CODEWITHSUNDEEP
multithreadingscalafuturethread-synchronization
Peltevis
Peltevis

Reputation: 15

Scala Future: Wait for update occurring in another independent thread

I have a class that has its own internal thread running over a queue of actions, it just loops all the time doing stuff. For that it reads a queue of notifications that tell it what action to do next. There is a notification to tell the class to stop all actions and kill itself. When another thread calls the notifyClose notification, its added to the queue and takes priority number 1. I want the method that adds the notification to return a Future.success when the other thread has processed the close notification.

The code run as follows:

def loop(): Future[Unit] = {
  current = if(queue.contains(KILL)) disable()  // <--- disable will set the state to Closed
  else process()                                // <--- if queue is empty it will call loop(), 
                                                // if its not empty it will do de action and then call loop()
}

private def disable(): Future[Unit] = {
  state = Closed
  Future.unit
}

def close(): Future[Unit] = {
  queue.add(KILL)
  while (state != Closed) { /* Wait until state is NotAvailable */ }
  Future.successful()
}

I want a better way to wait till the state changes to Closed in the close() method. I fill that and empty loop is the worst idea ever.

The variable current holds the current Future being processed, maybe there is a way hooking my result to that Future? The thing is I don't know when disable() method will actually start. Any help will be appreciated.
Thanks for taking the time of reading.

Upvotes: 1

Views: 194

Answers (1)

Mario Galic
Mario Galic

Reputation: 48430

Try using Promise as a kind of state, for example

def loop(): Future[Unit] = {
  if(queue.contains(KILL)) Future.successful(disable.trySuccess())
  else process()                                
}

private val disable = Promise[Unit]()

def close(): Future[Unit] = {
  queue.add(KILL)
  disable.future
}

This way Future returned by close() will complete only when loop() actually invokes disable.trySuccess.

Upvotes: 1

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