10.3. Using async and wait

Sometimes, we want a thread to stop running concurrently. For instance, our current thread may need to wait for the result of a coroutine before continuing to run. Obviously, the main thread should not do that because our application would stop, but it may be necessary to do it in other threads.

In the example below we create a coroutine that will then stop to wait for another result:

val coroutineContext: coroutineContext =  Dispatchers.Default + SupervisorJob()
val scope: coroutinescope = coroutinescope(coroutineContext)

scope.launch() {
   val job_deferred: Deferred<Int> = async {
delay(1000L)
   	(..100).random()
   }

   val r = job_deferred.await()
   Log.i("coroutine await",r.toString())
}

First, r = job_deferred.await() is executed on thread worker1:

Executing await.
Source: Javier Salvador (Original image) License: CC BY-NC-ND 4.0

This causes the thread worker1 to stop until the coroutine started with await finishes and returns its result. Now we can see that the coroutine that has been launched is running on the thread called worker2.

The coroutine is running while the original thread waits.
Source: Javier Salvador (Original image) License: CC BY-NC-ND 4.0

Finally, when job_deferred completes its execution, the coroutine running it stops and worker1 resumes its execution having the result in variable r:

After await.
Source: Javier Salvador (Original image) License: CC BY-NC-ND 4.0