Kotlin Coroutines : Waiting for multiple threads to finish

Question

So looking at Coroutines for the first time, I want to process a load of data in parallel and wait for it to finish. I been looking around and seen RunBlocking and Await et

Answer 1

You can use async builder function to process a load of data in parallel:

class Presenter {
    private var job: Job = Job()
    private var scope = CoroutineScope(Dispatchers.Main + job) // creating the scope to run the coroutine. It consists of Dispatchers.Main (coroutine will run in the Main context) and job to handle the cancellation of the coroutine.

    fun runInParallel() {
        scope.launch { // launch a coroutine
            // runs in parallel
            val deferredList = listOf(
                    scope.asyncIO { processPages(urls, collection) },
                    scope.asyncIO { processPages(urls, collection2) },
                    scope.asyncIO { processPages(urls, collection3) }
            )

            deferredList.awaitAll() // wait for all data to be processed without blocking the UI thread

            // do some stuff after data has been processed, for example update UI
        }
    }

    private fun processPages(...) {...}

    fun cancel() {
        job.cancel() // invoke it to cancel the job when you don't need it to execute. For example when UI changed and you don't need to process data
    }
}

Extension function asyncIO:

fun  CoroutineScope.asyncIO(ioFun: () -> T) = async(Dispatchers.IO) { ioFun() } // CoroutineDispatcher - runs and schedules coroutines

GlobalScope.launch is not recommended to use unless you want the coroutine to be operating on the whole application lifetime and not cancelled prematurely.

Edit: as mentioned by Roman Elizarov you can try not to use awaitAll() function unless you want to update UI or do something else right away after all data are processed.