<@UBTGMFRCZ> I have a question about the ViewMode...

# compose

[Alexander Karkossa]

@cb 
I have a question about the ViewModels in the JetCaster app.
https://github.com/android/compose-samples/blob/master/Jetcaster/app/src/main/java/com/example/jetcaster/ui/home/HomeViewModel.kt#L46
The state/_state variable (MutableStateFlow) is set by a collect on a combine in a launch on the viewModelScope.

val state: StateFlow<HomeViewState>
    get() = _state

  init {
    viewModelScope.launch {
      combine(
        ...
      }.collect {
        _state.value = it
      }
    }
  }

Why don't use return value of the combine method directly to the state variable?

val state = combine(
    ...
  )

Can someone explain the benefits of viewModelScope.launch -> collect in the init?

[Alexander Sitnikov]

combine() returns just a Flow, whereas state's type is StateFlow. The important difference between the two is that StateFlow has a value property, that you can always read from a non-suspending function.

Having _state property in the view model allows you to write to it from any method of this view model. But if you know for sure, that state will only ever be computed from this single combine , you can use stateIn() operator, which turns any Flow into StateFlow :

state = combine(...)
    .stateIn(viewModelScope, SharingStarted.Eagerly, initialValue = false)

[Alexander Karkossa]

But what is the benefit of StateFlow over Flow, when the state is only used in compose with collectAsState and never used elsewhere?

[Alexander Sitnikov]

Generally speaking, StateFlow designed specifically to be used in UI situations. Flow.collectAsState requires you to provide initial value, and StateFlow.collectAsState reads value from state field - that way view knows less about business logic, which is good. Also, StateFlow collection never throws, but Flow collection can

[Alexander Sitnikov]

I don't know what will happen, if exception will be thrown in LaunchedEffect (which collectAsState uses under the hood), but I imagine nothing particularly good

[cb]

StateFlow is useful here, as you don’t need to provide a default value when using collectAsState() , as @Alexander Sitnikov explained.

[Alexander Karkossa]

OK i unterstand the intention, but it is much more complexity, only for prevent the default value. Is it worth it? collect the Flow within a couroutine to set the value into a StateFlow? In the jetcaster example, the stateIn (as @Alexander Sitnikov explained) would also work, until you don't change the value from anywhere else right?

[cb]

stateIn() didn’t exist when I wrote that code iirc, but yeah it should work the same