Updating UI using ViewModel and DataBinding
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 viewlastName
is editable on viewloadUser()
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:
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.vmtestapplication.MainActivity">
<data>
<import type="android.view.View" />
<!-- Only view model required -->
<variable
name="vm"
type="com.example.vmtestapplication.UserDataViewModel" />
</data>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:animateLayoutChanges="true"
android:orientation="vertical">
<!-- Primitive error message -->
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{vm.error}"
android:visibility="@{vm.error == null ? View.GONE : View.VISIBLE}"/>
<!-- Read only field (only `@`) -->
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{vm.firstName}" />
<!-- Two-way data binding (`@=`) -->
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@={vm.lastName}" />
</LinearLayout>
</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<String> firstName = new MediatorLiveData<>();
private final MutableLiveData<String> lastName = new MediatorLiveData<>();
private final MutableLiveData<String> error = new MutableLiveData<>();
/**
* Expose LiveData if you do not use two-way data binding
*/
public LiveData<String> getFirstName() {
return firstName;
}
/**
* Expose MutableLiveData to use two-way data binding
*/
public MutableLiveData<String> getLastName() {
return lastName;
}
public LiveData<String> getError() {
return error;
}
@MainThread
public void loadUser(String userId) {
// cancel previous running task
cancelLoadTask();
loadTask = new ListTask();
Observer<UserData> observer = new Observer<UserData>() {
@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<String, Void, UserData> {
private final MutableLiveData<UserData> data= new MediatorLiveData<>();
public LiveData<UserData> 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.