How do I cancel a Flow collection and start a whol...

# coroutines

[Remy Benza]

How do I cancel a Flow collection and start a whole new flow?

[Giorgos Neokleous]

You can format code blocks with triple ticks

[Remy Benza]

👍 got an idea for my question? 😄

[louiscad]

Cancel the coroutine calling collect and launch a new one. The operators of the family xxxLatest might also be what you're looking for @Remy Benza

[Remy Benza]

should I cancel the coroutine scope (tied to the activity) inside the viewmodel?

[Remy Benza]

since I have a date picker which sends an event to the viewmodel with the epoch instant I use to fetch the music tracks

[louiscad]

No, you should not cancel the root scope

[louiscad]

You can just cancel the Job returned by the call to launch or flow.launchIn(scope)

[louiscad]

You can make the user selection a Flow and then use flatMapLatest

[Remy Benza]

so this job I would cancel?

[Remy Benza]

when the user select a date via the date picker?

[louiscad]

Well, I'm not going too deep into your codebase details, but you can convert that listener that reflects user actions (selecting a date) using callbackFlow

[Remy Benza]

Thnx alot for you replies

[Remy Benza]

But I would need a callback flow in order for this to work right?

[Remy Benza]

Build a callbackflow once for an edittext in a searchbox with a debouncer

[louiscad]

There's multiple ways, but converting the listener to a Flow with callbackFlow would enable you to use flatMapLatest

[louiscad]

Reading this doc will certainly help you to see what are the building blocks you can leverage: https://kotlinlang.org/docs/reference/coroutines/flow.html

[Remy Benza]

yes thnx @louiscad, I'm quite new to flow as you may have noticed 😋

[louiscad]

Docs are often a great place to start when they're available, as is the case with first party Kotlin libraries 😉

[Remy Benza]

Yes, I've red the docs but then translating these general explanations to your own concrete implementation isn't a piece of cake that often

[Remy Benza]

but your advise helps a lot

[louiscad]

You also have some snippet examples in the KDoc here : https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/ 🙂

[Remy Benza]

ok @louiscad final question 😇, so I wrote the callbackflow extension function for the datepicker but can't seem to use .flatMapLatest

[Remy Benza]

only .map is valid

[Remy Benza]

.flatMapLatest gives me:
Type inference failed: inline fun <T, R> Flow<T>.flatMapLatest(crossinline transform: suspend (value: T) -> Flow<R>): Flow<R>
cannot be applied to
receiver: Flow<Long> arguments: (suspend (Long) -> Unit)

[Remy Benza]

In the .flatMapLatest docs I read: Returns a flow that switches to a new flow produced by transform function every time the original flow emits a value. When the original flow emits a new value, the previous flow produced by transform block is cancelled.

[Remy Benza]

this is indeed the behavior I'm looking for

[louiscad]

@Remy Benza Which Kotlin version are you using?

[Remy Benza]

'1.3.61'

[louiscad]

With Kotlin 1.4.10, a bunch of type inference bugs have been fixed.

[Remy Benza]

can't switch to 1.4 due to very large codebase and kotlinx serialization 0.14 migration work

[louiscad]

kotlinx what?

[louiscad]

kotlinx is a not a library

[Remy Benza]

https://github.com/Kotlin/kotlinx.serialization

[Remy Benza]

1.4 needs 1.0 for, so we can't just switch over

[louiscad]

This library has been updated as well, and there's a migration guide

[Remy Benza]

I know, the customer has other priorities now so..

[louiscad]

On the long term, it's not really viable to keep using outdated software, so better take the time to upgrade now

[louiscad]

But if it's a blocking point for his priorities, then…

[Remy Benza]

Im fully with you on this, but Im not making these decisions

[Remy Benza]

are you sure this is due to a bug?

[Remy Benza]

not a error in my code?

[louiscad]

Anyway, you might be able to get away by specifying all the type arguments to workaround these issues

[louiscad]

I don't know, I cannot dive too deep or I'll need a cut from what your client pays you 😛

[Remy Benza]

haha

[Remy Benza]

I'll send them your bill

[louiscad]

Haha, I don't believe it 🙃
Do explicit types work?

[Remy Benza]

but flatMap latest operator should be on the

dateSelectionAsFlow()

right? and the canceling the job there is also the right place?

[louiscad]

You can do a test with a simple JVM only playground

[louiscad]

To understand how these work

[louiscad]

Making a flow is quite easy, there's examples in the doc.

[Remy Benza]

yes, I know the basic stuff. flowOf() and emit / collect

[Remy Benza]

but this is a bit more envolved

[louiscad]

Then you can play with simple flows and use flatMapLatest with them

[louiscad]

You can hardcode a sample of what you could get from dateSelectionAsFlow()

[louiscad]

Then check all the blocks before joining them together once your understanding is sufficient

[Remy Benza]

yeah, I know I must invest a bit more time to learn the API's better

[Remy Benza]

but for today would love to fix this issue

[Remy Benza]

aside from the flatMapLatest operator

[Remy Benza]

doesn't my job.cancel() cancels the whole

lifecycleScope.launch {

[Remy Benza]

btw I'm on kotlinx coroutines 1.4

[Remy Benza]

but on kotlin 1.3.61

[louiscad]

If you use flatMapLatest, you should not need to do any manual coroutines cancelling for that purpose.

[Remy Benza]

.flatMapLatest does indeed work on a codebase with kotlin 1.4.10

[Remy Benza]

but im not seeing how a flow of date selection will retrigger my placedtracks flow in my viewmodel when all im doing is sending the epoch Long to the viewmodel and setting it in a LiveData value for the UI

[Remy Benza]

shouldn't I manually cancel the current flow via it's job

[louiscad]

flatMapLatest cancels the flow returned in the previous lambda automatically, that's what its doc specifies