Signal

A Signal is a callback mechanism used by both Source and Sink to manage forward/back-pressure. In both cases it is returned when a requested operation cannot immediately complete, but can at a later point in time. For example, when pulling from a Source, if no item is immediately available, a signal is returned that will be triggered when an item is available to pull.

val stream: Source[Int] = //...
stream.pull() match {
 case PullResult.Item(num) => //...
 case PullResult.Full(signal) => signal.notify {
   //when this callback function is called, it is guaranteed that an item is now available
   stream.pull()//...
 }
}

Signals are multi-listener, so that multiple consumers can attach callbacks to a single listener. Callbacks are fired in the order they are queued, and generally conditions for triggering a signal are re-checked for each listener (so that, for example, if one item is pushed to an empty Source, only one listener is signaled).