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ViewModel

abstract class ViewModel
kotlin.Any
   ↳ androidx.lifecycle.ViewModel

ViewModel is a class that is responsible for preparing and managing the data for an Activity or a Fragment. It also handles the communication of the Activity / Fragment with the rest of the application (e.g. calling the business logic classes).

A ViewModel is always created in association with a scope (an fragment or an activity) and will be retained as long as the scope is alive. E.g. if it is an Activity, until it is finished.

In other words, this means that a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel.

The purpose of the ViewModel is to acquire and keep the information that is necessary for an Activity or a Fragment. The Activity or the Fragment should be able to observe changes in the ViewModel. ViewModels usually expose this information via LiveData or Android Data Binding. You can also use any observability construct from you favorite framework.

ViewModel's only responsibility is to manage the data for the UI. It should never access your view hierarchy or hold a reference back to the Activity or the Fragment.

Typical usage from an Activity standpoint would be:

public class UserActivity extends Activity {
 
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.user_activity_layout);
          final UserModel viewModel = new ViewModelProvider(this).get(UserModel.class);
          viewModel.getUser().observe(this, new Observer<user>
 () {
              @Override
              public void onChanged(@Nullable User data) {
                  // update ui.
              }
          });
          findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                   viewModel.doAction();
              }
          });
      }
  }
  
</user>
ViewModel would be:
public class UserModel extends ViewModel {
      private final MutableLiveData<User> userLiveData = new MutableLiveData<>();
 
      public LiveData<User> getUser() {
          return userLiveData;
      }
 
      public UserModel() {
          // trigger user load.
      }
 
      void doAction() {
          // depending on the action, do necessary business logic calls and update the
          // userLiveData.
      }
  }
  

ViewModels can also be used as a communication layer between different Fragments of an Activity. Each Fragment can acquire the ViewModel using the same key via their Activity. This allows communication between Fragments in a de-coupled fashion such that they never need to talk to the other Fragment directly.

public class MyFragment extends Fragment {
      public void onStart() {
          UserModel userModel = new ViewModelProvider(requireActivity()).get(UserModel.class);
      }
  }
  

Summary

Public constructors

ViewModel is a class that is responsible for preparing and managing the data for an Activity or a Fragment.

Protected methods

open Unit

This method will be called when this ViewModel is no longer used and will be destroyed.

Extension properties

From androidx.lifecycle
CoroutineScope

CoroutineScope tied to this ViewModel.

Public constructors

<init>

ViewModel()

ViewModel is a class that is responsible for preparing and managing the data for an Activity or a Fragment. It also handles the communication of the Activity / Fragment with the rest of the application (e.g. calling the business logic classes).

A ViewModel is always created in association with a scope (an fragment or an activity) and will be retained as long as the scope is alive. E.g. if it is an Activity, until it is finished.

In other words, this means that a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel.

The purpose of the ViewModel is to acquire and keep the information that is necessary for an Activity or a Fragment. The Activity or the Fragment should be able to observe changes in the ViewModel. ViewModels usually expose this information via LiveData or Android Data Binding. You can also use any observability construct from you favorite framework.

ViewModel's only responsibility is to manage the data for the UI. It should never access your view hierarchy or hold a reference back to the Activity or the Fragment.

Typical usage from an Activity standpoint would be:

public class UserActivity extends Activity {
 
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.user_activity_layout);
          final UserModel viewModel = new ViewModelProvider(this).get(UserModel.class);
          viewModel.getUser().observe(this, new Observer<user>
 () {
              @Override
              public void onChanged(@Nullable User data) {
                  // update ui.
              }
          });
          findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                   viewModel.doAction();
              }
          });
      }
  }
  
</user>
ViewModel would be:
public class UserModel extends ViewModel {
      private final MutableLiveData<User> userLiveData = new MutableLiveData<>();
 
      public LiveData<User> getUser() {
          return userLiveData;
      }
 
      public UserModel() {
          // trigger user load.
      }
 
      void doAction() {
          // depending on the action, do necessary business logic calls and update the
          // userLiveData.
      }
  }
  

ViewModels can also be used as a communication layer between different Fragments of an Activity. Each Fragment can acquire the ViewModel using the same key via their Activity. This allows communication between Fragments in a de-coupled fashion such that they never need to talk to the other Fragment directly.

public class MyFragment extends Fragment {
      public void onStart() {
          UserModel userModel = new ViewModelProvider(requireActivity()).get(UserModel.class);
      }
  }
  

Protected methods

onCleared

protected open fun onCleared(): Unit

This method will be called when this ViewModel is no longer used and will be destroyed.

It is useful when ViewModel observes some data and you need to clear this subscription to prevent a leak of this ViewModel.