Hey everyone I want to make the second version of the code r kotlinlang #coroutines

fun getAll(): Flow<...> { //works in parallel, doesn't respect structural concurrency
    GlobalScope.launch {
        //start request...
    }
    return db.flow()//starts emitting immediately...
}

suspend fun getAll(): Flow<...> { //works sequentially
    withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
        launch {
            //start request...
        }
    }
    return db.flow()//start emitting after request is finished...
}

Dominaezzz

05/06/2020, 3:03 PM

second version of what code? Not sure what you're trying to achieve.

ar-g

05/06/2020, 3:35 PM

I meant second version of the function. The launch courutine, which executes request, runs sequentially and

db.flow()

starts emitting only after request finished.

Dominaezzz

05/06/2020, 3:38 PM

fun getAll(): Flow<...> = flow { //works sequentially
    withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
        //start request...
    }
    emitAll(db.flow())//start emitting after request is finished...
}

ar-g

05/06/2020, 3:42 PM

no, this one also waits until request will be executed

I want to subscribe to db immediately, and then execute network request. I'm not fully understand why second version of the function doesn't run in parallel. Because semantiacally it looks very similar 🤔

Dominaezzz

05/06/2020, 3:44 PM

fun getAll(): Flow<...> = flow { //works in parallel, but only once flow is collected
    coroutineScope {
        launch(<http://Dispatchers.IO|Dispatchers.IO>) {
            //start request...
        }
        emitAll(db.flow()) //starts emitting immediately...
    }
}

The second version doesn't run in parallel because

withContext

waits for the

launch

to finish because Structured Concurrency.

ar-g

05/06/2020, 3:51 PM

Thanks Dominic, this one works, I wasn't aware of

courutineScope

function. I expected that everything which is launched with different Dispatcher would be executed in parallel.

araqnid

05/06/2020, 4:34 PM

launch != withContext - launch starts something in parallel, withContext starts something and suspends until it has completed. Regardless of the dispatcher used to actually execute bits of it. (and the coroutineScope here enforces that both have completed)

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