Using a specific dispatcher when starting the coroutine

So far, we have seen how to create coroutines using async() and launch(), but in both cases we were using the default dispatcher. Consider the following code:

fun main(args: Array<String>) = runBlocking {
  val task = launch {
    printCurrentThread()
  }
  task.join()
}

Here, printCurrentThread(), as its name suggests, will just print the name of the current thread to the standard output:

fun printCurrentThread() {
  println("Running in thread [${Thread.currentThread().name}]")
}

By running this code, we see that by default the coroutine will run in DefaultDispatcher, which as the time of writing is the same dispatcher as CommonPool – but be aware that this could change in the future.

If we update main() to create a CoroutineDispatcher the same way we did before, and send it to launch(), we will see that the coroutine is being executed in the thread that we indicated, for example:

fun main(args: Array<String>) = runBlocking {
  val dispatcher = newSingleThreadContext(name = "ServiceCall")
  val task = launch(dispatcher) {
    printCurrentThread()
  }
  task.join()
}

The output looks like the following screenshot. Notice that the name of the thread is the same name that we set for the dispatcher:

Now we will do likewise in MainActivity:

private val dispatcher = newSingleThreadContext(name = "ServiceCall")
override fun onCreate(savedInstanceState: Bundle?) {
  super.onCreate(savedInstanceState)
  setContentView(R.layout.activity_main)
  launch(dispatcher) {
    // TODO Call coroutine here
  }
}