How can I assign a private mutableStateOf to a State variable in Android Jetpack?

Question

I hope to set _isRecording inside SoundViewModel class, and I hope to expose isRecording to UI.

But the Code A is wrong, how can I fix it?

Code A

    class SoundViewModel @Inject constructor(): ViewModel() {
    
        private var _isRecording by mutableStateOf(false)
        val isRecording: State<Boolean> by _isRecording  //It's wrong
        //val isRecording: State<Boolean> = _isRecording  //It's wrong
        ..
    }

Add content:

To nglauber: Thanks!

I think Code B will work well.

Which is better between your code and Code B?

Code B

class SoundViewModel @Inject constructor(): ViewModel() {

   var isRecording by mutableStateOf(false)
              private set

}


@Composable
fun YourComposable(soundViewModel: SoundViewModel) {
   //I can use soundViewModel.isRecording directly
}

Answer 1

When you use the keyword by, it's basically an alias to getValue/setValue. Therefore:

// _isRecording is a Boolean
private var _isRecording by mutableStateOf(false)
// _isRecording is a State<Boolean>
private var _isRecording = mutableStateOf(false)

This is why you're getting this error.

You should expose like this:

class SoundViewModel @Inject constructor(): ViewModel() {

    private var _isRecording = mutableStateOf(false)
    val isRecording: State<Boolean> = _isRecording
}

and consume like this:

@Composable
fun YourComposable(soundViewModel: SoundViewModel) {
    val isRecording by soundViewModel.isRecording

Answer 2

CLEARLY -- Code B is the best option. That's exactly what official Compose Docs recommend. The method with the underscore was adopted for LiveData objects, but when you shift to the Compose-compatible MutableState<T>, it is simply de-efficiencizing(?) your code, if you add that extra underscore step.

Check out the State-in-Compose for full insights.

Go with the simplest approach that gets the job done, always the rule of thumb.