I recently read something that I interpreted as be...

# coroutines

[Dennis Schröder]

I recently read something that I interpreted as being bad.

suspend fun someFunctionCreatesAFlow(context: Dispatchers.Default): Flow<String> = withContext(context) {
 flow {
    emit(somethingAsAString)
   } 
}

I guess it is because Flows kinda manage their own context with flow::flowOn(). Can anybody confirm this?

[Dennis Schröder]

So I guess this is the right way ... ?

suspend fun someFunctionCreatesAFlow(context: Dispatchers.Default): Flow<String> = coroutineScope {
 flow {
    emit(somethingAsAString)
   } .flowOn(context)
}

[Dominaezzz]

suspend fun someFunctionCreatesAFlow(context: Dispatchers.Default): Flow<String> = flow {
    emit(somethingAsAString)
   } .flowOn(context)

[Dennis Schröder]

Ohh well yeah but in my actual use case `somethingAsAString`is of type suspend CoroutineScope.() -> String. Therefore I need a scope

[octylFractal]

it's not bad necessarily, but iirc flow isn't a suspend function itself, only terminal operations, so it doesn't pass any of the context to the flow. technically that function doesn't need to be suspend at all

[Dennis Schröder]

But I forget to mention sorry

[octylFractal]

if you need a scope, create it inside flow { }

[octylFractal]

e.g.

flow {
  coroutineScope {
     ...
  }
}

[louiscad]

Don't make is suspending, just use flowOn.

[streetsofboston]

What Louis said. The creation of a Flow has/needs no context/scope. The collector of the Flow, however does need one, and that will be the same scope that is used when your Flow's emitter (that suspend-lambda) is called.

[Pablichjenkov]

If I call Flow.collect() from a scope that has a Dispatcher.UI type.
Will the flow chain process in this UI scope by default?
Am I forced to use flowOn(IOContext) to make it execute off the UI thread?

[streetsofboston]

@Pablichjenkov That is not necessary a problem.
As long as you don’t have blocking code in your Flow, you should be all set.
If you have suspending code inside your Flow, that is not a problem either, since suspending won’t block the UI thread

[Pablichjenkov]

Well by flow chain process I mean flows that have suspending functions inside and either IO or heavy computation blocks.
Where these suspending blocks will be executed if not passed any Dispatcher explicitly, Dispatcher.Default?

[louiscad]

All the info about that is already in kotlinx.coroutines docs. You can verify what you see in the docs, that should help you understand what to expect.

[octylFractal]

flows run on the same dispatcher as whatever calls the terminal operations, unless you use flowOn, then everything setup before that call runs in the context passed there (calls after flowOn still use the same dispatcher as the terminal op caller)

[Pablichjenkov]

@octylFractal I read what you just said, however, my concern was about the following:
You have 2 types of code in a flow, lets say.

// case 1
flow {

    suspendingBlockingFunc()

}
//and case 2
flow {

    nonSuspendingBlockingFunc() // Blocks either by IO or heavy CPU

}

If not using flowOn operator and calling collect() from Dispatcher.UI.
Case 1 flow will execute on UI killing it
Case 2 flow will execute on where? -> Default?

[octylFractal]

exactly what I said, the same dispatcher as collect()

[octylFractal]

that is, case 1 & 2 are the same, there is no difference in how they are executed besides what the function they call does

[Pablichjenkov]

I see it now. There is no magic behind it.

[Pablichjenkov]

thnks