Hi, I am a bit confused about Kotlin Coroutines Jobs. Is it the case that a job can only run once? I...

# getting-started

[Nat Strangerweather]

Hi, I am a bit confused about Kotlin Coroutines Jobs. Is it the case that a job can only run once? I wanted to launch the searchJob every time my search parameter changed in the example below. What happens is that the searchJob is called successfully the first time I pass my parameter, but never again after that. Could someone please have a look ?

[Nat Strangerweather]

fun onSearch(searchTitle: String) {
        _searchTitle.value = searchTitle
        println("on search started")
        searchJob = viewModelScope.launch {
            delay(1000L)
            println("search job started")
            getFeeds(searchTitle)
                .onEach { result ->
                    when (result) {
                        is Resource.Success -> {
                            _isRefreshing.value = false
                            _state.value = state.value.copy(
                                newsFeedItems = result.data ?: emptyList(),
                                isLoading = false,
                                isSuccess = true,
                                isError = false
                            )
                            println("is success")  }

[Nat Strangerweather]

The onSearch() function starts, but the searchJob function does not.

[Josh Eldridge]

How are you using the searchJob variable outside of this?

[Nat Strangerweather]

No, that's the only place.

[Josh Eldridge]

So you define a variable somewhere named searchJob and only assign it here?

You might want to launch the coroutine with an Coroutine exception handler passed into it to see if there is a hidden exception cancelling the coroutine

[Nat Strangerweather]

Yes, I define it and use it in the viewModel:

private var searchJob: Job? = null

I have been using a tutorial and adapting it to my own project...
I'll do what you suggest. So does it mean that the searchJob needs to be cancelled properly in order to be reused?

[Josh Eldridge]

I don't believe you can reuse job references, once it reaches its final state of cancelled or completed it is done. But the job reference shouldn't have any impact here as long as you aren't manually cancelling it somewhere else. Does the "search job started" not print a second time or is the contents inside of the flow not working?

[Nat Strangerweather]

The "search job started" does not print a second time.

[Nat Strangerweather]

But "on search started" does...

[Josh Eldridge]

That's really odd, you may want to override onCleared in the view model and see if the view model scope itself is getting cancelled. Because it looks like it should run a second time from my brief glance

[Nat Strangerweather]

Ok, thanks for your help. I will check it out.

[bezrukov]

what is `getFeeds`' return type? Is it flow? If yes, do you apply a terminal operator (e.g. collect) on it? I mean snippet you shared includes only onEach which is not terminal, and alone will have no effect

[bezrukov]

I would recommend to create a minimum reproducing sample of it, e.g. smth similar to

var searchJob: Job? = null
searchJob = viewModelScope.launch { 
   println("A: before delay")
   delay(1000)
   println("A: before delay")
}
searchJob = viewModelScope.launch { 
   println("B: before delay")
   delay(1000)
   println("B: before delay")
}

I'm pretty sure when you will minimize your case to what I wrote above you should be able to find the cause.

[Nat Strangerweather]

Yes, it's a flow. this is the whole snippet:

fun onSearch(searchTitle: String) {
        _searchTitle.value = searchTitle
        println("on search started")
        searchJob = viewModelScope.launch {
            delay(1000L)
            println("search job started")
            getFeeds(searchTitle)
                .onEach { result ->
                    when (result) {
                        is Resource.Success -> {
                            _isRefreshing.value = false
                            _state.value = state.value.copy(
                                newsFeedItems = result.data ?: emptyList(),
                                isLoading = false,
                                isSuccess = true,
                                isError = false
                            )
                            println("is success")
                        }

                        is Resource.Error -> {
                            _isRefreshing.value = false
                            _state.value = state.value.copy(
                                newsFeedItems = result.data ?: emptyList(),
                                isLoading = false,
                                isSuccess = false,
                                isError = true
                            )
                        }

                        is Resource.Loading -> {
                            _isRefreshing.value = true
                            _state.value = state.value.copy(
                                newsFeedItems = result.data ?: emptyList(),
                                isLoading = true,
                                isSuccess = false,
                                isError = false
                            )
                        }
                    }
                }.launchIn(this)
        }
    }

[Nat Strangerweather]

Thanks I'll try that too

[Nat Strangerweather]

Ok, I'm getting :

A: before delay
B: before delay
A: after delay
B: after delay
Clear

But only the first time. I assume my problem is elsewhere then. I'll keep investigating.

[bezrukov]

is viewModelScope normal androidx' scope? If no (e.g. if it's manually created), that might happen if you have an unhandled exception somewhere.
You can change

println("on search started")

to

println("on search started ${viewModelScope.coroutineContext.job.isActive}")

to check if VM is somehow cancelled second time

[Nat Strangerweather]

Yes it's

androidx.lifecycle.viewModelScope

The result of

println("on search started ${viewModelScope.coroutineContext.job.isActive}")

is "True" the first time and "False" the second time.

[Nat Strangerweather]

It's a HiltViewModel, I don't know if that has any importance here?

[Nat Strangerweather]

Is it possible that another viewModelScope can interfere? Because I have a function just before using viewModelScope too.

[Nat Strangerweather]

Interesting that replacing viewModelScope with GlobalScope means my app is working as expected. However, I believe GlobalScope is not recommended, right?

[bezrukov]

Yes, GlobalScope is not recommended, since the jobs created using that scope might outlive your fragment/activity.
I didn't work with Hilt, so can't help there but better to find who causes viewModelScope to cancel

[bezrukov]

I don't think another usage of viewModelScope may cause it - viewModelScope uses SupervisorJob, which means cancellation of one child job (e.g. due to an exception) won't affect siblings

[bezrukov]

if onCleared is called between your onSearch functions, just debug it, and see who calls it and why

[Nat Strangerweather]

Oh right, thanks, I deleted these questions because I thought they were not relevant.

[Nat Strangerweather]

On cleared is called when I navigate back to my list of options.

[Nat Strangerweather]

so the problem seems to be back navigation.

[Nat Strangerweather]

Thanks for helping me narrow it down.