Hi guys. What is the equivalent of “onEachLatest” ...

# coroutines

[gabin]

Hi guys. What is the equivalent of “onEachLatest” operator in Flow? I need to cancel previously running suspend fun in onEach block

[octylFractal]

Not sure what operator you're referring to, I can't find that in reactor or rxjava -- but perhaps collectLatest is what you want?

* The crucial difference from [collect] is that when the original flow emits a new value, [action] block for previous
 * value is cancelled.

[Marc Knaup]

Or mapLatest and simply returning the passed value.

[Marc Knaup]

Or just copy the implementation of onEach and add `Latest`:

fun <T> Flow<T>.onEachLatest(action: suspend (T) -> Unit): Flow<T> = transformLatest { value ->
    action(value)
    emit(value)
}

[gabin]

collectLatest is not exactly what I need. It’s a suspend function, and I’ve decided to switch to flow.onEach() {} . launchIn(scope) construction

[gabin]

I want to use this kind of pattern:

[gabin]

instead of scope.launch construction:

[Marc Knaup]

And updateUI can be safely canceled?
If you get too many events you could also use .conflate() before .onEach()

[gabin]

@Marc Knaup thx for your implementation but is it really missing in flow library?

[Marc Knaup]

A lot of operators could be added to the standard library 🙂

[gabin]

yes, updateUI can be canceled

[gabin]

I use debounce() instead of conflate

[Marc Knaup]

You could suggest adding it and provide a use case:
https://github.com/Kotlin/kotlinx.coroutines/issues/new

[Marc Knaup]

I’m still undecided between debounce and sample.

[Marc Knaup]

debounce causes an initial delay, doesn’t it?

[Marc Knaup]

Oh, but sample may be good for events but not for data 😅

[Marc Knaup]

A hybrid would be nice.

[gabin]

no, debounce takes the recent value in the time range

[Marc Knaup]

Yes, but it waits at least time range for that

[Marc Knaup]

So if you debounce 1 second you will get the first update at least in 1 second. That’s how I read the docs.
But let me test

[gabin]

e.g. if user types “abcde” I will get value only when he stops typing

[Marc Knaup]

Yeah for typing it makes sense

[Marc Knaup]

For button clicks it won’t

[gabin]

@Marc Knaup and what do you think to use for button clicks? simple flow with collectLatest{} or single() ?

[Marc Knaup]

None of the standard operators is suitable 😞

[gabin]

I am wondering what behavior do you want to achieve 😉

[Marc Knaup]

In RxJava it would be throttleLatest

[Marc Knaup]

It would basically prevent two events (e.g. button clicks) that are too close together.

[Marc Knaup]

I’ve added https://github.com/Kotlin/kotlinx.coroutines/issues/1927

[Marc Knaup]

(you can do the same for onEachLatest 😉)

[bezrukov]

@gabin you can use mapLatest as is (according to example you used)

events()
   .mapLatest { event -> updateUI(event) }
   .launchIn(scope)

[gabin]

@bezrukov map has slightly different purpose. It converts 1 type to another. I don’t want to achieve this

[bezrukov]

in your case you will convert it to unit, and it doesn't matter.
You can review collectLatest implementation, and it's done via mapLatest as well.

[bezrukov]

if naming confuses you, you can add extension
onEachLatest(action: ..) = mapLatest(action)

[gabin]

ok, thanks for proposition