Good morning (for some at least i guess :sweat_smile: ) I'm starting to use flows and I wonder how c...

# coroutines

[julioyg]

Good morning (for some at least i guess 😅 ) I'm starting to use flows and I wonder how can i specify the thread the function I pass onto collect is executed.
I'm trying:

launch(UI) {
  flow.collect {
    // i was hoping this was executed in UI
  }

should that be running in the UI Dispatcher?

[nrobi]

You can use kotlinx.coroutines.Dispatchers.Main for the UI

[sikri]

You don’t need to launch { flow.collect { } }, because you would have callabacks and too many scopes
Instead, you can launch flow in some scope, for example:

flow
    .onEach { doSmth(it) }
    .launchOn(Dispatcher.Main)

[julioyg]

thanks 🙂

[julioyg]

@sikri that doesn't consume the the value, does it?

[julioyg]

it just creates new flow that does that on when there is a new item

[nrobi]

Yes it does, launchOn() calls collect()

[sikri]

It consumes

[sikri]

There is a nice simple example
https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/launch-in.html

[julioyg]

ohhhhh! cool

[julioyg]

Thank you 🙂

[sikri]

Because other way we would have too many scopes

launch(UI) {
   flow.collect {
       too many scopes
   }
}

[sikri]

https://github.com/Kotlin/kotlinx.coroutines/blob/8d8b6eb8a1186ae987610a31ac441d107d04b034/kotlinx-coroutines-core/common/src/flow/terminal/Collect.kt

@ExperimentalCoroutinesApi // tentatively stable in 1.3.0
public fun <T> Flow<T>.launchIn(scope: CoroutineScope): Job = scope.launch {
    collect() // tail-call
}

[tseisel]

@julioyg When collecting a flow with collect, the passed block invoked for each collected item, and * always in the context of the current coroutine*.
Note that UI is deprecated in favor of Dispatchers.Main.
If you want some flow operations to be executed on another Thread/Dispatcher, you could use the flowOn operator. Note that unlike Rx `subscribeOn`/`observeOn`, flowOn is only applied to upstream operations.

[julioyg]

thanks 🙂