Note: Before you start, make sure you are running an emulator or device that supports API 28 or higher. In this codelab, you will explore the activity lifecycle in greater detail. A configuration change happens when the state of the device changes so radically that the easiest way for the system to resolve the change is to completely shut down and rebuild the activity. The API models Kotlin program structures at the symbol level according to Kotlin grammar. With that, in Kotlin when accessing a Java object, we can assume both Java objects as nullable or not. Lateinit is allowed for non-primitive data types only and the variable can't be of null … No variable by default can be set to ‘null’ in Kotlin. You use getInt() to get data out of the bundle, just as you used putInt() to put data into the bundle. Less boilerplate, more expressive code and, yes, it is null safe – if you choose the path of avoiding torture. Compile and run the app. Modify the app to use the Android lifecycle library, and convert the, Set up and use the Android Debug Bridge (, Open the DessertClicker app from the last codelab. The onStart() method is called just before the activity becomes visible. Since the FragmentActivity superclass implements LifecycleOwner, there's nothing more you need to do to make your activity lifecycle-aware. Click the Logcat tab to see that the onDestroy() callback was never run—your activity simply ended. In MainActivity, uncomment the onSaveInstanceState() method, run the app, click the cupcake, and rotate the app or device. It got selected as a candidate for the programming language of the year. However, when you run the code as above, it will null crash . When Android shuts down your app process, the, When your app goes into the background, just after, You can get data back out of the bundle in the, To retrieve data from the bundle with a key, use the. Kotlin had a pretty busy year in 2017. Another option is, use MOSHI, the Kotlin specific version of JSON Serializer. What an activity is, and how to create one in your app. ? You can rotate the emulator left or right with the rotation buttons, or with the, Examine the output in Logcat. Your app contains a physics simulation that requires heavy computation to display. But also notice that the dessert has returned to a cupcake. By supporting nullability in the type system, the compiler can detect possible NullPointerException errors at compile time and reduce the possibility of having them thrown at runtime. A configuration change can also occur when the device language changes or a hardware keyboard is plugged in. The lateinit keyword allows you to explicitly state that a variable is never null when it is read, even if it does not immediately contain a value. It compiles fine. Follow. So if the bundle is not null, you know you're "re-creating" the activity from a previously known point. Android Studio project: DessertClickerFinal. This is not really a null crash, but still, a crash that almost like a null. let takes the object it is invoked upon as the parameter and returns the result of the lambda expression. This is useful when we get data from bundle and we can then have it auto cast to the target object. Note: The DessertTimer class uses a background thread for the timer, with associated Runnable and Handler classes. When your app goes into the background, it's not destroyed, it's only stopped and waiting for the user to return to it. In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references). For Kotlin, Nullability is a type.At a higher level, a Kotlin type fits in either of the two. Groovy is not statically-typed, so there are no Null Pointer issues. Moshi ). There's one last special case in managing the activity and fragment lifecycle that is important to understand: how configuration changes affect the lifecycle of your activities and fragments. It's less important to keep the DessertClicker app, which the user might not have looked at for a few days, running smoothly in the background. For example, if you have a custom variable like revenue in the DessertClicker app, the Android OS doesn't know about this data or its importance to your activity. Note: There are two overrides for onSaveInstanceState(), one with just an outState parameter, and another that includes outState and outPersistentState parameters. It seems like the only way to null crash during runtime in Kotlin is to use !! Kotlin Android Extensions on fragments thought in the above case it will behave the same. But to the user, it doesn't look like the app has been closed.