I'm on the viewmodel of my screen, and I have a fu...

# coroutines

[Pablo]

I'm on the viewmodel of my screen, and I have a function on my viewmodel that it's called when user press a button. This is the function:

fun viewModelDoHeavyTask() {
    viewModelScope.launch(Dispatchers.IO) {
        heavyTaskUseCase.heavyTask()
    }
}

As you can see, that function, called another function on my HeavyTaskUseCase class. The question is... should heavyTaskUseCase.heavyTask() be a suspend function? if so, why? I don't have clear when a function that is being called from inside a corutine should be suspend and which benefits I get of it. Will I notice any improvement if I do it? Thanks

[Sam]

Good question! No, heavyTask() should not be a suspending function.
• The only benefit of adding the suspend modifier is that it gives you the ability to call other suspending functions. If you don't use any suspending functions, you don't need the suspend modifier, and you won't get any benefit from it.
• Not only that, but the suspend modifier is a way to signal to callers that this function is main-safe, and that it'll suspend its coroutine instead of blocking its thread. If you know the function is going to do heavy CPU work or thread-blocking I/O, adding the suspend modifier would be actively misleading to callers of the function.

[Pablo]

thank you, I'll wait to read more opinions. Your answer surprised me, I thought the correct whould be to mark it as a suspend function

[Robert Williams]

Possible counter: if heavyTask is actually implemented as a series of smaller tasks or can be rewritten this way, making it suspend is the only way to properly support cancellation:

suspend fun heavyTask() = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
  while (coroutineContext.isActive && hasMoreSubtasks) {
    doNextSubtask()
  }
}

It also makes it trivial to do stuff like executing subtasks in parallel if that makes sense for the use case

[ephemient]

if it were implemented in such a way, then it would be main-safe

[Pablo]

what you recommend ephemient?

[uli]

No answer for a while. Maybe because we do not know enough of heavyTask’s properties.
Is it io bound? Cpu bound? Can it be made to actually suspend? Is it composed of subtask that can easily be structured as @ephemient suggested?
Or simply, what does it do?