Coroutines and ViewModel

Most of the Kotlin samples in this book that need to do background processing will do so using coroutines. We will have suspend functions that either call other suspend functions or do work in a designated thread or thread pool:

suspend fun doSomethingCool(): AwesomeMix = withContext(Dispatchers.IO) {
  // TODO something that returns an AwesomeMix
}

A suspend function needs to be called inside of a CoroutineScope. While Kotlin offers a GlobalScope that can be used for anything, ideally you use a scope that is more closely tied to the work that is being done.

In Android UI development, that frequently will be a CoroutineScope associated with a ViewModel. There is a viewModelScope extension property available to ViewModel that provides a CoroutineScope tied to the lifetime of the ViewModel. In particular, if the ViewModel is no longer being used (i.e., it is being called with onCleared()), the viewModelScope is also cleaned up.

So, you wind up with code like this:

fun doSomethingCoolAndGetTheResultsOnTheMainApplicationThread() {
  viewModelScope.launch(Dispatchers.Main) {
    val mix = repo.doSomethingCool()

    // TODO something with mix on the main application thread
  }
}

(where repo is an object of the class that has the aforementioned doSomethingCool() function)

Dispatchers.Main will mean that while doSomethingCool() will run on a background thread, we will get the result delivered to us on the main application thread.

You can learn more about coroutines in the "Introducing Coroutines" chapter of Elements of Kotlin Coroutines!

Some of the Kotlin examples later in the book will demonstrate using coroutines.