Hi everyone, I'm trying to use MutableStateFlow, encapsulating a List of an object, but when I updat...

# coroutines

[Mijael Viricochea]

Hi everyone, I'm trying to use MutableStateFlow, encapsulating a List of an object, but when I update it with some new value, the collectLatest{} method doesn't work, can you help me please?
This method doesn't update the value

@ExperimentalCoroutinesApi
fun deleteSelected() {
    val value = dataPoints.value
    value.removeAll { it.selected }
    dataPoints.value = ArrayList(value)
}

[bezrukov]

because when you assign new list to it (dataPoints.value = ....) it checks objects for equality, and they're equal because value is mutable

[bezrukov]

I suggest switching to immutable List<> and use filter instead of removeAll:

dataPoints.value = dataPoints.value.filter { !it.selected }

[bezrukov]

If you strongly prefer to use ArrayList directly, you need to write:

val value = ArrayList(dataPoints.value)
value.removeAll { it.selected }
dataPoints.value = value

[Mijael Viricochea]

@bezrukov that worked, thank you.

[Mijael Viricochea]

@bezrukov

fun sweepAll() {
    val value = mutableListOf<DataPoint>()
    value.addAll(dataPoints.value)
    value.forEach {
        it.x = null
        it.y = null
    }
    dataPoints.value = value
}

I have this function and the value again is not changing, can you please tell me why is that?

[bezrukov]

because your points are mutable

[bezrukov]

stateflow works only with immutable data. To receive an update, value needs to be changed when you set dataPoints.value = value. But you made it.x = null on original datapoint, so it was changed in both lists

[bezrukov]

if you want to transfer mutable data over StateFlow you need to write some wrapper that will have a changed property. Something like:
data class Wrapper(val version: Int, data: List<DataPoints>) and on each update you need to copy this wrapper with increased version.
but it's hacky and better to use immutable data.