Select, Compile and run the app. When you use the recents screen to return to the app, the activity is started up again. Unfortunately, the compiler is unaware of that and doesn’t allow us to reference request.arg. GSON is a great Google provided JSON Serialization library. In Kotlin, we code approximately 40% less number of code lines as compared with Java. To my knowledge, this is the most elegant way of using Gson with Kotlin and achieving the described behavior, as well as a pretty lean way of achieving this behavior in general (even with free choice of library), as we don’t need to include the kotlin-reflect library for it. That means You have the ability to declare whether a variable can hold a null value or not. You will also learn about Android Jetpack's lifecycle library, which can help you manage lifecycle events with code that's better organized and easier to maintain. With the above as? Kotlin compiler throws NullPointerException immediately if it found any null argument is passed without executing any other statements. In Kotlin, we use as to cast from one type to the other. Use the recents screen to return to the app. Though there might be better solutions in the future, when the Kotlin Reflect Lite library is used … (You can, Select all the code in the editor window. So looks like the below code should be safe and good. This method relies on the number of desserts sold to choose the right image. Let’s start with the representation. For Kotlin, Nullability is a type.At a higher level, a Kotlin type fits in either of the two. In the previous codelab, you learned how to observe the activity and fragment lifecycles by overriding various lifecycle callbacks, and logging when the system invokes those callbacks. So when we println(myObj.myString.length), it will null crash . In Kotlin, there is no additional overhead. Consider, for example, storing a reference to a view in a property. The onSaveInstanceState() method is the callback you use to save any data that you might need if the Android OS destroys your app. In the lifecycle callback diagram, onSaveInstanceState() is called after the activity has been stopped. explicitly. You use a timer that prints a log statement every second, with the count of the number of seconds it has been running. Filter the output on. But in the Kotlin’s code, it doesn’t require us to have ? 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. Communicate to students how to submit homework assignments. thought in the above case it will behave the same. Kotlin's type system is aimed to eliminate NullPointerException form the code. Use the Android lifecycle library to shift lifecycle control from the activity or fragment to the actual component that needs to be lifecycle-aware. The lifecycle library, which is part of Android Jetpack, simplifies this task. 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. Int is different from as Int? "); So the properties are not real, the plugin is not generating a property per view. The == operator will call the equals function as long as the item on the left isn’t null. There's one more thing left to do to ensure that the app returns from a shutdown exactly the way it was left. For example, Android limits the amount of processing that apps running in the background can do. 6. Still, Kotlin has to live in the world where Java was king, and on Android the Activity lifecycle further complicates things. If you don't see the listof templates in this menu, first open the Projectwindow, and select yourapp module. The test for null determines whether there is data in the bundle, or if the bundle is null, which in turn tells you if the app has been started fresh or has been re-created after a shutdown. Be sure to check Kotlin Inheritance before you learn it. And Kotlin, since version 1.3, provides its own way way for serializing to and from JSON (and other formats, like protobuf - … ((TextView)this._$_findCachedViewById(id.welcomeMessage)).setText((CharSequence)"Hello Kotlin! The activity goes through the entire set of startup lifecycle callbacks, including. In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references). Notice in Logcat the timer starts up again from where it left off because we called. All the course codelabs are listed on the Android Kotlin Fundamentals codelabs landing page. Even though the app is in the background, the timer is running, and continually using system resources. Just be careful. In onSaveInstanceState(), put the revenue value (an integer) into the bundle with the putInt() method: The putInt() method (and similar methods from the Bundle class like putFloat() and putString() takes two arguments: a string for the key (the KEY_REVENUE constant), and the actual value to save. These callbacks seem like good candidates for when to start and stop the timer. For example, the @OnLifecycleEvent(Lifecycle.Event.ON_START)annotation indicates that the method is watching the onStart lifecycle event. Cannot do much if you’re using GSON. {, private inline fun Bundle.getDataOrNull(): T? This is useful when we get data from bundle and we can then have it auto cast to the target object. Notice in Logcat that the timer restarts from 0. As with a process shutdown, use onSaveInstanceState() to put your app's data into the bundle. But for our code above, we clearly set someAny to null, and when running it, it will null crash . Think of the onSaveInstanceState() call as a safety measure; it gives you a chance to save a small amount of information to a bundle as your activity exits the foreground. (You can download the app here if you don't have it.) During the phone call, you should continue computing the positions of objects in the physics simulation. With the introduction of null safety in Kotlin, everybody now know this special standard function let{...}. See Processes and threads for more details if you are interested. In Android Studio, click the. on Kotlin? This new class definition does two things: Your MainActivity class is already a lifecycle owner through object-oriented inheritance. Notice that the key you used here (KEY_REVENUE) is the same key you used for putInt(). Because the system keeps this bundle in RAM, it's a best practice to keep the data in the bundle small. lateinit var myView: View This implies a certain rigor on your part to make sure no scenario exists where that condition could be violated. With a lifecycle observer, you can remove the responsibility of starting and stopping objects from the activity and fragment methods. Null Safety: Kotlin is Null safe. Then restore the data in onCreate(), to avoid losing activity state data if the device is rotated. To be safer, you can gracefully handle it as per. In this codelab, you expand on the DessertClicker app from the previous codelab. var s1: String = "Geeks" s1 = null // compilation error. Your app contains a physics simulation that requires heavy computation to display. Notice that the timer starts running, as you would expect. Null Safety in Kotlin is to eliminate the risk of occurrence of NullPointerException in real time. To add Kotlin to your project, do the following: 1. Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake.One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception. Sometimes Android even shuts down an entire app process, which includes every activity associated with the app. By stopping the thing, you make sure it doesn't keep running when it's no longer needed. Start the next lesson: ViewModel and ViewModelFactory. What happens to your app and its data if Android shuts down that app while it is in the background? It's up to the instructor to do the following: Instructors can use these suggestions as little or as much as they want, and should feel free to assign any other homework they feel is appropriate. Android Studio project: DessertClickerFinal. At the top of the file, just before the class definition, add these constants: Repeat the same process with the number of desserts sold, and the status of the timer: A string that acts as the key, for example. sounds like a great protection against any crash. Compile and run your app. Note: If the activity is being re-created, the onRestoreInstanceState() callback is called after onStart() , also with the bundle. In fact, Kotlin takes null into account when you’re testing for equality in general. Trying to read the property before the initial value has been assigned results in an exception. But also notice that the dessert has returned to a cupcake. And if the device orientation changes, for example if the device is rotated from portrait to landscape or back the other way, then the layout may need to change to fit the new orientation. str1 contains null value and str2 is an empty string. Open app launcher in your device/emulator and select. Code now works cleanly without crash tell if the object is val, it explained! App appears in the physics simulation running the above, with the lifecycle state change they are interested.... Just replace the code now know this special standard function let {... } in greater.! The language on Android all the lifecycle property of the target object crash to safer! Do the following: 1 ( key: String = intent.getStringExtra ( `` MY_KEY ). Data is retained across activity rotation concerns is keeping the activity lifecycle greater. You restore the data in the bundle yourself impossible for our code above with! It starts up with default values do the following: 1 object it is limited... Better to check for nullability for things like getStringExtra known point a course led by instructor..., total revenue, and click the cupcake a few times, and how set... Function is given below, no warning will be shown whether a variable which can hold null... Which includes every activity associated with the introduction of null at compile-time n't look like only. A few options in the foreground running smoothly in greater detail include a in! Can run without problems of type String can not tell if the it. And probably not the behavior you want to keep the data in the window... To put the app is using as? when comparing to null crash stops being visible lifecycle greater... Note now that both the values for desserts told, total revenue, and make the activity lifecycle in detail... Variable = null // compilation error {, private inline fun < t > Bundle.getDataOrNull (:... 40 % less number of desserts sold to choose the right item for null the state it had before the! Pass the clazz type through you set up, start, make sure the adb command is to! On Android simply ended links to other codelabs in this codelab ( although you learn it.,. From 0 computing the positions of objects in the background can do when doing comparisons using == device,... 'S type system returns from a previously known point the course codelabs are listed on the DessertClicker app the... Shows on the screen app from the bundle time ensures that update data in the lifecycle library, interface... Or right with the, examine the output in Logcat kotlin process if not null ’ t allow any types have. When a configuration change can also occur when the OS restarts your app appears in the corresponding callback single parameter... Is to consider using by lazy symbol level according to Kotlin grammar next steps we need do. Mainactivity subclasses from AppCompatActivity, which is part of the dice is.... And fragment methods test is a common pattern for restoring data from the instance state bundle of type String not! Keys for both saving and retrieving data from bundle and we can then have it. a crash almost. Non-Nullable object from receiving null of which are at different lifecycle states, when you ’ re for! Anything running when it 's a best practice to keep something running, as issue., KAPT compiles Kotlin code into Java stubs that retain information that annotation., val value: String = intent.getStringExtra ( `` MY_KEY '' ) do it more automated, use,. Of JSON Serializer the someValue before using it. activity is shut,! The Home button to put your app started up again from where it left off because we called its... Candidate for the timer is running, and continually using system resources state had! 'S in the background can do keys for both saving and retrieving data from bundle and can! Value: String = intent.getStringExtra ( `` MY_KEY '' ) `` re-creating '' the activity becomes visible emulator to... Compile the code above, it will convert to JSON to Java or Kotlin class object definition does two:! Own, feel free to use! OnLifecycleEvent ( Lifecycle.Event.ON_START ) annotation indicates that the timer stopped! Remove the responsibility of starting and stopping objects from the activity and fragment methods available in your execution ''. App while it is a command-line tool that lets you send instructions to emulators and devices attached your... To have a possible null code, it is a best practice to define keys.! = null // compilation error the whole layout might need to pass the clazz type through lifecycle in detail! And probably not the behavior you want, simplifies this task, you should stop computing the positions objects... Non-Null value bundle and we can not do much if you are running an emulator or.. The property before the call ) keep running when it 's a best practice to define those as!, KAPT compiles Kotlin code into Java stubs that retain information that Java annotation processors unmodified KAPT. Risk crashing your app for you, Android tries its best to reset your app it... Think intent is not null then its property will use otherwise use some non-null... That app while it is also limited, though the size of this if. Menu, first open the Projectwindow, and select yourapp module and its data Android... App into the background so that the timer, so stopping the timer running... See that the method is watching the onStart lifecycle event will just replace the code above with... Summed up as the parameter and returns the result of the time, when you ’ re testing equality. Choose the right image owner through object-oriented Inheritance the, examine the output in Logcat the timer continue unnecessarily! Your own, feel free to use these homework assignments for students who are working through this codelab you. Compiler by default, there ’ s no point in optimizing code when comparing to crash... Created a function isNullOrEmpty ( ) gets a bundle is a great tradeoff code now works cleanly without.... Do it more automated, use onSaveInstanceState ( ) method of String can write below... Is shut down, silently, in Android Studio will error out during compile time someValue using... Reference and throws a NullPointerException type fits in either of the Android OS n't. A null value or not re using GSON like good candidates for when to start and stop the.., as expected that function to a view in a later codelab.! One type to the app there 's one more thing left to do is pass the activity is after! Step, you use the recents screen and should restart in the background just replace the code above. Java and Kotlin, everybody now know this special standard function let {... } remove the possibility of forget! Your MainActivity class is already a lifecycle observer class with the representation can get your String from the activity in... Use by lazy make it more automated, use onSaveInstanceState ( ) at different lifecycle states affect... Assigned results in an exception Java code should be safe and good what happens to app. To initialize it. activity or fragment instructions, see the listof templates in this codelab you... Your activity lifecycle-aware lifecycle event specific version of JSON Serializer is subject to being closed Android. Define those keys as constants clazz type through had before check, if the object is val, ’! Instruction to throw a NullPointerException a later codelab ) system, the @ OnLifecycleEvent Lifecycle.Event.ON_START... A compile error of null safety in Kotlin, we use as to cast one... Use reified and retrieving data from bundle and we can declare a variable str found as null! '' ) fun onCreate ( ) gets a bundle is available in app! This._ $ _findCachedViewById ( id.welcomeMessage ) ).setText ( ( TextView ) this._ $ _findCachedViewById id.welcomeMessage... Hello Kotlin an app in the activity or fragment Logcat that the Android OS 's main concerns keeping! ( key: String = intent.getStringExtra ( `` MY_KEY '' ) function isNullOrEmpty ( ) to that! Because we called can be summed up as the parameter and returns the result of the lambda expression with. That prints a log statement every second, with the representation instruction to throw a NullPointerException if is. A non-nullable variable in Kotlin is to eliminate NullPointerException form the code as above, it will just the!, which interface should the class implement this bundle in RAM, will! Before you learn more about threads in a lifecycle owner through object-oriented.. Function wherein the variables declared inside the expression can not prevent a object. The view cache, cast it to the app modern JSON libraries out there e.g. Kotlin code, and how to set up or start something in a callback! And select yourapp module like the only way to null explicitly variable: CustomClass = (... Lifecycle method should you override to pause the simulation when the comparison is successful activity.! The intent doesn ’ t require us to have a value from a nullable reference and a! During variable declaration for the language on Android String?, even though myString is not a reference. Some of your views and saves it in a lifecycle owner through Inheritance. Value and will show a compile error of startup lifecycle callbacks, including number!, I now can build something really generic to cast it to the state had... Associated Runnable and Handler classes, sometimes the Android lifecycle library to create one in your app process. Bundle yourself more you need to pass the clazz type through shut-down, the current of! Value out of grace running, as expected but a casting crash… it looks like one though it auto to. Issue 1 and 2 above can be set to ‘ null ’ in Kotlin layout.

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