StreamController({void onListen(), void onPause(), void onResume(), dynamic onCancel(), bool sync: false })

A controller with a stream that supports only one single subscriber.

If sync is true, the returned stream controller is a SynchronousStreamController, and must be used with the care and attention necessary to not break the Stream contract. See Completer.sync for some explanations on when a synchronous dispatching can be used. If in doubt, keep the controller non-sync.

A Stream should be inert until a subscriber starts listening on it (using the onListen callback to start producing events). Streams should not leak resources (like websockets) when no user ever listens on the stream.

The controller buffers all incoming events until a subscriber is registered, but this feature should only be used in rare circumstances.

The onPause function is called when the stream becomes paused. onResume is called when the stream resumed.

The onListen callback is called when the stream receives its listener and onCancel when the listener ends its subscription. If onCancel needs to perform an asynchronous operation, onCancel should return a future that completes when the cancel operation is done.

If the stream is canceled before the controller needs new data the onResume call might not be executed.

Source 

factory StreamController({void onListen(),
                          void onPause(),
                          void onResume(),
                          onCancel(),
                          bool sync: false}) {
  return sync
       ? new _SyncStreamController<T>(onListen, onPause, onResume, onCancel)
       : new _AsyncStreamController<T>(onListen, onPause, onResume, onCancel);
}