ViewModel: A deep dive (GDG Heraklion)

At Google I/O we got the Android architecture components.
One of its most interesting concepts is the new ViewModel.
How does it work? How do I plug it into my app? Can I use it without any of the other components?
Can I plug it into an existing MVVM or how would I build MVVM with Google's ViewModel?

Transcript

1. The ViewModel @PreusslerBerlin a deep dive Danny Preussler
3. The ”architecture components”
4. The ”architecture components” •Room •Lifecycle Observer •LiveData •ViewModel
5. The ”architecture components” ViewModel like in Model-View-ViewModel ViewModel?
6. ViewModel in MVVM world
7. ViewModel in data binding Rx XML ViewModel
8. ViewModel in data binding <TextView … android:id="@+id/all_shows_item_title" android:text="@{viewModel.title}" /> <android.support.v7.widget.CardView … android:onClick="@{() -> viewModel.onClicked()}"> <data> <variable name="viewModel" type="com.vmn.playplex.main.allshows.SeriesViewModel"/> </data>
9. in new suggested architecture
10. Is MVP dead?
11. Is MVP dead? It’s like Java and Kotlin: •MVP will stay for quite some time •There is a new cooler kid in town that won’t leave Put the ViewModel behind Presenter
12. Is MVP dead? It’s like Java and Kotlin: •MVP will stay for quite some time •There is a new cooler kid in town that won’t leave Put the ViewModel behind Presenter
13. Is MVP dead? It’s like Java and Kotlin: •MVP will stay for quite some time •There is a new cooler kid in town that won’t leave Put the ViewModel behind Presenter
14. Is MVP dead? It’s like Java and Kotlin: •MVP will stay for quite some time •There is a new cooler kid in town that won’t leave Put the ViewModel behind Presenter
15. In architecture components •A ViewModel provides the data for a specific UI •The ViewModel does not know about the View! •Survives configuration change
16. In architecture components •A ViewModel provides the data for a specific UI •The ViewModel does not know about the View! •Survives configuration change
17. In architecture components •A ViewModel provides the data for a specific UI •The ViewModel does not know about the View! •Survives configuration change
18. In architecture components •Remember configuration change can be: •Rotation •Any other resize i.e. split screen •Language change
19. life cycle: rotation onCreate onStart onResume onPause onStop onDestroy onCreate onStart onResume ViewModel
20. life cycle: finish onCreate onStart onResume onPause onStop onDestroy ViewModel
21. How to use compile 'android.arch.lifecycle:runtime:1.0.0-alpha4' compile 'android.arch.lifecycle:extensions:1.0.0-alpha4’ // annotationProcessor // 'android.arch.lifecycle:compiler:1.0.0-alpha4'
22. How to use class MyViewModel extends ViewModel {
23. How to use class MyViewModel extends AndroidViewModel {
24. How to use class MyViewModel extends ObservableViewModel { Coming soon Jose Alcérreca, Google https://medium.com/@dpreussler/add-the-new-viewmodel-to-your-mvvm-36bfea86b159
25. How to use public void onStopped() { … } No more life cycle forwarding!
26. How to use public void onCreate(...) { model = ViewModelProviders .of(this) .get(MyViewModel.class); }
27. What if… constructor arguments needed?
28. How to use class MyViewModelFactory implements ViewModelProvider.Factory { @Inject MyUseCase useCase; @Override public MyViewModel create(Class modelClass) { return new MyViewModel(useCase); } }
29. How to use ViewModelProviders .of(this, new MyViewModelFactory()) .get(MyShowsViewModel.class);
30. How to use •Always try to build your own Factory •Default factory uses newInstance() which is some hundred times slower than new calls (reflection) https://speakerdeck.com/dpreussler/comparing-dependency-injection- frameworks-and-internals-for-android
31. How to use •Always try to build your own Factory •Default factory uses newInstance() which is some hundred times slower than new calls (reflection) https://speakerdeck.com/dpreussler/comparing-dependency-injection- frameworks-and-internals-for-android
32. What if… I need to clean something when destroyed?
33. What if… class MyViewModel extends ViewModel { @Override protected void onCleared() { super.onCleared(); cleanupSubscriptions(); }
34. How does it survive orientation change?
35. How does it actually work? class HolderFragment extends Fragment { public HolderFragment() { setRetainInstance(true); } …
36. How does it actually work? String HOLDER_TAG = "android.arch.lifecycle.state.StateProviderH olderFragment";
37. How does it know the activity is finishing?
38. How does it actually work? @Override public void onDestroy() { super.onDestroy(); mViewModelStore.clear(); }
39. Can I do it differently? ViewModel is not life cycle aware?
40. It just refuses to die
41. remember Never hold View or Activity references in the ViewModel!
42. Could you Write that?
43. Could you write that? •What if asked for ViewModel but fragment transaction not done yet? •You might end up with duplicates
44. Could you write that? •What if asked for ViewModel but fragment transaction not done yet? •You might end up with duplicates
45. Could you write that? static class HolderFragmentManager { private Map<Activity, HolderFragment> mNotCommittedActivityHolders = new HashMap<>(); private Map<Fragment, HolderFragment> mNotCommittedFragmentHolders = new HashMap<>(); private ActivityLifecycleCallbacks mActivityCallbacks = new EmptyActivityLifecycleCallbacks() { @Override public void onActivityDestroyed(Activity activity) { HolderFragment fragment = mNotCommittedActivityHolders.remove(activity); …
46. Could you write that? •What if activity dies before fragment transaction? •You might end up with memory leaks
47. Could you write that? private ActivityLifecycleCallbacks mActivityCallbacks = new EmptyActivityLifecycleCallbacks() { @Override public void onActivityDestroyed(Activity activity) { HolderFragment fragment = mNotCommittedActivityHolders.remove(activity); if (fragment != null) { Log.e(LOG_TAG, "Failed to save a ViewModel for " + activity); } } }; …
48. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(this) .get(MyViewModel.class);
49. result Different ViewModels
50. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(this) .get(MyViewModel.class);
51. result •Fragment and Activity share the same FragmentManager •But implementation uses Activity’s FragmentManager but ChildFragmentManager for Fragments
52. result •Fragment and Activity share the same FragmentManager •But implementation uses Activity’s FragmentManager but ChildFragmentManager for Fragments
53. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(getActivity()) .get(MyViewModel.class);
54. result Same ViewModel
55. Other uses cases communication layer between activities and fragments or fragments and fragments
56. Does that mean all problems are solved?
57. all problems solved? ViewModels provide a convenient way to retain data across configuration changes but they are not persisted if the application is killed by the operating system https://developer.android.com/topic/libraries/architecture/viewmodel.html#viewm odel_vs_savedinstancestate
58. but but WHY?
59. Why? The data saved via onSaveInstanceState is kept in the system process memory and the Android OS allows you to keep only a very small amount of data so it is not a good place to keep actual data for your app. TransactionTooLargeException anyone?
60. MeaNS ViewModels gives us rotation But takes away recreation
61. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
62. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
63. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
64. but but EditText might have restored it’s state but the ViewModel will not now about it
65. but but Where to store the UI state?
66. store the UI state In Bundles!
67. but but Who owns the UI state?
68. store the UI state The ViewModel
69. store the UI state
70. lets tweak it class MyModelFactory implements ViewModelProvider.Factory { … public MyModelFactory(Bundle bundle) { this.bundle = bundle; } @Override public MyViewModel create(Class modelClass) { MyViewModel viewModel = new MyViewModel(); viewModel.readFrom(bundle); return viewModel; }
71. lets tweak it @Override public void onSaveInstanceState(Bundle bundle){ super.onSaveInstanceState(bundle); viewModel.writeTo(bundle); }
72. What if Activity needs multiple version of same ViewModel.class?
73. Extend IT so we can have unique class names ?!
74. But what if count is dynamic ?
75. SO unknown size of elements ?
76. CALLED RecyclerVIEW!
77. list item ViewModels In MVVM: •each item in RecyclerView should be backed by a ViewModel •ListViewModel and ItemViewModels •ItemViewModels normally don’t retrieve their own state so no need to extend Googles ViewModel
78. list item ViewModels In MVVM: •each item in RecyclerView should be backed by a ViewModel •ListViewModel and ItemViewModels •ItemViewModels normally don’t retrieve their own state so no need to extend Googles ViewModel
79. list item ViewModels In MVVM: •each item in RecyclerView should be backed by a ViewModel •ListViewModel and ItemViewModels •ItemViewModels normally don’t retrieve their own state so no need to extend Googles ViewModel
80. Let’s do it anyway
81. list item ViewModels public <T extends ViewModel> T get( Class<T> modelClass)
82. list item ViewModels •Key could be position? public <T extends ViewModel> T get( String key, Class<T> modelClass)
83. What about Changes?
84. What about Removal?
85. list item ViewModels “ViewModelProvider, which will create ViewModels via the given Factory and retain them in a ViewModelStore “
86. public class ViewModelStore { private final HashMap<String, ViewModel> mMap = new HashMap<>(); final void put(String key, ViewModel viewModel) {…} final ViewModel get(String key) { …} public final void clear() {…} }
87. public class ViewModelStore { private final HashMap<String, ViewModel> mMap = new HashMap<>(); final void put(String key, ViewModel viewModel) {…} final ViewModel get(String key) { …} public final void clear() {…} }
88. Store can not be overridden
89. list item ViewModels RecyclerView items and Googles ViewModel don’t fit together
90. What if Two different activities need same ViewModel.class?
91. This Does not sound like MVVM
92. Let’s do it anyway
93. Can I do it differently? •Let’s assume we build transactional flow spreading over multiple Activities and wand to keep data in ViewModel •Can not use the retained fragment approach •Need to override how the ViewModel is stored •Need to override the ViewModelStore is stored
94. Viewmodelstoreowner “A responsibility of an implementation of this interface is to retain owned ViewModelStore during the configuration changes and call ViewModelStore#clear(), when this scope is going to be destroyed.“
95. Viewmodelstoreowner public interface ViewModelStoreOwner { ViewModelStore getViewModelStore(); }
96. Viewmodelproviders /** * Utilities methods for {@link ViewModelStore} class. */ public class ViewModelProviders {
97. ViewModelproviders public static ViewModelProvider of( Fragment fragment, Factory factory) { return new ViewModelProvider( ViewModelStores.of(fragment), factory); }
98. Viewmodelstores /** * Factory methods for ViewModelStore class. */ public class ViewModelStores {
99. ViewModelstores ViewModelStore of(Fragment fragment) { return holderFragmentFor(fragment) .getViewModelStore(); }
100. lets do it new ViewModelProvider( new LongLivingViewModelStoreOwner(), new BundleAwareViewModelFactory (bundle, provider)) .get(MyViewModel.class);
101. How to extend life? Toothpick: A scope tree based Dependency Injection (DI) library https://github.com/stephanenicolas/toothpick
102. lets do it class LongLivingViewModelStoreOwner implements ViewModelStoreOwner { @Override public ViewModelStore getViewModelStore() { return Toothpick.openScope(SCOPE_NAME) .getInstance(ViewModelStore.class); } public void discardViewModel() { Toothpick.closeScope(SCOPE_NAME); }
103. let`s sum up •Well designed API •Know the life time: Never have Activivity or View reference in ViewModel •Be aware of Recreation •Careful with RecyclerView •Still alpha
104. Want to know more •https://medium.com/@dpreussler/add-the-new- viewmodel-to-your-mvvm-36bfea86b159 •https://proandroiddev.com/customizing-the-new- viewmodel-cf28b8a7c5fc •http://hannesdorfmann.com/android/arch- components-purist •https://blog.stylingandroid.com/architecture- components-viewmodel/ •https://www.youtube.com/watch?v=QrbhPcbZv0I
105. Southpark copyright Disclaimer
106. viacom.tech We are hiring
107. The ViewModel @PreusslerBerlin goo.gl/6gNTDr