ViewModel

The ViewModel class is designed to store and manage UI-related data in a lifecycle conscious way. The ViewModel class allows data to survive configuration changes such as screen rotations.

The Android framework manages the lifecycles of UI controllers, such as activities and fragments. The framework may decide to destroy or re-create a UI controller in response to certain user actions or device events that are completely out of your control.
If the system destroys or re-creates a UI controller, any transient UI-related data you store in them is lost. For example, your app may include a list of users in one of its activities. When the activity is re-created for a configuration change, the new activity has to re-fetch the list of users. For simple data, the activity can use the onSaveInstanceState() method and restore its data from the bundle in onCreate(), but this approach is only suitable for small amounts of data that can be serialized then deserialized, not for potentially large amounts of data like a list of users or bitmaps.

Another problem is that UI controllers frequently need to make asynchronous calls that may take some time to return. The UI controller needs to manage these calls and ensure the system cleans them up after it’s destroyed to avoid potential memory leaks. This management requires a lot of maintenance, and in the case where the object is re-created for a configuration change, it’s a waste of resources since the object may have to reissue calls it has already made.

Implement a ViewModel

Architecture Components provides ViewModel helper class for the UI controller that is responsible for preparing data for the UI. ViewModel objects are automatically retained during configuration changes so that data they hold is immediately available to the next activity or fragment instance.

Example:

class MyViewModel : ViewModel() {
    private val users: MutableLiveData<List<User>> by lazy {
        MutableLiveData<List<User>>().also {
            loadUsers()
        }
    }

    fun getUsers(): LiveData<List<User>> {
        return users
    }

    private fun loadUsers() {
        // Do an asynchronous operation to fetch users.
    }
}

class MyActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
      // Use the 'by viewModels()' Kotlin property delegate from the activity-ktx artifact

      val model: MyViewModel by viewModels()
      model.getUsers().observe(this, Observer<List<User>>{ users ->
          // update UI
      })
    }
}

If the activity is re-created, it receives the same MyViewModel instance that was created by the first activity. When the owner activity is finished, the framework calls the ViewModel objects’s onCleared() method so that it can clean up resources.

Caution: A ViewModel must never reference a view, Lifecycle, or any class that may hold a reference to the activity context.

ViewModel objects can contain LifecycleObservers, such as LiveData objects. However ViewModel objects must never observe changes to lifecycle-aware observables, such as LiveData objects. If the ViewModel needs the Application context, for example to find a system service, it can extend the AndroidViewModel class and have a constructor that receives the Application in the constructor, since Application class extends Context.

The lifecycle of a ViewModel

ViewModel objects are scoped to the Lifecycle passed to the ViewModelProvider when getting the ViewModel. The ViewModel remains in memory until the Lifecycle it’s scoped to goes away permanently: in the case of an activity, when it finishes, while in the case of a fragment, when it’s detached.

You usually request a ViewModel the first time the system calls an activity object’s onCreate() method. The system may call onCreate() several times throughout the life of an activity, such as when a device screen is rotated. The ViewModel exists from when you first request a ViewModel until the activity is finished and destroyed.

Replacing Loaders with ViewModel

Loader classes like CursorLoader are frequently used to keep the data in an app’s UI in sync with a database. You can use ViewModel, with a few other classes, to replace the loader.

In one common approach to using loaders, an app might use a CursorLoader to observe the contents of a database. When a value in the database changes, the loader automatically triggers a reload of the data and updates the UI:

ViewModel works with Room and LiveData to replace the loader. The ViewModel ensures that the data survives a device configuration change. Room informs your LiveData when the database changes, and the LiveData, in turn, updates your UI with the revised data.