Coroutine launch() while inside another coroutine

Question

I have the following code (pseudocode)

fun onMapReady()
{
    //do some stuff on current thread (main thread)

    //get data from server
    GlobalScope.launch(Dispatchers.IO){

        getDataFromServer { result->

            //update UI on main thread
            launch(Dispatchers.Main){
                updateUI(result) //BREAKPOINT HERE NEVER CALLED
            }
        }

    }
}

As stated there as a comment, the code never enters the coroutine dispatching onto main queue. The below however works if I explicitly use GlobalScope.launch(Dispatchers.Main) instead of just launch(Dispatchers.Main)

fun onMapReady()
{
    //do some stuff on current thread (main thread)

    //get data from server
    GlobalScope.launch(Dispatchers.IO){

        getDataFromServer { result->

            //update UI on main thread
            GlobalScope.launch(Dispatchers.Main){
                updateUI(result) //BREAKPOINT HERE IS CALLED
            }
        }

    }
}

Why does the first approach not work?

Answer 1

I believe the problem here is that getDataFromServer() is asynchronous, it immediately returns and therefore you invoke launch(Dispatchers.Main) after you exited from the GlobalScope.launch(Dispatchers.IO) { ... } block. In other words: you try to start a coroutine using a coroutine scope that has finished already.

My suggestion is to not mix asynchronous, callback-based APIs with coroutines like this. Coroutines work best with suspend functions that are synchronous. Also, if you prefer to execute everything asynchronously and independently of other tasks (your onMapReady() started 3 separate asynchronous operations) then I think coroutines are not at all a good choice.

Speaking about your example: are you sure you can't execute getDataFromServer() from the main thread directly? It shouldn't block the main thread as it is asynchronous. Similarly, in some libraries callbacks are automatically executed in the main thread and in such case your example could be replaced with just:

fun onMapReady() {
    getDataFromServer { result->
        updateUI(result)
    }
}

If the result is executed in a background thread then you can use GlobalScope.launch(Dispatchers.Main) as you did, but this is not really the usual way how we use coroutines. Or you can use utilities like e.g. runOnUiThread() on Android which probably makes more sense.