Hiya :wave:, I’m still trying to grasp coroutines ...

# android

[Martyna Maron]

Hiya 👋, I’m still trying to grasp coroutines and dispatchers, and I’ve encountered an issue in my Android app where the thread seemed to be blocked, which I’ve managed to fix but don’t quite understand why 😕 Any explanation would be appreciated!

In onResume of my main fragment, I’ve got code like this (further down the chain, I don’t call this directly like this in onResume):

suspend fun onSessionStart() = withContext(Dispatcher.javascript) {

        for(feature in featuresList) {
            feature.setUpSynchronously()

            myActivity.lifecycleScope.launch(Dispatcher.javascript) {
                feature.setUpAsync()
            }
        }
    }

Right after I call this method above, I call this:

suspend fun start() = withContext(Dispatcher.javascript) {
        //...
    }

Note that setUpAsync() may call a method which switches to Dispatcher.network, but doesn’t launch a new coroutine.

My problem is that on a fresh app launch, this code runs fine, but when I trigger onSessionStart again, when the app is still alive, I never get to start() and the app stalls. I can see by the logs that all of the setUpAsync() calls finish, but they don’t seem to return 🤷🏻‍♀️

Removing the context when launching the async set up, seems to fix the problem:

myActivity.lifecycleScope.launch {
                feature.setUpAsync()
            }

But I don’t understand why, any idea?

[Andrew]

Hey @Martyna Maron, I've never seen Dispatcher.javascript before - is this a custom dispatcher in your project?

[Martyna Maron]

yeah, same for the network one 👍 just maps to a custom ExecutorService

[Anshulupadhyay03]

Hey @Martyna Maron did you go through this https://developer.android.com/kotlin/coroutines/coroutines-best-practices ?

[Martyna Maron]

Don’t think I have, thanks!

[Colton Idle]

@Martyna Maron you might also have more luck asking this question in #coroutines

[gildor]

What kind custom execution service?

[gildor]

Also how do you call this onSessionStart from onResume?

[gildor]

Also setUpSynchronously looks quite suspicious, like some blocking method

[Natsuki(开元米粉实力代购)]

the most suspicious part is Dispatchers.javascript ,
launch should return immediately, even may before setUpAsync called, i guess the thread underline javascript is blocked