launchWhenX dropping events

gistfile1.txt

class MainFragment : Fragment(R.layout.main_fragment) {

    companion object {
        fun newInstance() = MainFragment()
    }

    private lateinit var viewModel: MainViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        viewModel = ViewModelProvider(this).get(MainViewModel::class.java)

        lifecycleScope.launchWhenStarted {
            viewModel.flow.collect {
                print("********** View: launch when started collector received $it in lifecycle state ")
                println(lifecycle.currentState.name)
            }
        }

        // Notify the view model of a lifecycle event just to cause
        // emissions during different lifecycle states.  In practice
        // the view model will emit values down the flow
        // without care to the observer's lifecycle state.
        // This just forces the conditions
        // necessary to demonstrate the issue.
        lifecycle.addObserver(LifecycleEventObserver { source: LifecycleOwner, event: Lifecycle.Event ->
            println("********** event: ${event.name}")
            viewModel.lifecycleEvent(event.name)
        }
        )
    }
}

class MainViewModel : ViewModel() {

    private var counter = AtomicInteger(0)

    private val channel = Channel<String>(Channel.BUFFERED)
    val flow = channel.receiveAsFlow()

    fun lifecycleEvent(eventName: String) {
        viewModelScope.launch {
            // Send the event name down the flow to make some traffic
            val i = counter.getAndIncrement()
            val event = "$eventName $i"
            println("********** ViewModel: Emitting $event on the flows")
            channel.send(event)
        }
    }
}


Note that if you run this you'll see the following logs:
********** event: ON_STOP
********** ViewModel: Emitting ON_STOP 4 on the flows
********** View: launch when started collector received ON_STOP 4 in lifecycle state CREATED
********** event: ON_DESTROY
********** ViewModel: Emitting ON_DESTROY 5 on the flows
********** event: ON_CREATE
********** ViewModel: Emitting ON_CREATE 6 on the flows
********** View: launch when started collector received ON_CREATE 6 in lifecycle state STARTED

Note the event forced out during ON_STOP was received in the CREATED lifecycle state. This makes sense as the observer is in on_stop. However, this could be bad if the event is a navigation event.

Also, note that the ON_DESTROY event that was forced out was dropped! That is, no observer collected it.  This is really bad.