How can you emulate `ContentLoadingProgressBar`’s behavior in a `Flow<T>`? Specifically, if I ...

# coroutines

[zak.taccardi]

How can you emulate `ContentLoadingProgressBar`’s behavior in a Flow<T>? Specifically, if I begin observing a Flow<ViewState> and it emits no ViewState.Loaded in the first 300ms, I want to emit a ViewState.Loading while ensuring that this ViewState.Loading emits before any ViewState.Loaded from the source Flow<ViewState>?

Kind of equivalent to:

val sourceFlow: Flow<ViewState> = ..

sourceFlow
  .onStart {
    delay(300)
    // if `sourceFlow` has not emitted any items yet
    emit(ViewState.Loading)
  }

https://developer.android.com/reference/androidx/core/widget/ContentLoadingProgressBar

[louiscad]

You cannot use onStart for this. You want to race a timer against a flow, so you'll need concurrency, which can be done in channelFlow where you can forward the source flow to the channel, and keep the timer running while this is started (until your timer/delay elapses of course).

[louiscad]

If you have a great name for an operator like you want, please tell me

[ursus]

Dont know flow but

Observable.merge(
    Observable.just(Loading),
    actualStream)
    .switchMap { 
        if it is Loading Observable.timer(someDelay).map { it }     
        else Observable.just(it)

[elizarov]

sourceFlow
    .onStart { emit(Loading) }
    .mapLatest { 
        if (it is Loding) delay(someDelay)
        it
    }

[louiscad]

Neat.

[zak.taccardi]

Crazy how simple that is.

[zak.taccardi]

You'd have to store some state so only the first emission delays, but very nice

[louiscad]

You can use the withIndex() operator, and return it.value in mapLatest instead of it.