Updating UI using ViewModel and DataBinding

Question

I am trying learn View-model android ,in my first phase of learning i am trying to update UI (Textview) by using view-model and data-binding. In View model i am having aynctask

Answer 1

I suggest to follow next basic principles:

• don't overload data objects by business or presentation logic
• only view model required to obtain data in presentation layer
• view model should expose only ready to use data to presentation layer
• (optional) background task should expose LiveData to deliver data

Implementation notes:

• firstName is read only on view
• lastName is editable on view
• loadUser() is not threadsafe
• we have error message when call save() method until data is not loaded

Don't overload data objects by business or presentation logic

Suppose, we have UserData object with first and last name. So, getters it's (usually) all what we need:

public class UserData {

    private String firstName;
    private String lastName;

    public UserData(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }
}

Only view model required to obtain data in presentation

To follow this suggestion we should to use only view model in data binding layout:

Note: you can use a few view models in one layout, but not raw data

View model should expose only ready to use data to presentation

This mean, you shouldn't to expose complex data objects (UserData in our case) directly from view model. Preferable to expose privative types which view can use as-is. In example below we don't need to hold UserData object because it used only to loading grouped data. We, probably, need to create UserData to save it but it depends on your repository implementation.

public class UserDataViewModel extends ViewModel {

    private ListTask loadTask;

    private final MutableLiveData firstName = new MediatorLiveData<>();
    private final MutableLiveData lastName = new MediatorLiveData<>();
    private final MutableLiveData error = new MutableLiveData<>();

    /**
     * Expose LiveData if you do not use two-way data binding
     */
    public LiveData getFirstName() {
        return firstName;
    }

    /**
     * Expose MutableLiveData to use two-way data binding
     */
    public MutableLiveData getLastName() {
        return lastName;
    }

    public LiveData getError() {
        return error;
    }

    @MainThread
    public void loadUser(String userId) {
        // cancel previous running task
        cancelLoadTask();
        loadTask = new ListTask();
        Observer observer = new Observer() {
            @Override
            public void onChanged(@Nullable UserData userData) {
                // transform and deliver data to observers
                firstName.setValue(userData == null? null : userData.getFirstName());
                lastName.setValue(userData == null? null : userData.getLastName());
                // remove subscription on complete
                loadTask.getUserData().removeObserver(this);
            }
        };
        // it can be replaced to observe() if LifeCycleOwner is passed as argument
        loadTask.getUserData().observeForever(observer);
        // start loading task
        loadTask.execute(userId);
    }

    public void save() {
        // clear previous error message
        error.setValue(null);
        String fName = firstName.getValue(), lName = lastName.getValue();
        // validate data (in background)
        if (fName == null || lName == null) {
            error.setValue("Opps! Data is invalid");
            return;
        }
        // create and save object
        UserData newData = new UserData(fName, lName);
        // ...
    }

    @Override
    protected void onCleared() {
        super.onCleared();
        cancelLoadTask();
    }

    private void cancelLoadTask() {
        if (loadTask != null)
            loadTask.cancel(true);
        loadTask = null;
    }
}

Background task should expose LiveData to deliver data

public class ListTask extends AsyncTask {

    private final MutableLiveData data= new MediatorLiveData<>();

    public LiveData getUserData() {
        return data;
    }

    @Override
    protected void onPostExecute(UserData userData) {
        data.setValue(userData);
    }

    @Override
    protected UserData doInBackground(String[] userId) {
        // some id validations
        return loadRemoiteUser(userId[0]);
    }
}

---

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private UserDataViewModel viewModel;

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

        // get view model
        viewModel = ViewModelProviders.of(this).get(UserDataViewModel.class);
        // create binding
        ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        // set view model to data binding
        binding.setVm(viewModel);
        // don't forget to set LifecycleOwner to data binding
        binding.setLifecycleOwner(this);

        // start user loading (if necessary)
        viewModel.loadUser("user_id");
        // ...
    }
}

PS: try to use RxJava library instead of AsyncTask to perform background work.