Lifecycle, LiveData, ViewModel Architecture Components Hugo Visser @botteaap [email protected] 

Architecture Components ● Guide ○ Reference architecture ○ Sample app ● Components ○ Lifecycle ○ LiveData ○ ViewModel ○ Room → http://littlerobots.nl/blog/cupboard-room/ https://developer.android.com/arch 

Components ● Best architecture is the one that works for you ○ Separation of concerns ○ Testable ○ Consistent ○ ● Implement it using Architecture Components ○ Solve common Android problems ○ Pick and choose the ones you need 

Lifecycle 

Why? ● Components need to respect the Activity / Fragment lifecycle ● Components need to know about the lifecycle state 

@Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); myLocationListener = new MyLocationListener(this::handleLocation); } @Override protected void onStart() { super.onStart(); myLocationListener.onStart(); // start other things } @Override protected void onStop() { myLocationListener.onStop(); // stop other things super.onStop(); } 

public class ArchActivity extends LifecycleActivity { private MyLocationListener myLocationListener; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); myLocationListener = new MyLocationListener(getLifecycle(), this::handleLocation); } } 

public class MyLocationListener implements LifecycleObserver { public MyLocationListener(Lifecycle lifecycle, Callback callback) { lifecycle.addObserver(this); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { } } 

public class MyLocationListener implements LifecycleObserver { public MyLocationListener(Lifecycle lifecycle, Callback callback) { lifecycle.addObserver(this); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { } } 

public class MyLocationListener implements LifecycleObserver { public MyLocationListener(Lifecycle lifecycle, Callback callback) { lifecycle.addObserver(this); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { } } 

public class MyLocationListener implements LifecycleObserver { public MyLocationListener(Lifecycle lifecycle, Callback callback) { lifecycle.addObserver(this); } @OnLifecycleEvent(Lifecycle.Event.ON_START) void onStart() { } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) void onStop() { } } 

public class ArchActivity extends LifecycleActivity { private MyLocationListener myLocationListener; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); myLocationListener = new MyLocationListener(getLifecycle(), this::handleLocation); } } 

public class ArchActivity extends LifecycleActivity { private MyLocationListener myLocationListener; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); myLocationListener = new MyLocationListener(getLifecycle(), this::handleLocation); } } 

public class MyLocationListener implements LifecycleObserver { private final Lifecycle mLifecycle; public MyLocationListener(Lifecycle lifecycle, Callback callback) { mLifecycle = lifecycle; lifecycle.addObserver(this); } public void enable() { if (mLifecycle.getCurrentState().isAtLeast(Lifecycle.State.STARTED)) { // do things } else { // defer things } } } 

Lifecycle ● Get the Lifecycle class from the LivecyleOwner and observe it ● Annotate methods with @OnLifecycleEvent ● Easy to check current state and compare it ● Components are self-contained, Activity boilerplate reduced! 

LiveData 

Reactive data holder ● Respond to data change, don’t poll → reactive ● Lifecycle aware component → it relies on Lifecycle 

LiveData 

Observable 

Observable io.reactivex. 

Observable java.util. 

Observable android.database. 

Observable android.databinding. 

¯\_(ツ)_/¯ 

LiveData 

@Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); // LiveData LocationLiveData locationLiveData = new LocationLiveData(); // Automatically unregistered when the activity is destroyed // If this has an initial value, it won't be delivered yet! locationLiveData.observe(this, this::handleLocation); } private void handleLocation(Location location) { // won't receive any callbacks until the activity is started or resumed } 

public class LocationLiveData extends LiveData implements LocationListener { @Override protected void onActive() { startListeningForLocation(); } @Override protected void onInactive() { stopListeningForLocation(); } @Override public void onLocationChanged(Location location) { // this value will be cached and delivered to all observers setValue(location); } } 

LiveData ● Pass it to functions, model ● Can defer work until there are observers ● MutableLiveData for simple cases (but expose as LiveData) 

ViewModel 

Why? ● Scope view model to Activity or Fragment → single instance ● Retain model across life cycle changes (rotation) ● Share model across fragments (using the activity context) 

public class MyViewModel extends ViewModel { private MutableLiveData> mUsers; public LiveData> getUsers() { if (mUsers == null) { mUsers = new MutableLiveData<>(); loadUsers(); } return mUsers; } private void loadUsers() { // do async operation to fetch users and later use // mUsers.postValue(users) } @Override protected void onCleared() { // Cancel pending operations } } 

public class MyActivity extends FragmentActivity { public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // The Activity acts as the scope. // If the model does not exist in this scope, it’s created. // Otherwise the existing model is returned MyViewModel model = ViewModelProviders.of(this).get(MyViewModel.class); model.getUsers().observe(this, users -> { // update UI }); } } 

ViewModel ● Easy way to retain data or objects scoped to Activity or Fragment ● Should not contain references to Views ● Combination w/ LiveData 

Architecture components ● Allow you to tackle common architectural problems specific to Android ● Mix and match ● More to come! 

Thanks! @botteaap hugo@littlerobots https://www.youtube.com/watch?v=vOJCrbr144o https://d.android.com/arch