Is it possible to observe in a viewmodel the value of a variable stored in a singleton? maybe can be...

# android

[Pablo]

Is it possible to observe in a viewmodel the value of a variable stored in a singleton? maybe can be observed with a flow? how can this be done?

object customSingleton {
    var customVar = "Hello World"
}

How can that be observed in a viewmodel?

[Michael Krussel]

A flow is probably the best solution.

But other options

1. poll for changes
2. RxJava
3. A callback that you can register and unregister.
And many more. But you either need to poll or the object needs to notify anyone interested when it changes.

[Pablo]

can you show me how to achieve it with flow?

[Michael Krussel]

Look up how to use StateFlow. But general way, would be to have a private MutableStateFlow, make the customVar getter and setter use the value from MutableStateFlow and then expose another property of type StateFlow that can be collected. Then in the view model, collect the state flow in a job launched from the view model's scope.

[Pablo]

it's really hard for me to use flows

[Pablo]

can't get the point

[Pablo]

val currentScreenId = MutableStateFlow<Int>(CustomSingleton.customVar)

val currentScreenIdState: StateFlow<Int> = currentScreenId.stateIn(
    viewModelScope,
    SharingStarted.WhileSubscribed(5_000),
    CustomSingleton.customVar
)

[Pablo]

Is that code correct? should I put the init value CustomSingleton.customVar in these two places? When I update the variable on the singleton will the composables listeing for this stateflow be recreated?

[Pablo]

well, something is not going well, because, when I update the variable (CustomSingleton.customVar = 1), the composable is not being recomposed

[Pablo]

I'm getting the variable this way in the composable:

val currentSectionId by viewModel.currentScreenIdState.collectAsStateWithLifecycle()

[Michael Krussel]

The flow goes in CustomSingleton.

object CustomSingleton {
    private val _customVarFlow = MutableStateFlow("Hello World")
    val customVarFlow = _customVarFlow.asStateFlow()
    var customVar: String
        get() = _customVarFlow.value
        set(value) {
            _customVarFlow.value = value
        }
}

class MyViewModel: ViewModel() {
    init {
        viewModelScope.launch { 
            CustomSingleton.customVarFlow.collect { currentVal ->
                // do something
            }
        }
    }
}

[Pablo]

using collect in the vm instead of collectasstatewithlifecycle in the composable is a good idea?

[Michael Krussel]

collectAsStateWithLifecycle is an option. You asked about the view model listening to it. So it all depends on what you are doing with it.