Hi all, I'm trying to wrap my head around how coro...

# android

[David Ng]

Hi all, I'm trying to wrap my head around how coroutine/flow works.
I have the following in my viewmodel

private val _status = MutableLiveData<Status>()
val status: LiveData<Status>
    get = _status

fun getStatus(id: Int) {
    viewModelScope.launch {
        repository.getStatus(id)    // This return Flow<Status>
            .collect { _status = it}
    }
}

And I use data binding on my layout

<TextView
    android:text="@{viewModel.status}" />

The UI doesn't get updated to the latest value as the status come through from repository, the Main thread is being held up.
If I switch the coroutine to use Dispatcher.IO, I got "java.lang.IllegalStateException: Cannot invoke setValue on a background thread" as the status need to be set on Main thread.
I've tried using BroadcastChannel

private val statusChannel = BroadcastChannel<Status>(Channel.CONFLATED)
val status = statusChannel.asFlow().asLiveData()

fun getStatus(id: Int) {
    viewModelScope.launch(<http://Dispatcher.IO|Dispatcher.IO>) {
        repository.getStatus(id)    // This return Flow<Status>
            .collect { statusChannel.offer(it) }
    }
}

But the idea of converting the channel back to flow feels odd.
What's the best way to approach this?

[Karthikeyan1241997]

You can use _status.postValue(it) instead of _status.value = it

[David Ng]

Oh I didn't realized .postValue can be used in non-UI thread. That's all I need, thanks!

[Karthikeyan1241997]

Or you can switch context inside collect lambda using withContext(Dispatchers.Main) since collect is a suspend function and directly set the value instead of postValue

[David Ng]

Yup that works as well! I do have a question regarding withContext, is there any overhead of switching context multiple times? Or it's negligible?

[Ian Lake]

postValue() will always skip a frame (it uses a technique that waits for vsync), while switching to Dispatchers.Main will not necessarily do so (it isn't tied to vsync), making the later generally the better choice

[David Ng]

I'm convinced, will use withContext instead 🙂

[Ian Lake]

Of course, you should really avoid this problem entirely - what you actually want is the use a MutableStateFlow of the current ID, then use flatMapLatest to transform that into your getStatus Flow, then use asLiveData() to convert that into a LiveData for your UI layer

[Ian Lake]

As that will cancel the previous getStatus collect automatically when the ID changes

[Ian Lake]

(in your current code, if you call getStatus twice, the previous collect will continue)

[David Ng]

Haven't come across MutatableStateFlow yet, will definitely do some reading about it. It's my first kotlin project, still learning the framework 😄

[David Ng]

Thanks for the suggestion!

[Ian Lake]

Kind of the Kotlin Flow version of https://developer.android.com/topic/libraries/architecture/livedata#transform_livedata

[Ian Lake]

Thinking about it, your BroadcastChannel + asFlow approach for the ID is probably better than a MutableStateFlow since MutableStateFlow requires an initial state (which you could simulate with a filterNotNull if you wanted)

[David Ng]

Hmm I'm actually giving it a stab at the MutableStateFlow

private val _status = MutableStateFlow<Status>()
val status: LiveData<Status>
    get = _status.asLiveData()

fun getStatus(id: Int) {
    viewModelScope.launch {
        repository.getStatus(id)    // This return Flow<Status>
            .collect { _status = it}
    }
}

And the repository layer would use .flatMapLatest to emit the latest flow?

[David Ng]

Is BroadcastChannel generally the approach to this problem? Feels a little odd to convert it to Flow again then LiveData

[David Ng]

Yup you're correct on the initial stated required for MutableStateFlow it won't build with the above code as MutableStateFlow<Status>() is expecting a parameter

[Ian Lake]

I think you misunderstand. I meant something like

private val idChannel = BroadcastChannel<Int>(Channel.CONFLATED)
val status = idChannel.asFlow().flatMapLatest { id ->
  repository.getStatus(id)
}.asLiveData()

fun setId(id: Int) {
  idChannel.offer(id)
}

[David Ng]

Ah yup, that makes perfect sense!

[Ian Lake]

If your id is an argument to your Fragment that doesn't change, you could make this even easier by using the Saved State module for ViewModel: https://developer.android.com/topic/libraries/architecture/viewmodel-savedstate

class SavedStateViewModel(state: SavedStateHandle) : ViewModel() {
  val status = state.getLiveData("id").asFlow().flatMapLatest { id ->
    repository.getStatus(id)
  }.asLiveData()
}

This takes advantage of the fact that the SavedStateHandle is automatically populated by the arguments of the Fragment when you use Fragment 1.2.0 or higher, meaning you don't ever need to set the ID - it is automatically pulled from the arguments when the ViewModel is created

[David Ng]

I've come across SavedStateHandle but didn't use it as I started off the project using safe args plugin. They seems to work differently. Is SavedStateHandle the way forward to pass arguments?

[Ian Lake]

They're both the same Bundle under the hood, so as long as the String you pass into getLiveData() is the same android:name of your argument in your graph, they'll read the same value

[Ian Lake]

You might consider starring the existing feature request for Safe Args + `SavedStateHandle`: https://issuetracker.google.com/issues/136967621

[David Ng]

That's definitely something I'm looking for thanks!

[Aidooyaw1992]

i really like the way they are making things simpler these days

[Aidooyaw1992]

i ran away from native android but I'm back now