What if I need to setup callback wrapped in the co...

# coroutines

[Eugen Martynov]

What if I need to setup callback wrapped in the coroutine and only after call some action. Can I achieve it only with async? Or I need to use more thread synchronisation.

Problem: I need to add listener for DB update but DB save happens async and I can not hook into it.

val waithForDBSave = async { setupListenerAndWait() } 
db.save() // I want to wait before executing here to be sure listener was setup
waitForDBSave.await()

[Eugen Martynov]

I probably should use semaphore

[Dmitry Khalanskiy [JB]]

Hi! If it is guaranteed that setupListenerAndWait() will not suspend until the listener is installed, you can do this:

val waithForDBSave = async(start = CoroutineStart.UNDISPATCHED) {

[Eugen Martynov]

It is written like

suspendCoroutine { continuation ->
        val saveListener = object : SaveListener {
            override fun onSaved() {
                    continuation.resume(Unit)
            }
        }
        db.addListener(saveListener)
    }

[Dmitry Khalanskiy [JB]]

suspendCoroutine won't suspend until its block finishes executing, so UNDISPATCHED should work.

[Eugen Martynov]

So, your suggestion should work

[Eugen Martynov]

Thank you!

[Nitesh Singh]

could you explain me solution ?

[Eugen Martynov]

Sure! Without coroutines it is sequential execution code. Coroutines are adding points for suspension - where execution stops until it is resumed. Then it runs sequential again till the next suspension point. Every sequential execution runs on a thread. So proposal is to make start of async undispatched - so it should execute on the same thread until the next suspension point. And since listener setup happens there that guarantees that code which is suppose to notify listener will happen in sequential manner after.

[Zach Klippenstein (he/him) [MOD]]

Probably better to use launch instead of async if your callback doesn't have a result.

[Eugen Martynov]

Thanks! Off topic - After long discussion here we are going to change API so there is more clean api to make things in sync

[Nitesh Singh]

i didn't understand why you are using this code ? for whom?