Managing android process instance state.

Android development challenges are mainly managing the lifecycle of SDK components.
One of them is persisting the state of UI when configuration changes happen. ViewModel + LiveData component solves this.
However, It stills a problem on how to recover the state after a system kill process of our app.

In this talk, you will see how to manage this and why you should care.
Given some insights into how lifecycles work, we are going to compare some homemade ways of solutions against the saved state module for ViewModel from Jetpack.
And finally, how to integrate this in a painless way using Dagger.

Transcript

1. With architecture components Managing process instance state Cristian Garrido
2. Agenda • Lifecycle basics • Understanding onSaveInstanceState • Basic implementation with MVVM • Introducing Saved State module for ViewModel • Abstracting factory injection
3. Lifecycle basics
4. • https://github.com/xxv/android-lifecycle • https://github.com/JoseAlcerreca/android-lifecycles
5. • Configuration changes • Process Lifecycle
6. Configuration changes • Screen rotation • Langage changes • Window size changes • Etc…
7. Screen rotation Activity.onCreate state=null Fragment.onAttach Fragment.onCreated state=null Activity.onStart Fragment.onViewCreated state=null Fragment.onActivityCreated state=null Fragment.onStart Activity.onResume Fragment.onResume
8. Screen rotation Activity.onPause Fragment.onPause Activity.onStop Fragment.onStop Activity.onSaveInstanceState state=Bundle[…] Fragment.onSaveInstanceState state=Bundle[…] Activity.onDestroyed Fragment.onViewDestroyed Fragment.onDestroy Fragment.onDetach
9. Screen rotation Activity.onCreated state=Bundle[…] Fragment.onAttach Fragment.onCreate state=Bundle[…] Activity.onStart Fragment.onViewCreated state=Bundle[…] Fragment.onActivityCreated state=Bundle[…] Fragment.onStart Activity.onResume Fragment.onResume
10. Process Lifecycle • Foreground activity • Visible activity • Background activity • Empty Process
11. Process Kill Activity.onCreate state=null Fragment.onAttach Fragment.onCreated state=null Activity.onStart Fragment.onViewCreated state=null Fragment.onActivityCreated state=null Fragment.onStart Activity.onResume Fragment.onResume
12. Process Kill Activity.onPause Fragment.onPause Activity.onStop Fragment.onStop Activity.onSaveInstanceState state=Bundle[…] Fragment.onSaveInstanceState state=Bundle[…]
13. Process Kill Activity.onPause Fragment.onPause Activity.onStop Fragment.onStop Activity.onSaveInstanceState state=Bundle[…] Fragment.onSaveInstanceState state=Bundle[…]
14. Process Kill Activity.onCreated state=Bundle[…] Fragment.onAttach Fragment.onCreate state=Bundle[…] Activity.onStart Fragment.onViewCreated state=Bundle[…] Fragment.onActivityCreated state=Bundle[…] Fragment.onStart Activity.onResume Fragment.onResume
15. Activities and singletons
16. How do we add info to the instance state?
17. override fun onCreate(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle) override fun onRestoreInstanceState(savedInstanceState: Bundle) Activity
18. Fragment override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View override fun onViewCreated(view: View, savedInstanceState: Bundle?) override fun onActivityCreated(savedInstanceState: Bundle?) override fun onViewStateRestored(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle)
19. Fragment.viewOwnerLifecycle override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View override fun onViewCreated(view: View, savedInstanceState: Bundle?) override fun onActivityCreated(savedInstanceState: Bundle?) override fun onViewStateRestored(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle)
20. Fragment override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View override fun onViewCreated(view: View, savedInstanceState: Bundle?) override fun onActivityCreated(savedInstanceState: Bundle?) override fun onViewStateRestored(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle)
21. Fragment override fun onActivityCreated(savedInstanceState: Bundle?) override fun onViewStateRestored(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle)
22. Fragment override fun onActivityCreated(savedInstanceState: Bundle?) override fun onViewStateRestored(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle)
23. Fragment // From FragmentManager.moveToState if (newState > Fragment.CREATED) { fragmentStateManager.createView(mContainer); fragmentStateManager.activityCreated(); fragmentStateManager.restoreViewState(); }
24. onSaveInstanceState, what does it actually do for us ?
25. onSaveInstanceState • Save view hierarchy “recursively” • State is unique for each UI components • Stable behaviour since API 28 (Pie)
26. onSaveInstanceState • Save view hierarchy “recursively” • State is unique for each UI components • Stable behaviour since API 28 (Pie) • Since Pie: After onStop() • Previous Pie: Before onStop() no guarantees about it will occur before or after onPause().
27. onSaveInstanceState outState=Bundle[{ android:viewHierarchyState=Bundle[{ android:views={ 16908290=android.view.AbsSavedState$1@456a31, 2131165223=androidx.appcompat.widget.Toolbar$SavedState@6cf5516, 2131165225=android.view.AbsSavedState$1@456a31, 2131165231=android.view.AbsSavedState$1@456a31, 2131165257=android.view.AbsSavedState$1@456a31, 2131165262=android.view.AbsSavedState$1@456a31 } }], android:lastAutofillId=1073741824, android:fragments=android.app.FragmentManagerState@7381e97 }]
28. Activity.intent & Fragment.arguments • Not managed by onSaveInstanceState • Can survive configuration changes and process kill. • But only if these were started with.
29. Managing UI state: divide and conquer
30. Managing UI state: divide and conquer
31. Managing UI state: divide and conquer • ViewModel + LiveData • onSaveInstanceState() • Local persistance
37. https://developer.android.com/topic/libraries/architecture/saving-states#options_for_preserving_ui_state
38. How to test a process kill
39. Testing • No SDK or library to make automated test • Unit testing only inputs and outputs • Manual testing
40. Don’t keep activities option This option make automatically destroy activities that passe to a background state. Nevertheless, The process tied to your application still alive.
41. Killing your app through Logcat Try it out: • Send application in background with HOME button • click Terminate application button on Logcat tab in Android Studio • then re-launch the app from the launcher. Note: In Android Studio 4.0, the Terminate button issues am force-stop, and so we need to use am kill variant.
42. Killing your app with ADB Try it out: • Send application in background with HOME button • Execute: $ adb shell am kill <your package name> • Then re-launch the app from the launcher.
43. Killing your with Venom Send a exitProcess(0) in a new activity. Making the old activity stack regenerate by itself. https://www.reddit.com/r/androiddev/comments/g2cdid/
44. Playground time!
45. Target • Make a Fragment UI state restorable after a kill process.
46. Planning • Step 1: Create a basic ViewModel implementation • Step 2: Make a implementation to survive the process kill
47. • Step 1: Create a basic implementation
48. class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.main_activity) if (savedInstanceState == null) { supportFragmentManager.beginTransaction() .replace(R.id.container, MainFragment.newInstance()) .commitNow() } } }
49. class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.main_activity) if (savedInstanceState == null) { supportFragmentManager.beginTransaction() .replace(R.id.container, MainFragment.newInstance()) .commitNow() } } }
50. class MainFragment : Fragment() { override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) viewModel.songResults.observeNotNull(viewLifecycleOwner) { bindSongs(it) } } override fun onViewStateRestored(savedInstanceState: Bundle?) { super.onViewStateRestored(savedInstanceState) searchButton.setOnClickListener { viewModel.search(searchField.text.toString()) } } private fun bindSongs(songs: List<Song>) = … }
51. class MainFragment : Fragment() { override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) viewModel.songResults.observeNotNull(viewLifecycleOwner) { bindSongs(it) } } override fun onViewStateRestored(savedInstanceState: Bundle?) { super.onViewStateRestored(savedInstanceState) searchButton.setOnClickListener { viewModel.search(searchField.text.toString()) } } private fun bindSongs(songs: List<Song>) = … }
52. class MainViewModel( private val repository: SongRepository ) : ViewModel() { private val _songResults = MutableLiveData<List<Song>>() val songResults: LiveData<List<Song>> get() = _songResults fun search(name: String) { _songResults.value = repository.findSongs(name) } }
53. class MainViewModel( private val repository: SongRepository ) : ViewModel() { private val _songResults = MutableLiveData<List<Song>>() val songResults: LiveData<List<Song>> get() = _songResults fun search(name: String) { _songResults.value = repository.findSongs(name) } }
54. class MainViewModel( private val repository: SongRepository ) : ViewModel() { private val _songResults = MutableLiveData<List<Song>>() val songResults: LiveData<List<Song>> get() = _songResults fun search(name: String) { _songResults.value = repository.findSongs(name) } }
55. It survives configuration changes
56. It does not survives process kill , yet…
57. We need to persist this using the instance state.
58. • Step 2: Make a implementation to survive the process kill
59. class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) override fun onSaveInstanceState(outState: Bundle) ... }
60. class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) savedInstanceState?.let { bindSongs(it.getParcelableArrayList<Song>("awesome")) } } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) val results = viewModel.songResults.value outState.putParcelableArrayList("awesome", results)) } ... }
61. class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) savedInstanceState?.let { bindSongs(it.getParcelableArrayList<Song>("awesome")) } } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) val results = viewModel.songResults.value outState.putParcelableArrayList("awesome", results)) } ... }
62. class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) savedInstanceState?.let { bindSongs(it.getParcelableArrayList<Song>("awesome")) } } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) val results = viewModel.songResults.value outState.putParcelableArrayList("awesome", results)) } ... }
63. Two reasons to do not do this • TransactionTooLargeException • ViewModel is not aware of the restoration
64. class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) viewModel.onActivityCreated(savedInstanceState) } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) viewModel.onSaveInstanceState(outState) } ... } class MainFragment : Fragment() { ... override fun onActivityCreated(savedInstanceState: Bundle?) { super.onActivityCreated(savedInstanceState) viewModel.onActivityCreated(savedInstanceState) } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) viewModel.onSaveInstanceState(outState) } ... }
65. class MainViewModel(...) : ViewModel() { fun onSaveInstanceState(outState: Bundle) { outState.putParcelableArrayList("awesome",songResults.value) } fun onViewStateRestored(savedInstanceState: Bundle?) { _songResults.value = state?.getParcelableArrayList("awesome") } }
66. class MainViewModel(...) : ViewModel() { private val instanceState: Bundle = Bundle() fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun onActivityCreated(savedInstanceState: Bundle) { instanceState.putAll(savedInstanceState) instanceState.getString("name") ?.let(::search) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
67. class MainViewModel(...) : ViewModel() { private val instanceState: Bundle = Bundle() fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun onActivityCreated(savedInstanceState: Bundle) { instanceState.putAll(savedInstanceState) instanceState.getString("name") ?.let(::search) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
68. class MainViewModel(...) : ViewModel() { private val instanceState: Bundle = Bundle() fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun onActivityCreated(savedInstanceState: Bundle) { instanceState.putAll(savedInstanceState) instanceState.getString("name") ?.let(::search) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
69. class MainViewModel(...) : ViewModel() { private val instanceState: Bundle = Bundle() fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun onActivityCreated(savedInstanceState: Bundle) { instanceState.putAll(savedInstanceState) instanceState.getString("name") ?.let(::search) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
70. class MainViewModel(...) : ViewModel() { private val instanceState: Bundle = Bundle() fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun onActivityCreated(savedInstanceState: Bundle) { instanceState.putAll(savedInstanceState) instanceState.getString("name") ?.let(::search) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
71. class MainViewModel(private val instanceState: Bundle, …) : ViewModel() { init { instanceState.getString("name") ?.let(::search) } }
72. class MainViewModel(private val instanceState: Bundle, …) : ViewModel() { init { instanceState.getString("name") ?.let(::search) } fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
73. It survives the process kill!!
74. Introducing Saved State module for ViewModel
76. public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner, OnBackPressedDispatcherOwner public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner AppCompat components
77. AppCompat components public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner, OnBackPressedDispatcherOwner public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner
78. ComponentActivity / Fragment @Override protected void onCreate(@Nullable Bundle savedInstanceState) { … mSavedStateRegistryController.performRestore(savedInstanceState); } protected void onSaveInstanceState(@NonNull Bundle outState) { … mSavedStateRegistryController.performSave(outState); }
79. class MainViewModel(private val instanceState: Bundle, …) : ViewModel() { init { instanceState.getString("name") ?.let(::search) } fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
80. class MainViewModel(private val instanceState: Bundle, …) : ViewModel() { init { instanceState.getString("name") ?.let(::search) } fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun search(name: String) { instanceState.putString("name", name) _songResults.value = repository.getSongsByName(name) } }
81. class MainViewModel(private val handle: SavedStateHandle, …) : ViewModel() { init { handle.get<String>("name") ?.let(::search) } fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun search(name: String) { handle.set("name", name) _songResults.value = repository.getSongsByName(name) } }
82. class MainViewModel(private val handle: SavedStateHandle, …) : ViewModel() { init { handle.get<String>("name") ?.let(::search) } fun onSaveInstanceState(outState: Bundle) { outState.putAll(instanceState) } fun search(name: String) { handle.set("name", name) _songResults.value = repository.getSongsByName(name) } }
83. class MainViewModel(private val handle: SavedStateHandle,…) : ViewModel() { init { handle.get<String>("name") ?.let(::search) } fun search(name: String) { handle.set("name", name) _songResults.value = repository.getSongsByName(name) } }
84. class MainViewModel(private val handle: SavedStateHandle,…) : ViewModel() { init { handle.get<String>("name") ?.let(::search) } fun search(name: String) { handle.set("name", name) _songResults.value = repository.getSongsByName(name) } }
85. class MainViewModel(private val handle: SavedStateHandle,…) : ViewModel() { private var name by savedState<String>("name", handle) init { name?.let(::search) } fun search(name: String) { this.name = name _songResults.value = repository.getSongsByName(name) } }
86. class MainViewModelFactory(private val instanceState: Bundle?,…) : ViewModelProvider.Factory { override fun <T : ViewModel?> create(modelClass: Class<T>): T { return MainViewModel(instanceState ?: Bundle(),…) as T } }
87. class MainViewModelFactory(private val instanceState: Bundle?,…) : ViewModelProvider.Factory { override fun <T : ViewModel?> create(modelClass: Class<T>): T { return MainViewModel(instanceState ?: Bundle(),…) as T } }
88. class MainViewModelFactory(private val owner: SavedStateRegistryOwner,…) : AbstractSavedStateViewModelFactory(owner, null) { override fun <T : ViewModel?> create( key: String, modelClass: Class<T>, handle: SavedStateHandle ): T { return MainViewModel(handle,…) as T } }
89. class MainViewModelFactory(private val owner: SavedStateRegistryOwner,…) : AbstractSavedStateViewModelFactory(owner, null) { override fun <T : ViewModel?> create( key: String, modelClass: Class<T>, handle: SavedStateHandle ): T { return MainViewModel(handle,…) as T } }
90. class MainViewModelFactory(private val owner: SavedStateRegistryOwner,…) : AbstractSavedStateViewModelFactory(owner, null) { override fun <T : ViewModel?> create( key: String, modelClass: Class<T>, handle: SavedStateHandle ): T { return MainViewModel(handle,…) as T } }
91. @Module interface MainActivityModule { @ContributesAndroidInjector fun injectorForMainActivity(): MainActivity @ContributesAndroidInjector(modules = [MainFragmentModule::class]) fun injectorForMainFragment(): MainFragment } @Module interface MainFragmentModule { @Binds fun bindSavedStateRegistryOwner( fragment: MainFragment ) : SavedStateRegistryOwner }
92. @Module interface MainActivityModule { @ContributesAndroidInjector fun injectorForMainActivity(): MainActivity @ContributesAndroidInjector(modules = [MainFragmentModule::class]) fun injectorForMainFragment(): MainFragment } @Module interface MainFragmentModule { @Binds fun bindSavedStateRegistryOwner( fragment: MainFragment ) : SavedStateRegistryOwner }
93. @Module interface MainActivityModule { @ContributesAndroidInjector fun injectorForMainActivity(): MainActivity @ContributesAndroidInjector(modules = [MainFragmentModule::class]) fun injectorForMainFragment(): MainFragment } @Module interface MainFragmentModule { @Binds fun bindSavedStateRegistryOwner( fragment: MainFragment ) : SavedStateRegistryOwner }
94. Recap • UI component has none knowledge how the instance state is saved
95. Recap • UI component has none knowledge how the instance state is saved • The instance state is only a map or a variable on the ViewModel
96. Recap • UI component has none knowledge how the instance state is saved • The instance state is only a map on the ViewModel • A lot of code was removed out of fragment and vm • Code for the factory still being needed
97. Code for the factory still being needed
98. @AssistedInject
99. class MainViewModel(private val handle: SavedStateHandle,…) : ViewModel() { …. class Factory( private val owner: SavedStateRegistryOwner, defaultArgs: Bundle?, … ) : AbstractSavedStateViewModelFactory(owner, defaultArgs) { override fun <T : ViewModel?> create( key: String, modelClass: Class<T>, handle: SavedStateHandle ): T { return MainViewModel(handle,…) as T } } } }
100. class MainViewModel @AssistedInject constructor( private val repository: SongRepository, @Assisted private val handle: SavedStateHandle ) : ViewModel() { … @AssistedInject.Factory interface AssistedFactory: ViewModelAssistedFactory<MainViewModel> }
101. class MainViewModel @AssistedInject constructor( private val repository: SongRepository, @Assisted private val handle: SavedStateHandle ) : ViewModel() { … @AssistedInject.Factory interface AssistedFactory: ViewModelAssistedFactory<MainViewModel> }
102. class SavedStateViewModelFactory<VM, AssistedFactory> @Inject constructor( owner: SavedStateRegistryOwner, defaultArgs: Bundle?, private val factory: AssistedFactory ) : AbstractSavedStateViewModelFactory(owner, defaultArgs) where VM: ViewModel, AssistedFactory: ViewModelAssistedFactory<VM> { override fun <VM : ViewModel?> create( key: String, modelClass: Class<VM>, handle: SavedStateHandle ): VM = factory.create(handle) as VM }
103. class SavedStateViewModelFactory<VM, AssistedFactory> @Inject constructor( owner: SavedStateRegistryOwner, defaultArgs: Bundle?, private val factory: AssistedFactory ) : AbstractSavedStateViewModelFactory(owner, defaultArgs) where VM: ViewModel, AssistedFactory: ViewModelAssistedFactory<VM> { override fun <VM : ViewModel?> create( key: String, modelClass: Class<VM>, handle: SavedStateHandle ): VM = factory.create(handle) as VM }
104. class MainFragment : DaggerFragment() { @Inject lateinit var factory : ViewModelFactory<MainViewModel, MainViewModel.AssistedFactory> private val viewModel: MainViewModel by viewModels { factory } }
105. ViewModelFactory<MainViewModel, MainViewModel.AssistedFactory>
106. class MainViewModel @AssistedInject constructor( private val repository: SongRepository, @Assisted private val handle: SavedStateHandle ) : ViewModel() { … @AssistedInject.Factory interface AssistedFactory: ViewModelAssistedFactory<MainViewModel> }
107. class MainViewModel @ViewModelInject constructor( private val repository: SongRepository, @Assisted private val handle: SavedStateHandle ) : ViewModel()
108. @AndroidEntryPoint class MainFragment : Fragment() { private val viewModel: MainViewModel by viewModels() }
109. Questions? Follow me on twitter @crgarridos Cristian Garrido Code: https://git.io/Je8CL
110. Thanks!