![]() ![]() The system doesn't persist instances of ViewModel to long-term storage. ![]() However, if you or the system exit the app, then the timer resets.Ĭaution: The system persists instances of ViewModel in memory throughout the life of a lifecycle owner, such as a fragment or activity. Navigate to another app and then return. ![]() Run the app (choose Step 2 in the Run Configurations dropdown) and confirm the timer doesn't reset when you perform either of the following actions: Remember that when a device is rotated, the activity is destroyed but any instances of ViewModel associated with it are not. The new instance of the owner re-connects to the existing ViewModel, as illustrated by the following diagram:Ĭaution: The scope of an activity or fragment goes from created to finished (or terminated), which you must not confuse with destroyed. A ViewModel is not destroyed if its owner is destroyed for a configuration change, such as screen rotation. The framework keeps the ViewModel alive as long as the scope of the LifecycleOwner is alive. This refers to an instance of LifecycleOwner. = new ViewModelProvider(this).get(ChronometerViewModel.class) Open ChronoActivity2 and examine how the class retrieves and uses a ViewModel: ChronometerViewModel chronometerViewModel Persisting the state of the Chronometer using a ViewModel A ViewModel is also better suited to managing tasks related to network communication, as well as data manipulation and persistence. Activities and fragments are short-lived objects which are created and destroyed frequently as a user interacts with an app. As the previous step demonstrates, an activity is a poor choice to manage app data. You can use a ViewModel to retain data across the entire lifecycle of an activity or a fragment. This timer is reset when a configuration change, such as screen rotation, destroys an activity. In the previous step, you ran an activity that displays a timer. In this step, you use a ViewModel to persist state across screen rotations and address the behaviour you observed in the previous step. You can use a ViewModel because instances of this class survive configuration changes, such as screen rotation. You now need to update the app to persist state across screen rotations. Note: If you're using an emulator or device running Android 8.0 or higher, you'll need to enable rotation in settings: Rotate the screen and notice that the timer resets!.The app runs and displays a screen similar to the following screenshot: Run the Step 1 run configuration on a device or emulator.Once you have run the command, you should see run configurations for each step: idea/runConfigurations/ĬOPY runConfigurations\*. idea/runConfigurationsĬp runConfigurations/*. Copy over the Android Studio run configurations that will be used in this codelab.Open the project Android Studio version 2.5 or newer.$ git clone you can download the repository as a Zip file: In this step, you download the code for the entire codelab and then run a simple example app. ![]() You begin with a sample app and add code through a series of steps, integrating the various architecture components as you progress. In this codelab, you implement examples of each of the components described above. To proceed, you must be familiar with the Android activity lifecycle. This codelab has been designed for Android developers with some basic experience. To read an introductory guide to this topic, see LiveData. LiveData manages observer subscriptions by pausing subscriptions to stopped LifecycleOwner objects, and cancelling subscriptions to LifecycleOwner objects that are finished. LiveData respects the complex lifecycles of your app components, including activities, fragments, services, or any LifecycleOwner defined in your app.
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