@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?

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)

But what is the benefit of

StateFlow

over

Flow

, when the

state

is only used in compose with

collectAsState

and never used elsewhere?

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

StateFlow

is useful here, as you don’t need to provide a default value when using

collectAsState()

, as @Alexander Sitnikov explained.

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?

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