Which Flow method do you happen to use more than the other o

Question

Which Flow method do you happen to use more than the other one `flatMapConcat ` two `flatMapMerge ` three i dont remember using any of them

Orhan Tozan · Answer

Wow majority of people never seem to flatMap sweat smile I wonder how you guys do it when you need to bring a Flow into a map

florent · Answer

Flatmaplatest

Orhan Tozan · Answer

Interesting if you like to use flatMapLatest do you also prefer mapLatest over map

ephemient · Answer

depends on the use case if it s a stream of events maybe skipping values is bad if it s a stream of the latest updated state maybe skipping values is fine

florent · Answer

I don t think there is a maplatest is it

Orhan Tozan · Answer

< florent> there is

Orhan Tozan · Answer

< ephemient> even with state I don t want to use mapLatest because you want to show the user the intermediary states like State Loading

ephemient · Answer

if there s a non loading state that arrived while your UI thread was busy why wouldn t you just jump directly to showing that

ephemient · Answer

also flatMapConcat == flatMapMerge concurrency = 1

ephemient · Answer

again depends on the use case but I ve found many uses for flatMapLatest few uses for flatMapMerge and never had any use for flatMapConcat

Orhan Tozan · Answer

< ephemient> because I first have to compute the arrived non loading state heavily say something that takes 10 seconds with mapLatest the user wouldn t get any state not even State Loading

ephemient · Answer

```input flatMapLatest input gt flow emit State Loading val result = heavyComputation input emit result ```

ephemient · Answer

that will always show loading and cancel the previous computation whenever upstream input emits a new value

Orhan Tozan · Answer

Or let s say your upstream flow emits your current position coordinates every 5 seconds You want to map this position flow to something that takes the coordinates and maps to a world location This operation to convert coordinates to world location takes 10 seconds Now correct me if I m wrong but if you do this with mapLatest the final flow would NEVER emit something

Orhan Tozan · Answer

< ephemient> ```flow emit x=1 y=2 delay 5000 emit x=1 y=2 delay 5000 emit x=1 y=3 delay 5000 mapLatest coordinates gt val worldLocationName = compute coordinates takes 10 seconds worldLocationName ```

Orhan Tozan · Answer

This flow never emits if im not mistaken

ephemient · Answer

so don t use mapLatest

Orhan Tozan · Answer

but in this case the flow would still represent State right

Orhan Tozan · Answer

they aren t events

ephemient · Answer

I never said that should imply mapLatest just possibly flatMapLatest

ephemient · Answer

of course if your producer is faster than your consumer then mapLatest doesn t complete

Orhan Tozan · Answer

Doesnt the same point apply to flatMapLatest Except it s for returning flows

ephemient · Answer

not if you emit the loading state immediately

Orhan Tozan · Answer

I meant the above code example but then the for flatMapLatest

ephemient · Answer

looking around I don t have any usages of mapLatest in the client I can see some places where we might use it in our backend but it s not using coroutines yet so whatever

ephemient · Answer

I gave an example of how I would use it though it s exactly the right tool for that job

Orhan Tozan · Answer

Because don t you want to rather show old data to the user than nothing most of the time

ephemient · Answer

if it s as a result of a user action no

Orhan Tozan · Answer

Could you give a example

Orhan Tozan · Answer

Of the usecase

ephemient · Answer

user presses a button which shows them a new post which has to be fetched from the server continuing to show the old post is just confusing

Orhan Tozan · Answer

Hmm in that case showing nothing would be indeed better than showing the old post

Orhan Tozan · Answer

What about a chat conversation list

Orhan Tozan · Answer

Imagine a repository with alot of raw chat data and another repository with user profiles using them together to construct conversation previews

Orhan Tozan · Answer

Now imagine a situation where there are 10 groupchats and everyone is spamming in the chat

Orhan Tozan · Answer

Following it from the conversation list there is a high chance the UI would be out of sync with mapLatest flatmaplatest because the consumer the constant changing new raw conversation state is constantly being recomputed to show a conversation preview thus the user probably only finally seeing the conversation list when the spam fest is over with

ephemient · Answer

having a single state object for that is going to be messy regardless

ephemient · Answer

ideally you could show partial computations along the way although how to do that may be tricky

Orhan Tozan · Answer

Ok let s say each conversation has his own flow

Orhan Tozan · Answer

Still if in that one conversation alot of people are spamming the chat you would still miss some message info with mapLatest

Orhan Tozan · Answer

Only after the spamfest is over

ephemient · Answer

if you re looking at a chat overview it seems reasonable to skip updates you can t keep up with as long as you don t show an inconsistent state

ephemient · Answer

```latestMessageInEachConversation = conversationsFlow flatMapLatest conversations gt conversations map conversation gt conversation map it latestMessage onSubscription emit LOADING combineLatest ``` or something along those lines

Orhan Tozan · Answer

Do you think it s reasonable to have a chat overview that doesn t change talking about last message of conversations for 10 seconds because a spamfest happened for 10 seconds I think between those 10 seconds the user would rather see the messages that were being spammed in the middle of the conversation than still seeing the very old messages

ephemient · Answer

^ it will show any updates that the collector is capable of collecting

louiscad · Answer

`flatMapLatest` is by far the most useful variant as it works with infinite cancellable flows which are very common compared to finite Flows you d need for `flatMapConcat` to be any useful

Orhan Tozan · Answer

Why is there not a Flow lt Flow lt T gt gt flattenLatest flattenConcat and flattenMerge exist For example sometimes my `combine` wants to return a Flow so you get a Flow in a Flow I can t flattenLatest this unfortunately

louiscad · Answer

Try writing it maybe you ll find out wink

Orhan Tozan · Answer

` flattenLatest ` `Unresolvered reference flattenLatest` Found out it doesn t exist

louiscad · Answer

You said that twice now

louiscad · Answer

I meant writing the code so it exists for you not writing the name of something that is not implemented yet

Orhan Tozan · Answer

Probably would take some effort first to understand the operators looking at flattenMerge s implementation I don t know really either what it does

Orhan Tozan · Answer

But I think you are trying to say that flattenLatest would be impossible

louiscad · Answer

I m not saying that

louiscad · Answer

I m only telling you to give it a try With `transformLatest` you can write it quite easily I think

Orhan Tozan · Answer

I m just trying to understand why it hasn t been implemented while flattenConcat and flattenMerge have been

louiscad · Answer

It s most likely priorities or ambiguity leading to tough API design questions You ll answer your own question if you try writing that function

louiscad · Answer

Also if you succeed you might send a PR to kotlinx coroutines or open an issue with the snippet

louiscad · Answer

Might also be lack of use case or that there s a better way to do what you want to do In Kotlin and in kotlinx coroutines there s no API with no use case