFT;
-    assert(!_needsEvent(currentEventIdBit));
-  }
-
-  /**
-   * Marks the listener as unsubscibed.
-   *
-   * Returns the number of unresumed pauses for the listener.
-   */
-  int _setUnsubscribed() {
-    assert(_isSubscribed);
-    int timesPaused = _state >> _LISTENER_PAUSE_COUNT_SHIFT;
-    _state = _LISTENER_UNSUBSCRIBED;
-    return timesPaused;
-  }
-
-  void _incrementPauseCount() {
-    _state += 1 << _LISTENER_PAUSE_COUNT_SHIFT;
-  }
-
-  void _decrementPauseCount() {
-    assert(isPaused);
-    _state -= 1 << _LISTENER_PAUSE_COUNT_SHIFT;
-  }
-
-  _sendData(T data);
-  _sendError(error);
-  _sendDone();
-}
-
 /** Superclass for provider of pending events. */
 abstract class _PendingEvents {
+  // No async event has been scheduled.
+  static const int _STATE_UNSCHEDULED = 0;
+  // An async event has been scheduled to run a function.
+  static const int _STATE_SCHEDULED = 1;
+  // An async event has been scheduled, but it will do nothing when it runs.
+  // Async events can't be preempted.
+  static const int _STATE_CANCELED = 3;
+
   /**
-   * Timer set when pending events are scheduled for execution.
+   * State of being scheduled.
    *
-   * When scheduling pending events for execution in a later cycle, the timer
-   * is stored here. If pending events are executed earlier than that, e.g.,
-   * due to a second event in the current cycle, the timer is canceled again.
+   * Set to [_STATE_SCHEDULED] when pending events are scheduled for
+   * async dispatch. Since we can't cancel a [runAsync] call, if schduling
+   * is "canceled", the _state is simply set to [_STATE_CANCELED] which will
+   * make the async code do nothing except resetting [_state].
+   *
+   * If events are scheduled while the state is [_STATE_CANCELED], it is
+   * merely switched back to [_STATE_SCHEDULED], but no new call to [runAsync]
+   * is performed.
    */
-  Timer scheduleTimer = null;
+  int _state = _STATE_UNSCHEDULED;
 
   bool get isEmpty;
 
-  bool get isScheduled => scheduleTimer != null;
+  bool get isScheduled => _state == _STATE_SCHEDULED;
+  bool get _eventScheduled => _state >= _STATE_SCHEDULED;
 
-  void schedule(_StreamImpl stream) {
+  /**
+   * Schedule an event to run later.
+   *
+   * If called more than once, it should be called with the same dispatch as
+   * argument each time. It may reuse an earlier argument in some cases.
+   */
+  void schedule(_EventDispatch dispatch) {
     if (isScheduled) return;
-    scheduleTimer = new Timer(Duration.ZERO, () {
-      scheduleTimer = null;
-      stream._handlePendingEvents();
+    assert(!isEmpty);
+    if (_eventScheduled) {
+      assert(_state == _STATE_CANCELED);
+      _state = _STATE_SCHEDULED;
+      return;
+    }
+    runAsync(() {
+      int oldState = _state;
+      _state = _STATE_UNSCHEDULED;
+      if (oldState == _STATE_CANCELED) return;
+      handleNext(dispatch);
     });
+    _state = _STATE_SCHEDULED;
   }
 
   void cancelSchedule() {
-    assert(isScheduled);
-    scheduleTimer.cancel();
-    scheduleTimer = null;
+    if (isScheduled) _state = _STATE_CANCELED;
   }
 
-  void handleNext(_StreamImpl stream);
+  void handleNext(_EventDispatch dispatch);
+
+  /** Throw away any pending events and cancel scheduled events. */
+  void clear();
 }
 
 
 /** Class holding pending events for a [_StreamImpl]. */
 class _StreamImplEvents extends _PendingEvents {
   /// Single linked list of [_DelayedEvent] objects.
   _DelayedEvent firstPendingEvent = null;
   /// Last element in the list of pending events. New events are added after it.
   _DelayedEvent lastPendingEvent = null;
 
   bool get isEmpty => lastPendingEvent == null;
 
-  bool get isScheduled => scheduleTimer != null;
-
   void add(_DelayedEvent event) {
     if (lastPendingEvent == null) {
       firstPendingEvent = lastPendingEvent = event;
     } else {
       lastPendingEvent = lastPendingEvent.next = event;
     }
   }
 
-  void handleNext(_StreamImpl stream) {
+  void handleNext(_EventDispatch dispatch) {
     assert(!isScheduled);
     _DelayedEvent event = firstPendingEvent;
     firstPendingEvent = event.next;
     if (firstPendingEvent == null) {
       lastPendingEvent = null;
     }
-    event.perform(stream);
+    event.perform(dispatch);
+  }
+
+  void clear() {
+    if (isScheduled) cancelSchedule();
+    firstPendingEvent = lastPendingEvent = null;
   }
 }
 
-
-class _DoneSubscription<T> implements StreamSubscription<T> {
-  _DoneHandler _handler;
-  Timer _timer;
-  int _pauseCount = 0;
-
-  _DoneSubscription(this._handler) {
-    _delayDone();
-  }
-
-  void _delayDone() {
-    assert(_timer == null && _pauseCount == 0);
-    _timer = new Timer(Duration.ZERO, () {
-      if (_handler != null) _handler();
-    });
-  }
-
-  bool get _isComplete => _timer == null && _pauseCount == 0;
-
-  void onData(void handleAction(T value)) {}
-
-  void onError(void handleError(error)) {}
-
-  void onDone(void handleDone()) {
-    _handler = handleDone;
-  }
-
-  void pause([Future signal]) {
-    if (_isComplete) return;
-    if (_timer != null) {
-      _timer.cancel();
-      _timer = null;
-    }
-    _pauseCount++;
-    if (signal != null) signal.whenComplete(resume);
-  }
-
-  void resume() {
-    if (_isComplete) return;
-    if (_pauseCount == 0) return;
-    _pauseCount--;
-    if (_pauseCount == 0) {
-      _delayDone();
-    }
-  }
-
-  bool get isPaused => _pauseCount > 0;
-
-  void cancel() {
-    if (_isComplete) return;
-    if (_timer != null) {
-      _timer.cancel();
-      _timer = null;
-    }
-    _pauseCount = 0;
-  }
-
-  Future asFuture([var futureValue]) {
-    // TODO(floitsch): share more code.
-    _FutureImpl<T> result = new _FutureImpl<T>();
-
-    // Overwrite the onDone and onError handlers.
-    onDone(() { result._setValue(futureValue); });
-    onError((error) {
-      cancel();
-      result._setError(error);
-    });
-
-    return result;
+class _MultiplexerLinkedList {
+  _MultiplexerLinkedList _next;
+  _MultiplexerLinkedList _previous;
+
+  void _unlink() {
+    _previous._next = _next;
+    _next._previous = _previous;
+    _next = _previous = this;
+  }
+
+  void _insertBefore(_MultiplexerLinkedList newNext) {
+    _MultiplexerLinkedList newPrevious = newNext._previous;
+    newPrevious._next = this;
+    newNext._previous = _previous;
+    _previous._next = newNext;
+    _previous = newPrevious;
   }
 }
 
-class _SingleStreamMultiplexer<T> extends _MultiStreamImpl<T> {
+/**
+ * A subscription used by [_SingleStreamMultiplexer].
+ *
+ * The [_SingleStreamMultiplexer] is a [Stream] which allows multiple
+ * listeners at a time. It is used to implement [Stream.asBroadcastStream].
+ *
+ * It is itself listening to another stream for events, and it forwards all
+ * events to all of its simultanous listeners.
+ *
+ * The listeners are [_MultiplexerSubscription]s and are kept as a linked list.
+ */
+// TODO(lrn): Change "implements" to "with" when automatic mixin constructors
+//            are implemented.
+class _MultiplexerSubscription<T> extends _BufferingStreamSubscription<T>
+                                  implements _MultiplexerLinkedList {
+  static const int _STATE_NOT_LISTENING = 0;
+  // Bit that alternates between event firings. If bit matches the one currently
+  // firing, the subscription will not be notified.
+  static const int _STATE_EVENT_ID_BIT = 1;
+  // Whether the subscription is listening at all. This should be set while
+  // it is part of the linked list of listeners of a multiplexer stream.
+  static const int _STATE_LISTENING = 2;
+  // State bit set while firing an event.
+  static const int _STATE_IS_FIRING = 4;
+  // Bit set if a subscription is canceled while it's firing (the
+  // [_STATE_IS_FIRING] bit is set).
+  // If the subscription is canceled while firing, it is not removed from the
+  // linked list immediately (to avoid breaking iteration), but is instead
+  // removed after it is done firing.
+  static const int _STATE_REMOVE_AFTER_FIRING = 8;
+
+  // Firing state.
+  int _multiplexState;
+
+  _SingleStreamMultiplexer _source;
+
+  _MultiplexerSubscription(this._source,
+                           void onData(T data),
+                           void onError(Object error),
+                           void onDone(),
+                           bool cancelOnError,
+                           int nextEventId)
+      : _multiplexState = _STATE_LISTENING | nextEventId,
+        super(onData, onError, onDone, cancelOnError) {
+    _next = _previous = this;
+  }
+
+  // Mixin workaround.
+  _MultiplexerLinkedList _next;
+  _MultiplexerLinkedList _previous;
+
+  void _unlink() {
+    _previous._next = _next;
+    _next._previous = _previous;
+    _next = _previous = this;
+  }
+
+  void _insertBefore(_MultiplexerLinkedList newNext) {
+    _MultiplexerLinkedList newPrevious = newNext._previous;
+    newPrevious._next = this;
+    newNext._previous = _previous;
+    _previous._next = newNext;
+    _previous = newPrevious;
+  }
+  // End mixin.
+
+  bool get _isListening => _multiplexState >= _STATE_LISTENING;
+  bool get _isFiring => _multiplexState >= _STATE_IS_FIRING;
+  bool get _removeAfterFiring => _multiplexState >= _STATE_REMOVE_AFTER_FIRING;
+  int get _eventId => _multiplexState & _STATE_EVENT_ID_BIT;
+
+  void _setRemoveAfterFiring() {
+    assert(_isFiring);
+    _multiplexState |= _STATE_REMOVE_AFTER_FIRING;
+  }
+
+  void _startFiring() {
+    assert(!_isFiring);
+    _multiplexState |= _STATE_IS_FIRING;
+  }
+
+  /// Marks listener as no longer firing, and toggles its event id.
+  void _endFiring() {
+    assert(_isFiring);
+    _multiplexState ^= (_STATE_IS_FIRING | _STATE_EVENT_ID_BIT);
+  }
+
+  void _setNotListening() {
+    assert(_isListening);
+    _multiplexState = _STATE_NOT_LISTENING;
+  }
+
+  void _onCancel() {
+    assert(_isListening);
+    _source._removeListener(this);
+  }
+}
+
+/**
+ * A stream that sends events from another stream to multiple listeners.
+ *
+ * This is used to implement [Stream.asBroadcastStream].
+ *
+ * This stream allows listening more than once.
+ * When the first listener is added, it starts listening on its source
+ * stream for events. All events from the source stream are sent to all
+ * active listeners. The listeners handle their own buffering.
+ * When the last listener cancels, the source stream subscription is also
+ * canceled, and no further listening is possible.
+ */
+// TODO(lrn): change "implements" to "with" when the VM supports it.
+class _SingleStreamMultiplexer<T> extends Stream<T>
+                                  implements _MultiplexerLinkedList,
+                                             _EventDispatch<T> {
   final Stream<T> _source;
   StreamSubscription<T> _subscription;
-
-  _SingleStreamMultiplexer(this._source);
-
-  void _callOnPauseStateChange() {
-    if (_isPaused) {
-      if (_subscription != null) {
-        _subscription.pause();
+  // Alternates between zero and one for each event fired.
+  // Listeners are initialized with the next event's id, and will
+  // only be notified if they match the event being fired.
+  // That way listeners added during event firing will not receive
+  // the current event.
+  int _eventId = 0;
+
+  bool _isFiring = false;
+
+  // Remember events added while firing.
+  _StreamImplEvents _pending;
+
+  _SingleStreamMultiplexer(this._source) {
+    _next = _previous = this;
+  }
+
+  bool get _hasPending => _pending != null && !_pending.isEmpty;
+
+  // Should be mixin.
+  _MultiplexerLinkedList _next;
+  _MultiplexerLinkedList _previous;
+
+  void _unlink() {
+    _previous._next = _next;
+    _next._previous = _previous;
+    _next = _previous = this;
+  }
+
+  void _insertBefore(_MultiplexerLinkedList newNext) {
+    _MultiplexerLinkedList newPrevious = newNext._previous;
+    newPrevious._next = this;
+    newNext._previous = _previous;
+    _previous._next = newNext;
+    _previous = newPrevious;
+  }
+  // End of mixin.
+
+  StreamSubscription<T> listen(void onData(T data),
+                               { void onError(Object error),
+                                 void onDone(),
+                                 bool cancelOnError }) {
+    if (onData == null) onData = _nullDataHandler;
+    if (onError == null) onError = _nullErrorHandler;
+    if (onDone == null) onDone = _nullDoneHandler;
+    cancelOnError = identical(true, cancelOnError);
+    _MultiplexerSubscription subscription =
+        new _MultiplexerSubscription(this, onData, onError, onDone,
+                                     cancelOnError, _eventId);
+    if (_subscription == null) {
+      _subscription = _source.listen(_add, onError: _addError, onDone: _close);
+    }
+    subscription._insertBefore(this);
+    return subscription;
+  }
+
+  /** Called by [_MultiplexerSubscription.remove]. */
+  void _removeListener(_MultiplexerSubscription listener) {
+    if (listener._isFiring) {
+      listener._setRemoveAfterFiring();
+    } else {
+      _unlinkListener(listener);
+    }
+  }
+
+  /** Remove a listener and close the subscription after the last one. */
+  void _unlinkListener(_MultiplexerSubscription listener) {
+    listener._setNotListening();
+    listener._unlink();
+    if (identical(_next, this)) {
+      // Last listener removed.
+      _cancel();
+    }
+  }
+
+  void _cancel() {
+    StreamSubscription subscription = _subscription;
+    _subscription = null;
+    subscription.cancel();
+    if (_pending != null) _pending.cancelSchedule();
+  }
+
+  void _forEachListener(void action(_MultiplexerSubscription listener)) {
+    int eventId = _eventId;
+    _eventId ^= 1;
+    _isFiring = true;
+    _MultiplexerLinkedList entry = _next;
+    // Call each listener in order. A listener can be removed during any
+    // other listener's event. During its own event it will only be marked
+    // as "to be removed", and it will be handled after the event is done.
+    while (!identical(entry, this)) {
+      _MultiplexerSubscription listener = entry;
+      if (listener._eventId == eventId) {
+        listener._startFiring();
+        action(listener);
+        listener._endFiring(); // Also toggles the event id.
       }
-    } else {
-      if (_subscription != null) {
-        _subscription.resume();
+      entry = listener._next;
+      if (listener._removeAfterFiring) {
+        _unlinkListener(listener);
       }
     }
-  }
-
-  /**
-    * Subscribe or unsubscribe on [_source] depending on whether
-    * [_stream] has subscribers.
-    */
-  void _onSubscriptionStateChange() {
-    if (_hasListener) {
-      assert(_subscription == null);
-      _subscription = _source.listen(this._add,
-                                     onError: this._addError,
-                                     onDone: this._close);
-    } else {
-      // TODO(lrn): Check why this can happen.
-      if (_subscription == null) return;
-      _subscription.cancel();
-      _subscription = null;
-    }
+    _isFiring = false;
+  }
+
+  void _add(T data) {
+    if (_isFiring || _hasPending) {
+      _StreamImplEvents pending = _pending;
+      if (pending == null) pending = _pending = new _StreamImplEvents();
+      pending.add(new _DelayedData(data));
+    } else {
+      _sendData(data);
+    }
+  }
+
+  void _addError(Object error) {
+    if (_isFiring || _hasPending) {
+      _StreamImplEvents pending = _pending;
+      if (pending == null) pending = _pending = new _StreamImplEvents();
+      pending.add(new _DelayedError(error));
+    } else {
+      _sendError(error);
+    }
+  }
+
+  void _close() {
+    if (_isFiring || _hasPending) {
+      _StreamImplEvents pending = _pending;
+      if (pending == null) pending = _pending = new _StreamImplEvents();
+      pending.add(const _DelayedDone());
+    } else {
+      _sendDone();
+    }
+  }
+
+  void _sendData(T data) {
+    _forEachListener((_MultiplexerSubscription listener) {
+      listener._add(data);
+    });
+    if (_hasPending) {
+      _pending.schedule(this);
+    }
+  }
+
+  void _sendError(Object error) {
+    _forEachListener((_MultiplexerSubscription listener) {
+      listener._addError(error);
+    });
+    if (_hasPending) {
+      _pending.schedule(this);
+    }
+  }
+
+  void _sendDone() {
+    _forEachListener((_MultiplexerSubscription listener) {
+      listener._setRemoveAfterFiring();
+      listener._close();
+    });
   }
 }
+
+
+/**
+ * Simple implementation of [StreamIterator].
+ */
+class _StreamIteratorImpl<T> implements StreamIterator<T> {
+  // Internal state of the stream iterator.
+  // At any time, it is in one of these states.
+  // The interpretation of the [_futureOrPrefecth] field depends on the state.
+  // In _STATE_MOVING, the _data field holds the most recently returned
+  // future.
+  // When in one of the _STATE_EXTRA_* states, the it may hold the
+  // next data/error object, and the subscription is paused.
+
+  /// The simple state where [_data] holds the data to return, and [moveNext]
+  /// is allowed. The subscription is actively listening.
+  static const int _STATE_FOUND = 0;
+  /// State set after [moveNext] has returned false or an error,
+  /// or after calling [cancel]. The subscription is always canceled.
+  static const int _STATE_DONE = 1;
+  /// State set after calling [moveNext], but before its returned future has
+  /// completed. Calling [moveNext] again is not allowed in this state.
+  /// The subscription is actively listening.
+  static const int _STATE_MOVING = 2;
+  /// States set when another event occurs while in _STATE_FOUND.
+  /// This extra overflow event is cached until the next call to [moveNext],
+  /// which will complete as if it received the event normally.
+  /// The subscription is paused in these states, so we only ever get one
+  /// event too many.
+  static const int _STATE_EXTRA_DATA = 3;
+  static const int _STATE_EXTRA_ERROR = 4;
+  static const int _STATE_EXTRA_DONE = 5;
+
+  /// Subscription being listened to.
+  StreamSubscription _subscription;
+
+  /// The current element represented by the most recent call to moveNext.
+  ///
+  /// Is null between the time moveNext is called and its future completes.
+  T _current = null;
+
+  /// The future returned by the most recent call to [moveNext].
+  ///
+  /// Also used to store the next value/error in case the stream provides an
+  /// event before [moveNext] is called again. In that case, the stream will
+  /// be paused to prevent further events.
+  var _futureOrPrefetch = null;
+
+  /// The current state.
+  int _state = _STATE_FOUND;
+
+  _StreamIteratorImpl(final Stream<T> stream) {
+    _subscription = stream.listen(_onData,
+                                  onError: _onError,
+                                  onDone: _onDone,
+                                  cancelOnError: true);
+  }
+
+  T get current => _current;
+
+  Future<bool> moveNext() {
+    if (_state == _STATE_DONE) {
+      return new _FutureImpl<bool>.immediate(false);
+    }
+    if (_state == _STATE_MOVING) {
+      throw new StateError("Already waiting for next.");
+    }
+    if (_state == _STATE_FOUND) {
+      _state = _STATE_MOVING;
+      _futureOrPrefetch = new _FutureImpl<bool>();
+      return _futureOrPrefetch;
+    } else {
+      assert(_state >= _STATE_EXTRA_DATA);
+      switch (_state) {
+        case _STATE_EXTRA_DATA:
+          _state = _STATE_FOUND;
+          _current = _futureOrPrefetch;
+          _futureOrPrefetch = null;
+          _subscription.resume();
+          return new FutureImpl<bool>.immediate(true);
+        case _STATE_EXTRA_ERROR:
+          Object prefetch = _futureOrPrefetch;
+          _cancel();
+          return new FutureImpl<bool>.error(prefetch);
+        case _STATE_EXTRA_DONE:
+          _cancel();
+          return new FutureImpl<bool>.immediate(false);
+      }
+    }
+  }
+
+  /** Clears up the internal state when the iterator ends. */
+  void _clear() {
+    _subscription = null;
+    _futureOrPrefetch = null;
+    _current = null;
+    _state = _STATE_DONE;
+  }
+
+  void cancel() {
+    StreamSubscription subscription = _subscription;
+    if (_state == _STATE_MOVING) {
+      _FutureImpl<bool> hasNext = _futureOrPrefetch;
+      _clear();
+      hasNext._setValue(false);
+    } else {
+      _clear();
+    }
+    subscription.cancel();
+  }
+
+  void _onData(T data) {
+    if (_state == _STATE_MOVING) {
+      _current = data;
+      _FutureImpl<bool> hasNext = _futureOrPrefetch;
+      _futureOrPrefetch = null;
+      _state = _STATE_FOUND;
+      hasNext._setValue(true);
+      return;
+    }
+    _subscription.pause();
+    assert(_futureOrPrefetch == null);
+    _futureOrPrefetch = data;
+    _state = _STATE_EXTRA_DATA;
+  }
+
+  void _onError(Object error) {
+    if (_state == _STATE_MOVING) {
+      _FutureImpl<bool> hasNext = _futureOrPrefetch;
+      // We have cancelOnError: true, so the subscription is canceled.
+      _clear();
+      hasNext._setError(error);
+      return;
+    }
+    _subscription.pause();
+    assert(_futureOrPrefetch == null);
+    _futureOrPrefetch = error;
+    _state = _STATE_EXTRA_ERROR;
+  }
+
+  void _onDone() {
+     if (_state == _STATE_MOVING) {
+      _FutureImpl<bool> hasNext = _futureOrPrefetch;
+      _clear();
+      hasNext._setValue(false);
+      return;
+    }
+    _subscription.pause();
+    _futureOrPrefetch = null;
+    _state = _STATE_EXTRA_DONE;
+  }
+}