Get last item emited by a Flow and dont receive updates

Question

What is the best way to obtain the last element emitted by a flow without receiving updates.

Question: I use a flow to observe changes in certain shared preferences, but sometimes I want to know the current value of that preference. I always use two functions, one to observe the values in a flow and the other to capture the data directly, is there any way to archive the same behavior with just the observer function?

suspend fun getBreadcrumb(): Breadcrumb =
        withContext(Dispatchers.IO) context@ {
            ...
        }

fun observeBreadcrumb(): Flow<Breadcrumb> {
        ....
    }

Straggle

Answer 1

Consider using StateFlow

StateFlow is a new API that was recently added to the coroutines standard library. You can consume it like a regular flow, but it also has a value property that gets the most recent value.

At the moment, the only way to send a new value to a state flow is to update its value property. You can't yet turn a regular flow into a state flow, though there is an open proposal for it.

Let's say you send values to the flow like this:

val breadcrumbs = MutableStateFlow(someValue)

launch {
    while (isActive) {
        waitForSomeCondition()
        breadcrumbs.value = getLatestValue()
    }
}

You can retrieve the latest value whenever you want, for example like this:

launch {
    while (isActive) {
        delay(1000)
        println("Current value is ${breadcrumbs.value}")
    }
}

But you can also collect it like a regular flow, and receive the new value each time it changes:

launch {
    breadcrumbs.collect {
        println("Current value is ${it}")
    }
}

This is an experimental API, so there are likely to be improvements and changes down the line.

Costumer

Does collect actually stop the flow emitting? – Retroversion

Answer 2

last() or lastOrNull() should be used to achieve this.

Requiescat

@NigamPatro how is this not an answer? – Saucer

Answer 3

You could expose your Flow as a LiveData with Flow's asLiveData extension function:

class MyViewModel: ViewModel {
    ...
    val myFlowAsLiveData: LiveData<Breadcrumb> = myFlow.asLiveData()
    ...
}

Then you can observe it as usual and if you need its current value just use its value property:

myViewModel.myFlowAsLiveData.value

Ponderous

I tried this but it dont seems to work i alwsys return null – Straggle

Does your livedata has any active observers? According to docs livedata created by builder receives value only then. – Wakerly

Answer 4

Did you try with coroutine function mapLatest

val breadcrumbs = MutableStateFlow(someValue)

breadcrumbs.mapLatest { breadcrumb ->
  println("Latest value is ${breadcrumb}")
}

Cauline