async and await

Why async & await are used?

The launch() function returns a Job object that can be used to await termination of the coroutine or to cancel. But there’s no way to return a result from the coroutine that was started using launch(). If you want to execute a task asynchronously and get the response, then use async() instead of launch().

The async() function takes the same parameters as launch(), so we can configure the context and start properties of coroutines that are created using async() and launch() in the same way. The difference, though, is that async() returns a Deferred<T> future object which has an await() method, among other methods, to check the status of the coroutine, cancel, and so on. A call to await() will block the flow of execution but not the thread of execution. Thus, the code in the caller and the code within the coroutine started by async() can run concurrently. The call to await() will eventually return the result of the coroutine started using async(). If the coroutine started using async() throws an exception, then that exception will be propagated to the caller through the call to await().

Using async & await

We’ll use async() and await() in the next chapter, but here, let’s get a quick taste of these functions. In the code that follows we get the number of cores available on a system asynchronously. The Dispatchers.Default argument is optional and, if left out, the coroutine will run in the dispatcher of the scope it inherits. In this example, we have a single-threaded scope and thus the coroutine will run in the same thread as the caller. But when run in a scope of a multi-threaded dispatcher, the coroutine will execute in any of the threads of that dispatcher.