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

Question

Giorgos Neokleous · Answer

You can format code blocks with triple ticks

Remy Benza · Answer

+1 got an idea for my question smile

louiscad · Answer

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 · Answer

should I cancel the coroutine scope tied to the activity inside the viewmodel

Remy Benza · Answer

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 · Answer

No you should not cancel the root scope

louiscad · Answer

You can just cancel the `Job` returned by the call to `launch` or `flow launchIn scope `

louiscad · Answer

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

Remy Benza · Answer

so this job I would cancel

Remy Benza · Answer

when the user select a date via the date picker

louiscad · Answer

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 · Answer

Thnx alot for you replies

Remy Benza · Answer

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

Remy Benza · Answer

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

louiscad · Answer

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

louiscad · Answer

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 · Answer

yes thnx < louiscad> I m quite new to flow as you may have noticed yum

louiscad · Answer

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

Remy Benza · Answer

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 · Answer

but your advise helps a lot

louiscad · Answer

You also have some snippet examples in the KDoc here <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines flow flow > slightly smiling face

Remy Benza · Answer

ok < louiscad> final question innocent so I wrote the callbackflow extension function for the datepicker but can t seem to use flatMapLatest

Remy Benza · Answer

only map is valid

Remy Benza · Answer

flatMapLatest gives me Type inference failed inline fun lt T R gt Flow lt T gt flatMapLatest crossinline transform suspend value T gt Flow lt R gt Flow lt R gt cannot be applied to receiver Flow lt Long gt arguments suspend Long gt Unit

Remy Benza · Answer

In the flatMapLatest docs I read Returns a flow that switches to a new flow produced by <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines flow flat map latest html kotlinx coroutines flow$flatMapLatest kotlinx coroutines flow Flow kotlinx coroutines flow flatMapLatest T 20kotlin SuspendFunction1 kotlinx coroutines flow flatMapLatest T 20kotlinx coroutines flow Flow kotlinx coroutines flow flatMapLatest R transform|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 · Answer

this is indeed the behavior I m looking for

louiscad · Answer

< Remy Benza> Which Kotlin version are you using

Remy Benza · Answer

``` 1 3 61 ```

louiscad · Answer

With Kotlin 1 4 10 a bunch of type inference bugs have been fixed

Remy Benza · Answer

can t switch to 1 4 due to very large codebase and kotlinx serialization 0 14 migration work

louiscad · Answer

kotlinx what

louiscad · Answer

kotlinx is a not a library

Remy Benza · Answer

Remy Benza · Answer

1 4 needs 1 0 for so we can t just switch over

louiscad · Answer

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

Remy Benza · Answer

I know the customer has other priorities now so

louiscad · Answer

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

louiscad · Answer

But if it s a blocking point for his priorities then

Remy Benza · Answer

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

Remy Benza · Answer

are you sure this is due to a bug

Remy Benza · Answer

not a error in my code

louiscad · Answer

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

louiscad · Answer

I don t know I cannot dive too deep or I ll need a cut from what your client pays you stuck out tongue

Remy Benza · Answer

haha

Remy Benza · Answer

I ll send them your bill

louiscad · Answer

Haha I don t believe it upside down face Do explicit types work

Remy Benza · Answer

but flatMap latest operator should be on the ```dateSelectionAsFlow ``` right and the canceling the job there is also the right place

louiscad · Answer

You can do a test with a simple JVM only playground

louiscad · Answer

To understand how these work

louiscad · Answer

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

Remy Benza · Answer

yes I know the basic stuff flowOf and emit collect

Remy Benza · Answer

but this is a bit more envolved

louiscad · Answer

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

louiscad · Answer

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

louiscad · Answer

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

Remy Benza · Answer

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

Remy Benza · Answer

but for today would love to fix this issue

Remy Benza · Answer

aside from the flatMapLatest operator

Remy Benza · Answer

doesn t my job cancel cancels the whole ``` lifecycleScope launch ```

Remy Benza · Answer

btw I m on kotlinx coroutines 1 4

Remy Benza · Answer

but on kotlin 1 3 61

louiscad · Answer

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

Remy Benza · Answer

flatMapLatest does indeed work on a codebase with kotlin 1 4 10

Remy Benza · Answer

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 · Answer

shouldn t I manually cancel the current flow via it s job

louiscad · Answer

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