Hi All, having some trouble with cancelling coroutines in an Android app and wondering if you all co...

# coroutines

[Jeff Johnson]

Hi All, having some trouble with cancelling coroutines in an Android app and wondering if you all could help:


I have a class that launches coroutines and allows them to be cancelled when the Activity/Fragment they are called from is destroyed. However it is not working like I expect. When I back out of the fragment while the operation is running, the coroutine cancel does not take, and I get an NPE when trying to access a View that does not exist anymore.

open class CoroutineLauncher : CoroutineScope {

    private val dispatcher: CoroutineDispatcher = Dispatchers.Main
    private val supervisorJob = SupervisorJob()
    override val coroutineContext: CoroutineContext
        get() = dispatcher + supervisorJob

    fun launch(action: suspend CoroutineScope.() -> Unit) = launch(block = action)

    fun cancelCoroutines() {
        supervisorJob.cancelChildren() //coroutineContext.cancelChildren() has same results
    }
}

here is the usage

class MyFragment : Fragment {

  val launcher = CoroutineLauncher()

  fun onSomeEvent() {

    launcher.launch {

      val result = someSuspendFunction() // made suspend function by using withContext(<http://Dispatchers.IO|Dispatchers.IO>)

      if (!isActive) return

      // CAUSES CRASH
      myTextView.text = result.userText

    }

  }

  override fun onDestroyView() {
    super.onDestroyView()
    launcher.cancelCoroutines()
  }

}

I added log lines to ensure onDestroyView and cancelCoroutines are both being called before the crash. It is my understanding that withContext is cancellable and thus I shouldn’t even need to check for isActive . I feel like I’m missing something obvious, but I can’t figure out what it is

[Joffrey]

I would suspect this is because myTextView may not be available when you reach that line. If your coroutine is cancelled right before this call, it will still try to perform the assignment (cancellation is cooperative).
Maybe you should not try to update the UI from the `launch`’s context. Use a withContext to ensure it runs on the UI thread.
Another way would be to use cancelAndJoin() to make sure you wait for your coroutines to finish from onDestroyView

[Jeff Johnson]

@Joffrey You are definitely correct that myTextView is unavailable. My CoroutineLauncher uses Dispatchers.Main as its dispatcher so that should definitely run on the UI thread

[Jeff Johnson]

Also I have logs showing that cancelChildren is called before someSuspendFunction() is even called. Which since withContext is ‘cooperative’ should mean that the Text View ix never even accessed

[Jeff Johnson]

If anyone cares, onSomeEvent was being invoked after cancel was called. Since we call cancelChildren instead of cancel , the launcher does not refuse new Jobs, and since cancel already happened, the new coroutine runs like normal and crashes

[Joffrey]

Nicely spotted! Thanks for letting me know the final diagnostic, it's always interesting ☺️