Hi I have one small doubt about flows: `StateFlow`...

# android

[Danish Ansari]

Hi
I have one small doubt about flows: StateFlow to be specific

I have MutableStateFlow<UiState> and corresponding StateFlow<UiState> defined in my viewmodel

class MainViewModel:ViewModel() {
    private val mutableUiState = MutableStateFlow(UiState())
    val uiState = mutableUiState.asStateFlow()
    // ...
}

And my UiState is roughly defined like this

data class UiState(
    val propertyA: Int = 0,
    val propertyB: Boolean = false
) {
    private var mutableProperty: Int = 0

    val derivedProperty: Boolean
        get() = mutableProperty % 2 == 0

    fun someMethod(newProperty: Int) {
        mutableProperty = newProperty
        // some extra logic goes here
    }
}

Now the doubt is that in my viewmodel I’m calling mutableUiState.someMethod(someInteger) but I think uiState is not getting updated because I have used uiState.derivedProperty at some place in UI in compose and the UI is not updating.

Someone care to help what I’m doing wrong? Thanks

[krzysztof]

You should ask this in #flow

But main problem here is that MutableStateFlow is not aware of internal changes in your UiState class, hence no new emits of the value

[krzysztof]

you trigger new emit by assigning new value to .value of MutableStateFlow.

So in your ViewModel, you should do something like

mutableUiState.value = mutableUiState.someMethod(/*...*/)

where .someMethod should return a new instance of your UiState class, because that’s the type you declared to be emitted

[krzysztof]

so your someMethod could return copy() of your class

[Kevin Del Castillo]

Krzysztof already answered your question, just to add my two cents, the changes you made to your private var in your UI state are considered internal mutability, changes made in that way won't emit a new value in your Flow, you have to manually update it assigning a new value or using the update function which should always return a new instance and not do any internal mutation, you'll often see the latter combined with the copy method of data classes.

[Danish Ansari]

Thanks @Kevin Del Castillo and @krzysztof
That was a good explanation

[Danish Ansari]

Sure I’ll post it in #flow next time, if I have such query