Android Architecture Re-evolution

Walk through Android architecture and design patterns with a deep dive in the new ViewModel and LiveData Android Architecture Components

Transcript

1. Architecture Re-evolution ViewModel & LiveData
2. Happy place for nerds
3. Disclaimer: Code will be in
4. In the beginning, there was... GOD the GOD Activity View + Logic + Services
6. Code separated by features Services
7. Model View Presenter Passive View Presenter Model User events View Updates Update model State change events
8. Clean Architecture
9. Why? Stop 42
10. Architecture Components ViewModel & LiveData Stable v1.0.0 / min SDK 14 (ICS)
11. ViewModel Activity lifecycle is complex, Fragment lifecycle is a death trap. ViewModel is a class that contains view data and logic separated from lifecycle-bound entities like Activities and Fragments Why & What
12. class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) } } • No more reference to Views. Ever • A factory can be used for more complex ViewModels • Created once and shared with multiple Fragments ViewModel How class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) } } class MyViewModel : ViewModel() { } class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) titleTextView.text = viewModel.title } } class MyViewModel : ViewModel() { val title = "Great title" } class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) titleTextView.text = viewModel.title redButton.onClick { viewModel.redButtonClicked() } } } class MyViewModel : ViewModel() { val title = "Great title" fun redButtonClicked() { ... } }
13. • Survive rotation • Easy to use No persistence when app killed Store data with existing solutions God object candidate Follow same practices as for presenters ViewModel Pros & Cons
14. • Observable pattern like RxJava but Lifecycle-Aware • No more onPause, onStart, etc. LiveData We can’t push data from a ViewModel into an Activity or Fragment We need a data provider in our ViewModel so the View can subscribe to it Why & What LiveData is a lifecycle-aware observable that holds data and provides updates
15. onCreate() => LiveData is observed and value is updated but not emitted to any Observer onStart() => LiveData value is emitted to all Observers on main thread onStop() => LiveData value is updated but not emitted to any Observer onDestroy() => LiveData stops being observed LiveData How
16. class MyViewModel : ViewModel() { val userName = MutableLiveData<String>() } LiveData How class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) } } class MyViewModel : ViewModel() { val userName = MutableLiveData<String>() fun updateUserName(newUserName: String) { userName.value = newUserName // In main thread or userName.postValue(newUserName) // In background thread } } class MyActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java) viewModel.userName .observe(this, Observer { value -> updateUserTextView(value) }) } }
17. LiveData How class MyViewModel : ViewModel() { private val userLiveData = MutableLiveData<UserInfo>() val userName: LiveData<String> = Transformations.map(userLiveData, user -> { user.name + " " + user.lastName }) } Basic transformations with map and switchMap
18. LiveData & RxJava • Interoperability between LiveData and RxJava with the extensions: LiveDataReactiveStreams.fromPublisher() • Override ViewModel.onCleared() to clear any ongoing RxJava subscription
19. • Forget to use postValue() from a background thread • Fragments start observing in onCreateView() but you need to manually remove Observer in onDestroyView() Pitfalls
20. ViewModel & LiveData
21. Rijksmuseum
22. • User wants to see the location of a painting in the map • User wants to see her location on the map when moving • User wants to see a route on the map from her location to painting Let’s practice
23. Let’s practice Route User location Painting location MapViewState
24. MapViewModel class MapViewModel : ViewModel() { private val userLocation = MutableLiveData<Location>() private val paintingLocation = MutableLiveData<Location>() class MapViewModel : ViewModel() { private val userLocation = MutableLiveData<Location>() private val paintingLocation = MutableLiveData<Location>() fun userSelectedPainting(location: Location) { // Updated from UI paintingLocation.value = location } class MapViewModel : ViewModel() { private val userLocation = MutableLiveData<Location>() private val paintingLocation = MutableLiveData<Location>() fun userSelectedPainting(location: Location) { // Updated from UI paintingLocation.value = location } private val locations = MediatorLiveData<Pair<Location, Location>>().apply { class MapViewModel : ViewModel() { private val userLocation = MutableLiveData<Location>() private val paintingLocation = MutableLiveData<Location>() fun userSelectedPainting(location: Location) { // Updated from UI paintingLocation.value = location } private val locations = MediatorLiveData<Pair<Location, Location>>().apply { addSource(userLocation) { newLoc -> value = Pair(newLoc, value.second) } addSource(paintingLocation) { newLoc -> value = Pair(value.first, newLoc) } }
25. MapViewModel private val liveRoute: LiveData<Route> = Transformations .switchMap(locations, { (userLoc, paintingLoc) -> planRoute(userLoc, paintingLoc) }) private val liveRoute: LiveData<Route> = Transformations .switchMap(locations, { (userLoc, paintingLoc) -> planRoute(userLoc, paintingLoc) }) val mapViewState = MediatorLiveData<MapViewState>().apply { addSource(locations) { newLoc -> value = MapViewState(newLoc.first, newLoc.second, value.route) } addSource(liveRoute) { newRoute -> value = MapViewState(value.userLoc, value.paintingLoc, newRoute) } } data class MapViewState( val userLoc: Location?, val paintingLoc: Location?, val route: Route? )
26. MapActivity class MapActivity : AppCompatActivity() { public override fun onCreate() { val viewModel = ViewModelProviders.of(this).get(MapViewModel::class.java) viewModel.mapViewState .observe(this, Observer { newMapViewState -> updateMapView(newMapViewState) }) } }
27. Silver Bullet Syndrome
28. Questions? @q42 @sebaslogen