Alternative implementation of StreamQueue.fork which has no impact if it

lib/src/subscription_fork.dart

+library async.subcsription_fork;
+
+import "dart:async";
+import "package:async/async.dart" show DelegatingStreamSubscription;
+
+/// Allows creating forks of a single [StreamSubscription].
+///
+/// The forks are stream subscriptions that will provide the same events
+/// as the original subscription from the time where the fork was created.
+///
+/// The original subscription is canceled when all forks are canceled, after the
+/// first fork has been created, and it's paused when all forks are paused.
+class SubscriptionForker {
+  final StreamSubscription _source;
+  final Set<_ForkedStreamSubscription> _subscriptions = new Set();
+  bool _isClosed;
+
+  SubscriptionForker(StreamSubscription subscription) : _source = subscription {
+    _source.pause();
+    _source.onData(_onData);
+    _source.onError(_onError);
+    _source.onDone(_onDone);
+  }
+
+  void _onData(T data) {
+    for (var subscription in _subscriptions.toList(growable: false)) {
+      subscription._controller.add(data);
+    }
+  }
+
+  void _onError(error, StackTrace stackTrace) {
+    for (var subscription in _subscriptions.toList(growable: false)) {
+      subscription._controller.addError(error, stackTrace);
+    }
+  }
+
+  void _onDone() {
+    _isClosed = true;
+    for (var subscription in _subscriptions.toList(growable: false)) {
+      subscription._controller.close();
+    }
+  }
+
+  Future _cancel(_ForkedStreamSubscription subscription) {
+    _subscriptions.remove(subscription);
+    if (_subscriptions.isEmpty) {
+      _isClosed = true;
+      return _source.cancel();
+    } else if (!_source.isPaused) {
+      _checkPause();
+    }
+  }
+
+  void _pause(_ForkedStreamSubscription subscription) {
+    _checkPause();
+  }
+
+  void _checkPause() {
+    if (_subscriptions.every(_isSubscriptionPaused)) {
+      _source.pause();
+    }
+  }
+
+  void _resume(_ForkedStreamSubscription subscription) {
+    _source.resume();
+  }
+
+  static bool _isSubscriptionPaused(_ForkedStreamSubscription forked) {
+    return forked._controller.isPaused;
+  }
+
+  /// Create a new fork of the source subscription.
+  StreamSubscription<T> fork() {
+    if (_isClosed) return new StreamSubscription<T>.empty();
+    var subscription = new _ForkedStreamSubscription<T>(this);
+    _subscriptions.add(subscription);
+    if (_source.isPaused) _source.resume();
+    return subscription;
+  }
+ }
+
+/// A forkable stream subscription.
+///
+/// A stream subscription with [fork] method.
+///
+class ForkableSubscription<T> extends DelegatingStreamSubscription<T> {
+  final SubscriptionForker<T> _forker;
+
+  /// Create a forkable subscription from [subscription].
+  ///
+  /// This creates a [SubscriptionForker] and returns a subscription backed
+  /// by one fork of this subscription forker, along with a [fork] method
+  /// which allows creating more.
+  ForkableSubscription(StreamSubscription<T> subscription)
+      : this._(subscription is _ForkedStreamSubscription
+                   ? subscription._forker
+                   : new SubscriptionForker<T>(subscription));
+
+  ForkableSubscription._(SubscriptionForker<T> forker)
+      : _forker = forker, super(forker.fork());
+
+  StreamSubscription<T> fork() => _forker.fork();
+}
+
+class _ForkedStreamSubscription<T> extends DelegatingStreamSubscription<T> {
+  final SubscriptionForker _forker;
+  final StreamController _controller;
+  factory _ForkedStreamSubscription(SubscriptionForker<T> forker)  {
+    var instance;
+    var onChange = () => instance._onChange();
+    var controller = new StreamController(sync: true,
+                                          onPause: onChange,
+                                          onResume: onChange,
+                                          onCancel: onChange);
+    instance = new _ForkedStreamSubscription._(forker, controller);
+    return instance;
+  }
+
+  _ForkedStreamSubscription._(this._forker, controller)
+      : _controller = controller, super(controller.stream.listen(null));
+
+  _onChange() {
+    if (!_controller.hasListener) {
+      return _forker._cancel(this);
+    } else if (_controller.isPaused) {
+      _forker._pause(this);
+    } else {
+      _forker._resume(this);
+    }
+    return null;
+  }
+}