ViewModels : Detrás de Escena

Charla presentada junto a Fabio Negri (@Fabio_Negri) para la comunidad de Android Devs de Buenos Aires, en las oficinas de Accenture, como parte del cierre del año de la comunidad.
La presentación se basa en comprender cómo es que funcionan los ViewModels, cómo se logra mantener el estado de la información ante los cambios de configuración y cuáles son los errores más comunes a la hora de utilizar los Architecture Components.

Transcript

1. ViewModels
 Detrás de escena Android Devs Buenos Aires Jorge Nogueiras @nogueirasj @nogueiras Fabio Negri @Fabio_Negri @fabionegri
2. AGENDA ▸ Un poco de historia… ▸ MVVM ▸ ViewModel ▸ Mejores prácticas ▸ Testing
3. UN POCO DE HISTORIA… LOS COMPONENTES SON UN PUNTO DE PARTIDA. SI TU ARQUITECTURA YA FUNCIONA, NO ES NECESARIO QUE LA CAMBIES SavedStateHandle Coroutines viewModelScope ViewModel en NavGraph ViewModels en Databinding VAYAN MIGRANDO CUANTO ANTES A VIEWMODEL PORQUE SINO LES VA A CABER REFACTOR Codelabs Samples en android-architecture
4. Android Devs Buenos Aires MVVM
5. Android Devs Buenos Aires
6. Android Devs Buenos Aires • Mantiene el estado de la vista • Bajo acoplamiento con la vista • Sobrevive a cambios de configuración (ej: Rotación de pantalla) ViewModel
7. Android Devs Buenos Aires Ejemplo de Scope de ViewModel
8. Android Devs Buenos Aires ¿Cómo sabemos cuanto tiempo vive un ViewModel?
9. Android Devs Buenos Aires ¿Cómo definimos el scope de un ViewModel? import androidx.lifecycle.ViewModel class MyViewModel : ViewModel() { } // MyActivity.kt / MyFragment.kt val viewModel = ViewModelProviders.of(this)[MyViewModel::class.java]
10. Android Devs Buenos Aires (
11. Android Devs Buenos Aires /** * Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity * is alive. More detailed explanation is in {@link ViewModel}. * <p> * It uses the given {@link Factory} to instantiate new ViewModels. * * @param activity an activity, in whose scope ViewModels should be retained * @param factory a {@code Factory} to instantiate new ViewModels * @return a ViewModelProvider instance */ @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory) { Application application = checkApplication(activity); if (factory == null) { factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application); } return new ViewModelProvider(activity.getViewModelStore(), factory); }
12. Android Devs Buenos Aires /** * Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given * {@code fragment} is alive. More detailed explanation is in {@link ViewModel}. * <p> * It uses the given {@link Factory} to instantiate new ViewModels. * * @param fragment a fragment, in whose scope ViewModels should be retained * @param factory a {@code Factory} to instantiate new ViewModels * @return a ViewModelProvider instance */ @NonNull @MainThread public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) { Application application = checkApplication(checkActivity(fragment)); if (factory == null) { factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application); } return new ViewModelProvider(fragment.getViewModelStore(), factory); }
13. Android Devs Buenos Aires ViewModelProvider /** * An utility class that provides {@code ViewModels} for a scope. * <p> * Default {@code ViewModelProvider} for an {@code Activity} or a {@code Fragment} can be obtained * from {@link androidx.lifecycle.ViewModelProviders} class. */ @SuppressWarnings("WeakerAccess") public class ViewModelProvider { }
14. Android Devs Buenos Aires } public class ViewModelProvider { public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { mFactory = factory; mViewModelStore = store; } …
15. Android Devs Buenos Aires ViewModelStore public class ViewModelStore { private final HashMap<String, ViewModel> mMap = new HashMap<>(); final void put(String key, ViewModel viewModel) { ViewModel oldViewModel = mMap.put(key, viewModel); if (oldViewModel != null) { oldViewModel.onCleared(); } } public final void clear() { for (ViewModel vm : mMap.values()) { vm.clear(); } mMap.clear(); } … }
16. Android Devs Buenos Aires Ahora bien… ¿Cómo se persisten los ViewModels? En el caso de Activity: static final class NonConfigurationInstances { Object custom; ViewModelStore viewModelStore; } @Override @Nullable public final Object onRetainNonConfigurationInstance() { ……. NonConfigurationInstances nci = new NonConfigurationInstances(); nci.custom = custom; nci.viewModelStore = viewModelStore; return nci; }
17. Android Devs Buenos Aires Ahora bien… ¿Cómo se persisten los ViewModels? En el caso de Activity: public ComponentActivity() { … getLifecycle().addObserver(new LifecycleEventObserver() { @Override public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) { if (event == Lifecycle.Event.ON_DESTROY) { if (!isChangingConfigurations()) { getViewModelStore().clear(); } } } }); … }
18. Android Devs Buenos Aires Ahora bien… ¿Cómo se persisten los ViewModels? En el caso de Fragment: final class FragmentManagerImpl extends FragmentManager implements LayoutInflater.Factory2 { private FragmentManagerViewModel mNonConfig; } /** * FragmentManagerViewModel is the always up to date view of the Fragment’s * non configuration state */ class FragmentManagerViewModel extends ViewModel { … private final HashSet<Fragment> mRetainedFragments = new HashSet<>(); private final HashMap<String, FragmentManagerViewModel> mChildNonConfigs = new HashMap<>(); private final HashMap<String, ViewModelStore> mViewModelStores = new HashMap<>(); }
19. Android Devs Buenos Aires Activity/ Fragment ViewModelProviders.of(this) ViewModelStore + get(ViewModelClass::class) - put(ViewModel) ViewModelProvider ViewModel#1 ViewModel#2 … Configuration Change
20. Android Devs Buenos Aires Sharing ViewModels
21. Android Devs Buenos Aires class Data class MyDataViewModel : ViewModel() { val selected = MutableLiveData<Data>() fun select(data: Data) { selected.value = data } }
22. Android Devs Buenos Aires class MyDataListFragment : Fragment() { private lateinit var viewModel: MyDataViewModel private lateinit var selector: View override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) viewModel = ViewModelProviders.of(requireActivity())[MyDataViewModel::class.java] selector.setOnClickListener { data -> viewModel.select(data) } } }
23. Android Devs Buenos Aires class MyDataDetailsFragment : Fragment() { private lateinit var viewModel: MyDataViewModel override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) viewModel = ViewModelProviders.of(requireActivity())[MyDataViewModel::class.java] viewModel.selected.observe(viewLifecycleOwner, Observer { // Update the UI with the Data details }) } }
24. Android Devs Buenos Aires Errores comunes
25. Android Devs Buenos Aires OnConfigurationChange
26. Android Devs Buenos Aires LiveData para eventos
27. Android Devs Buenos Aires SingleLiveEvent
28. Android Devs Buenos Aires ViewPresenterModel
29. Android Devs Buenos Aires fun onViewCreated() { doSomeWork() } init { doSomeWork() }
30. Android Devs Buenos Aires fun onDestroy() { subscriptions.clear() } override fun onCleared() { subscriptions.clear() }
31. Android Devs Buenos Aires Leaks
32. Android Devs Buenos Aires class MyViewModel(private val activity: FragmentActivity)
33. Android Devs Buenos Aires class MyViewModel(private val fragment: Fragment)
34. Android Devs Buenos Aires val viewModel = MyViewModel(MyActivity()) class MyViewModel(private val view: MyView) interface MyView class MyActivity : FragmentActivity(), MyView
35. Android Devs Buenos Aires class MyViewModel(private val getMyData: GetMyData) class GetMyData(private val repository: MyDataRepository) interface MyDataRepository class RetrofitMyDataRepository(context: Context) : MyDataRepository class MyActivity : FragmentActivity(), MyView { val viewModel = MyViewModel(GetMyData(RetrofitMyDataRepository(this))) }
36. Android Devs Buenos Aires Testing
37. Android Devs Buenos Aires Estrategias
38. Android Devs Buenos Aires Observe LiveData @Test fun `observe initialised value`() { } //given , when val stateObserver: Observer<ViewState> = mock() val viewModel = MyViewModel() viewModel.states.observeForever(stateObserver) //then verify(stateObserver).onChanged(ViewState.Initialised)
39. Android Devs Buenos Aires Observe LiveData @Test fun `observe livedata and assert values`() { } //given val stateObserver: Observer<ViewState> = mock() val viewModel = MyViewModel() viewModel.states.observeForever(stateObserver) //when viewModel.onButtonClick() //then verify(stateObserver).onChanged(ViewState.ValuableData)
40. Android Devs Buenos Aires Assert LiveData value @Test fun `assert livedata value`() { } //given val viewModel = MyViewModel() //when viewModel.onButtonClick() //then assertThat(viewModel.states.value).isEqualTo(ValuableData)
41. Android Devs Buenos Aires @After fun tearDown() { viewModel.states.removeObserver(observer) }
42. Android Devs Buenos Aires testImplementation("androidx.arch.core:core-testing:2.1.0") @Rule @JvmField val instantTaskExecutorRule = InstantTaskExecutorRule()
43. Android Devs Buenos Aires Para Investigar… ViewModelState ViewModelScope (coroutines) ViewModel en Navigation Graph
44. Android Devs Buenos Aires ¿Preguntas?
45. Android Devs Buenos Aires ¡Muchas Gracias!
46. LINKS DE INTERÉS ▸ https://developer.android.com/topic/libraries/architecture/viewmodel ▸ https://medium.com/google-developer-experts/viewmodels-under-the-hood- f8e286c4cc72 ▸ https://proandroiddev.com/5-common-mistakes-when-using-architecture- components-403e9899f4cb
47. ViewModels
 Detrás de escena Android Devs Buenos Aires Jorge Nogueiras @nogueirasj @nogueiras Fabio Negri @Fabio_Negri @fabionegri