Viewmodel in action: DevFest Florida 2018

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? Let's do a deep dive into the topic

Transcript

1. DEVFEST FLORida ViewMODEL IN ACTION the new architecture components @PreusslerBerlin
3. Introducing The architecture components
4. starring Room LiFe cycle LiveData ViewModel Pagination (new)
5. ViewModel like in Model-View-ViewModel ?
6. ViewModel in MVVM world Model ViewModel View View ViewModel Model
7. in new suggested architecture
8. in new suggested architecture
9. Is MVP dead?
10. 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
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
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
13. Is MVP dead? •Start by putting the ViewModel behind Presenter
14. In architecture components •A ViewModel provides the data for a specific UI •The ViewModel does not know about the View! •Survives configuration change
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 implementation 'android.arch.lifecycle:extensions:1.0.0’ kapt 'android.arch.lifecycle:compiler:1.0.0’
22. How to use class MyViewModel() : ViewModel() {
23. How to use class MyViewModel(app: Application) : AndroidViewModel(app) {
24. How to use class MyViewModel() :ViewModelObservable() { Coming soon Jose Alcérreca, Google https://medium.com/@dpreussler/add-the-new-viewmodel-to-your-mvvm-36bfea86b159
25. How to use override fun onCreate(...) { model = ViewModelProviders .of(this) .get(MyViewModel::class.java) }
26. What if… constructor arguments needed?
27. How to use class MyViewModelFactory :ViewModelProvider.Factory(useCase: MyUseCase) { fun <T: ViewModel> create(aClass: Class<T>): T { return MyViewModel(useCase) as T } }
28. How to use ViewModelProviders .of(this, MyViewModelFactory(usecase)) .get(MyShowsViewModel::class.java)
29. 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
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 override fun onStopped() { … } No more life cycle forwarding!
32. What if… I need to clean something when destroyed?
33. What if… class MyViewModel() : ViewModel() { override fun 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. Tell us more
43. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(this) .get(MyViewModel::class.java)
44. Different ViewModels RESULT
45. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(this) .get(MyViewModel::class.java)
46. W A I T
47. result •Fragment and Activity share the same FragmentManager •But implementation uses Activity’s FragmentManager but ChildFragmentManager for Fragments
48. result •Fragment and Activity share the same FragmentManager •But implementation uses Activity’s FragmentManager but ChildFragmentManager for Fragments
49. What if Two Fragments of same Activity ask for same ViewModel.class via ViewModelProviders .of(getActivity()) .get(MyViewMode::class).java
50. result Same ViewModel
51. Tell us more
52. Other uses cases communication layer between activities and fragments or fragments and fragments
53. Other uses cases Replace Loaders (plus Room and LiveData/Rx)
54. Could you Write that?
55. Could you write that? •What if asked for ViewModel but fragment transaction not done yet? •You might end up with duplicates
56. Could you write that? •What if asked for ViewModel but fragment transaction not done yet? •You might end up with duplicates
57. Could you write that? static class HolderFragmentManager { private Map<Activity, HolderFragment> mNotCommittedActivityHolders = new HashMap<>(); private Map<Fragment, HolderFragment> mNotCommittedFragmentHolders = new HashMap<>(); …
58. Could you write that? •What if activity dies before fragment transaction? •You might end up with memory leaks
59. 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); } } }; …
60. Does that mean all problems are solved?
61. 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
62. but but WHY?
63. 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?
64. MeaNS ViewModels gives us rotation But takes away recreation
65. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
66. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
67. after The truth •Keep non-UI states in non-UI layer Not in bundle! •Use real caching strategies •Allows updating cache in background
68. but but EditText might have restored it’s state but the ViewModel will not now about it
69. but but Where to store the UI state?
70. store the UI state In Bundles!
71. but but Who owns the UI state?
72. store the UI state The ViewModel
73. store the UI state
74. lets tweak it class MyModelFactory(val bundle: Bundle?) :ViewModelProvider.Factory() { … fun <T: ViewModel> create(aClass: Class<T>): T { return MyViewModel().apply { readFrom(bundle) } as T } ...
75. lets tweak it override onSaveInstanceState(bundle: Bundle){ super.onSaveInstanceState(bundle); viewModel.writeTo(bundle); }
76. So what about this LIFE CYCLE THING?
77. LIFECYCLE is a class that holds the information about the lifecycle state of a component
78. LIFECYCLE OWNER is a single method interface that denotes that the class has a Lifecycle. Fragments and Activities in Support Library 26.1.0+ already implement
79. LIFECYCLE OBSERVER class MyListener() : LifecycleObserver { @OnLifecycleEvent(Event.ON_START) fun onStart() { … }
80. LIFECYCLE OBSERVER activity.lifecycle.addObserver(listener)
81. LIFECYCLE OBSERVER activity.lifecycle.addObserver(listener) An observer added with a Lifecycle will be automatically removed if the corresponding Lifecycle moves to Lifecycle.State#DESTROYED state.
82. AND LIFECYCLE OBSERVER GIVEs US… Process Lifecycle Owner
83. So SHOULD VIEWMODEL IMPLEMENT LIFE CYCLE OBSERVER?
84. NOOOOOOOOOOOO
85. ViewMODEL lives LOngER! Activity Activity ViewModel
86. IntroduciNG LIVE DATA Activity Activity LiveData LiveData ViewModel
87. IntroduciNG LIVE DATA •Observable similar to RxJava •Life cycle aware •Doesn’t emit when not needed •Memory leaks save
88. IntroduciNG LIVE DATA class MyViewModel(): ViewModel() { val message = MutableLiveData<String>()
89. IntroduciNG LIVE DATA myModel.message.observe( this, Observer { display(it) })
90. What if I USE Rx Java?
91. Rx Java FULL PICTURE Activity ViewModel (un)subscribe
92. Rx Java FULL PICTURE Activity ViewModel Life cycle aware class (un)subscribe (un)subscribe
93. What if I USE DATA BINDING?
94. Data binding full picture XML ViewModel bind
95. 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>
96. How do I…
97. .. show a toast class SeriesViewModel : Viewmodel() { … @Bindable var error = ObservableField<String>()
98. .. show a toast viewModel.error.addOnPropertyChangedCallback( object : OnPropertyChangedCallback() { override fun onPropertyChanged(…) { showToast(viewModel.error.get() } })
99. .. show a toast <FrameLayout app:showError="@{viewModel.errorAppeared}"> @BindingAdapter("showError") fun ViewGroup.onErrorAppeared(error: String?){ errorString?.let { showToast(context, error)) } }
100. Data binding full picture XML Activity ViewModel (un)bind bind Life cycle aware class (un)bind
101. WAYS TO OBSERVE DATA from VM? •Data binding Observable from xml or code, might need unregister •RxJava Observable from code, needs unregister •LiveData Observable, from code, no unregister, life cycle aware
102. OR DON’T USE ARCH COMP VIEWMODEL Activity Life cycle ware ViewModel Repository Activity Life cycle ware ViewModel
103. let`s sum up •Well designed API •Goal: common architecture language •Use the parts you need •Know about life cycle
104. let`s sum up View Model Its production ready!
105. 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 • https://www.youtube.com/watch?v=c9-057jC1ZA
106. Southpark copyright Disclaimer
107. viacom.tech
108. View Model IN ACTION @PreusslerBerlin