Why can't I cancel a Flow using Job.cancel() when I start it for two times? - kotlin

Question

In the App, I click "Start" button to display new information every 100ms, then click "Stop" button to stop display new information. it's work as my expection.
But If I click "Start" button for two times, then I click "Stop" button to hope to stop display new information, but the new information keep displaying, why? It seems that myJob.cancel() doesn't work.
class HandleMeter: ViewModel() {
var myInfo = mutableStateOf("Hello")
private lateinit var myJob: Job
private fun soundDbFlow(period: Long = 100) = flow {
while (true) {
emit(soundDb())
delay(period)
}
}
fun calCurrentAsynNew() {
myJob = viewModelScope.launch(Dispatchers.IO) {
soundDbFlow().collect { myInfo.value = it.toString() + " OK Asyn " + a++.toString() }
}
}
fun cancelJob(){
myJob.cancel()
}
...
}
#Composable
fun Greeting(handleMeter: HandleMeter) {
var info = handleMeter.myInfo
Column(
modifier = Modifier.fillMaxSize()
) {
Text(text = "Hello ${info.value}")
Button(
onClick = { handleMeter.calCurrentAsynNew() }
) {
Text("Start")
}
Button(
onClick = { handleMeter.cancelJob() }
) {
Text("Stop")
}
}
}

Answer 1

When you call handleMeter.calCurrentAsynNew() it starts a new coroutine and assigns the returned Job to myJob. When you click Start second time, it again starts a new coroutine and updates the value of myJob to this new Job instance but this doesn't cancel the previous coroutine, it's still running.
If you want to cancel previous coroutine when calCurrentAsynNew() is called, you will have to manually cancel it using myJob.cancel()
fun calCurrentAsynNew() {
if(::myJob.isInitialized)
myJob.cancel()
myJob = viewModelScope.launch {
soundDbFlow().collect { myInfo.value = it.toString() + " OK Asyn " + a++.toString() }
}
}
Instead of using lateinit var myJob: Job you could have also used var myJob: Job? = null and then to cancel it, use myJob?.cancel()
Dispatchers.IO is also not required in this case.