Can i use a viewmodel that is observed for two activities?

Question

I'm working on a new project that implements MVVM. Can I use a viewmodel that is observed for two activities ? or should I make one viewmodel for each activity ?

public class FormViewModel extends AndroidViewModel {

/*
This is my only ViewModel in the project
 */

private UserRepository userRepository;
//linked fields in xml for lib Data Binding
public String name, lastName, address, age;

//variables observed in the views
public MutableLiveData<String> responseMessageInsertUpdate = new MutableLiveData<>();
public MutableLiveData<String> responseStartUserFormActivity = new MutableLiveData<>();
public MutableLiveData<String> responseMessageDelete = new MutableLiveData<>();

public FormViewModel(Application application) {
    super(application);
    userRepository = new UserRepository(application);

}

//get all users from database that implements RoomDataBase, it´s observed em MainActivity
//and update recyclerview when database receive any change
public LiveData<List<User>> getAllUsers() {
    return userRepository.selectAllUsers();
}

/*
action of submit button defined (linked for lib Data Binding) in xml
makes change or user registration
 */
public void submitClick(User user) {
    int idade = 0;
    if (this.age != null) {
        if (!this.age.isEmpty()) {
            idade = Integer.parseInt(this.age);
        }
    }
    if (user != null) {
        user.setName(name);
        user.setLastName(lastName);
        user.setAddress(address);
        user.setAge(idade);

    } else {
        user = new User(name, lastName, address, idade);
    }

    //validation logic
    if (user.isFormValid()) {
        if (user.getId() > 0) {
            //update the user in the database
            userRepository.updateUser(user);
            //there is an observable of this MutableLiveData variable in UserFormActivity that shows this
            //message in a toast for the User when received a value
            responseMessageInsertUpdate.setValue("User data uploaded successfully.");
        } else {
            //insert the user on data base
            userRepository.insertUser(user);
            responseMessageInsertUpdate.setValue("User " + user.getName() + " stored successfully.");
        }

    } else {
        responseMessageInsertUpdate.setValue("Please, correctly fill in all the fields of the form to confirm the registration.");
    }
}


//action of btnNewForm linked for lib Data Binding in xml
public void newFormClick() {
    /*
    this MutableLiveData is observed for MainActivity and start a new UserFormActivity when receive
    value when the btnNewForm is pressed
     */
    responseStartUserFormActivity.setValue("startActivity");
}

//delete User from database
public void deleteUser(User user) {
    if (user != null) {
        userRepository.deleteUser(user);
        /*
        there is an observable of this MutableLiveData variable in MainActivity that shows this
        message in a toast for the user when received a value (when an user is deleted from database)
         */
        responseMessageDelete.setValue(user.getName() + " removed from list successfully.");
    }
}

//this method is called on UserFormActivity to show more details of an existing user in activity fields
public void showDataUserInActivity(User user) {
    //linked fields in xml for lib Data Binding that receive values from the object user
    name = user.getName();
    lastName = user.getLastName();
    address = user.getAddress();
    age = String.valueOf(user.getAge());
}

}

public class MainActivity extends AppCompatActivity {

/*
this activity shows all users in recyclerview
 */

private Context contexto = this;
private ActivityMainBinding binding;
private UserAdapter userAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
    FormViewModel formViewModel = ViewModelProviders.of(this).get(FormViewModel.class);
    binding.setViewModel(formViewModel);

    createRecyclerView();

    methodsViewModel();

}

//methods from ViewModel
private void methodsViewModel() {
    //observer that update recyclerview when database receive any change
    binding.getViewModel().getAllUsers().observe(this, new Observer<List<User>>() {
        @Override
        public void onChanged(@Nullable List<User> pessoas) {
            userAdapter.addUserToList(pessoas);
        }
    });

    //observer that starts a new UserFormActivity when btnNewForm is pressed
    //receive value in the method newFormClick from ViewModel
    binding.getViewModel().responseStartUserFormActivity.observe(this, new Observer<String>() {
        @Override
        public void onChanged(@Nullable String s) {
            startUserFormActivity();
        }
    });

    //observer that shows a message in a toast when the user is deleted from database
    //receive value in the method deleteUser from ViewModel
    binding.getViewModel().responseMessageDelete.observe(this, new Observer<String>() {
        @Override
        public void onChanged(@Nullable String message) {
            Toast.makeText(contexto, message, Toast.LENGTH_SHORT).show();
        }
    });

}

private void createRecyclerView() {
    RecyclerView rvUser = binding.rvPessoas;
    rvUser.setLayoutManager(new LinearLayoutManager(contexto));
    userAdapter = new UserAdapter(contexto, itemClick());
    rvUser.setAdapter(userAdapter);
}

private void startUserFormActivity() {
    Intent intent = new Intent(contexto, UserFormActivity.class);
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    contexto.startActivity(intent);
}

private void startUserFormActivity(User user) {
    Intent intent = new Intent(contexto, UserFormActivity.class);
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    intent.putExtra("user", user);
    contexto.startActivity(intent);
}

private UserAdapter.ItemClick itemClick() {
    return new UserAdapter.ItemClick() {
        @Override
        public void simpleClick(View view, final int position) {
            AlertDialog.Builder alertDialog = new AlertDialog.Builder(contexto);
            String[] options = {"Update", "Delete"};
            alertDialog.setItems(options, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialogInterface, int i) {
                    if (i == 0) {
                        //start a new UserFormActivity to change user attributes
                        startUserFormActivity(userAdapter.getUserFromList().get(position));
                    } else if (i == 1) {
                        //call the method deleteUser from ViewModel
                        binding.getViewModel().deleteUser(userAdapter.getUserFromList().get(position));
                    }
                }
            });

            alertDialog.show();
        }
    };
}

}

public class UserFormActivity extends AppCompatActivity {

private Context context = this;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    FormViewModel formViewModel = ViewModelProviders.of(this).get(FormViewModel.class);
    final ActivityFormUserBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_form_user);
    binding.setViewModel(formViewModel);

    if (getIntent().getSerializableExtra("user") != null) {
        User user = (User) getIntent().getSerializableExtra("user");
        formViewModel.showDataUserInActivity(user);
        //put user data in activity when action "update" is called in MainActivity
        binding.setUser(user);
    }
    /*
    Method from ViewModel
    Observer that shows a message in a toast and close the activity when the user is storage or updated from database
    receive value in the method submitClick from ViewModel
     */
    formViewModel.responseMessageInsertUpdate.observe(this, new Observer<String>() {
        @Override
        public void onChanged(@Nullable String s) {
            Toast.makeText(context, s, Toast.LENGTH_LONG).show();
            if (s.contains("successfully")) {
                finish();
            }
        }
    });

}

}

Here is my ViewModel and my two activities for more details. As I said it's a ViewModel that is observed for two activities. This ViewModel calls a repository that updates, inserts and deletes user data as well as also updates e sends messages to the views.

Answer 1

• It's completely OK to share a viewmodel among the views, in case if you're using the same data or it's a kind of centralised datastore.
• Otherwise implement separate model for each view as it increases code readability and hence efficiency.
• Happy to provide personalised solution if you could post some of your code snippets here. Happy coding