Implementing the Activity

Creating MainActivity

It’s time to implement the code for the UI, starting with the landing page, which is the main activity. We’ll first write the code to handle the airport code that the user enters into the text box and the button click. Then we’ll write the code to populate the RecyclerView with data obtained from the web service. That appears like a lot of work, but, surprisingly, we won’t need too much code to implement these actions. Let’s get started, one step at a time.

Coroutines in the UI

As the first step, let’s address head-on the need for coroutines in the UI. The UI will have to fetch the airport status from the web service, but we don’t want the UI to block and become inactive when the application is in the middle of network calls. To address this, we’ve already designed the getAirportStatus() top-level function in AirportStatus to be non-blocking, using the suspend keyword. From Chapter 16, Exploring Coroutines, we know that functions marked with suspend can’t be called from within arbitrary functions—they have to be called from within coroutines. There’s a catch, however—the UI code created by the Android Studio doesn’t use coroutines. So if we invoke getAirportStatus() from an event handler, the code will fail compilation. In short, we have to run the UI code in a coroutine context so that the event handlers can call functions that are marked with suspend.

We can easily run the UI code as a coroutine by making a small change to the generated MainActivity class—we’ll implement Kotlin’s CoroutineScope and override the coroutineContext property. With this change, the activity class can provide a coroutine scope for the execution of coroutines. Let’s make this change in the MainActivity.kt file: