Hello, I’d like to invite some feedback on a design I have. I am trying to model a stateful connect...

# coroutines

[Chris Fillmore]

Hello, I’d like to invite some feedback on a design I have.

I am trying to model a stateful connection using a StateFlow<Job> , so that connection lifecycles can be observed, cancelled, reconnected, etc. I have an Android app which connects to several websocket endpoints on different protocols, including plain websockets, socketio, action cable, graphql subscriptions, so being able to define a common pattern like this would be useful to me. More in 🧵

[Chris Fillmore]

I have the following test program:

fun main(args: Array<String>) = runBlocking {
  println("Program start")
  val jobLoop = LazyJobLoop(Dispatchers.Default)
  jobLoop.job.collect {
    withContext(it) {
      println("In withContext")
    }
  }
  Unit
}

interface JobLoop : CoroutineScope {
  val job: StateFlow<Job>
}

class LazyJobLoop(
  coroutineContext: CoroutineContext,
) : JobLoop, CoroutineScope by CoroutineScope(coroutineContext) {
  private val _job = MutableStateFlow(launchJob())
  override val job = _job.asStateFlow()

  private fun launchJob() = launch(start = CoroutineStart.LAZY) {
    println("In job")
  }
}

[Chris Fillmore]

This program fails with

Exception in thread "main" java.lang.IllegalStateException: Job is still new or active: LazyStandaloneCoroutine{New}@544a2ea6

[Chris Fillmore]

Of course the flaw in this design is that if I start the job lazily via`start = CoroutineStart.LAZY` and someone tries to use the Job in a CoroutineContext, then I will get the above exception

[Chris Fillmore]

But starting lazily is of course desirable to me, so that client code can just call jobLoop.job.value.start() when they want the job to start (or in my case, when I want to open the websocket connection.

Or it could be used like:

jobLoop.job
  .onEach { job ->
    // Maybe do some suspending work beforehand,
    // like fetch another resource, get a fresh token, whatever
    job.start()
  }
  .launchIn(myClientScope)

[Chris Fillmore]

Does anyone have any advice on how I could achieve what I want? Thanks for any feedback!

[Chris Fillmore]

Fwiw it has occurred to me that I could introduce some boilerplate to start the job normally, i.e. launch { } without the start parameter, then immediately suspend, but I would need to introduce something else to suspend on, complicating the design.