i have a best pracitice question: when having a st...

# compose

[Moritz Post]

i have a best pracitice question: when having a state that could be null (eg when observing a viewMode, in a composable) using the by delegate, how do you deal with the case that the value is null? An if check is an option but the value can not be smart-cast. What are you doing in that case? Example:

val user by viewModel.user.collectAsState(null)

if (user != null) {
  // user? can not be cast to user?
  // Message is "Smart cast to 'User' is impossible, because 'user' is a property that has open or custom getter."
}

For one state object you could also use let but that is very cumbersome when multiple values should be consumed. Using !! is also not so nice. Any tips?

[Csaba Szugyiczki]

We usually pass these state-s as parameters immediately after creating them from the ViewModel states. If you pass it as a Nullable parameter to another Composable, smart cast will work.

@Composable
fun Screen() {
  val user by viewModel.user.collectAsState(null)

    ScreenContent(user)
  }
}

@Composable
private fun ScreenContent(user: User?) {
  if (user != null) {
    // user here should be non-null
  }
}

[Moritz Post]

That is a valid approach indeed.

[Moritz Post]

I found that if you only need to read the state you can also just use the state.value immediatly. eg

val user = viewModel.user.collectAsState(null).value

Thereby you create the intermediate variable under the hood which can be smart-cast as expected.

[mgrazianodecastro]

Another solution, depending on the use case, you can keep the ScreenContent with non-null values, and only entering the screen when the state is not null - and let some loading screen until there. A way to do so it to handle this at the navigation level, listening to a channel from the view model, which will send an event to start a redirect when the state is ready

[goku]

Late to the party, but on top of @Moritz Post’s approach I created an extension function and it reads nicely

@Composable
fun <T> StateFlow<T>.collectAsStateValue(
): T = collectAsState().value

val user = viewModel.user.collectAsStateValue()

[Moritz Post]

@goku Sounds good but the method should rather be collectValue() as it is not tied to a state anymore.