Gabor Varadi - Reactive State Management with Jetpack Components

Gabor Varadi - Reactive State Management with Jetpack Components (2020)

1. ReactiveStateManagement
UsingJetpackComponents
Gabor Varadi
@zhuinden

2. What is theproblemwearetryingto
solve?

3. •Save/restoreminimalstateacrossprocess
death
•Executedataloadingasynchronouslybasedon
state
–Shouldbe keptacrossconfigurationchanges
•Minimize„movingparts”
–Mutationsshouldbe controlled
–Mutationsshouldbe observed

4. •Save/restoreminimalstateacrossprocess
death
•Executedataloadingasynchronouslybased
onstate
–Shouldbe keptacrossconfigurationchanges
•Minimize„movingparts”
–Mutationsshouldbe controlled
–Mutationsshouldbe observed

5. •Save/restoreminimalstateacrossprocess
death
•Executedataloadingasynchronouslybasedon
state
–Shouldbe keptacrossconfigurationchanges
•Minimize„movingparts”
–Mutationsshouldbe controlled
–Mutationsshouldbe observed

6. Processdeath
(aka: howAndroid appsactuallywork)

10. Core App Quality Guidelines
From https://developer.android.com/docs/quality-guidelines/core-app-quality#fn
„When returning to the foreground, the app
must restore the preserved state and any
significant stateful transaction that was
pending, such as changes to editable fields,
game progress, menus, videos, and other
sections of the app.

11. How to induce process death?
•Step 1: put app in background with HOME
•Step 2: press „Terminate application”
•Step 3: restart app from launcher

12. For apps you don’t own
https://play.google.com/store/apps/details?id=me.empirical.android.application.fillme
mory&hl=en

13. Commonmistakes
•Assumingthatstaticvariablesinitializedonone
screen(like dataloadingona Splashscreen) stay
setontheotherscreen(nope)
•Assumingthatif(savedInstanceState == null) {
is trueatleastonce(nope)
•Holding a referencetoa Fragmentinstance
withoutusingfindFragmentByTagfirst(for
example”ViewPagerAdapter.addFragment ()”)
(don’t)

14. Howtodetectprocessdeath?
if(savedInstanceState != null
&& lastNonConfigurationInstance == null) {
}
Orin BaseActivity:
companionobject{
var isRestored: Boolean = false
}
if(savedInstanceState !=null) {
if(!isRestored) {
isRestored= true
//here
}
}

15. Saving UI states

16. What needs to be persistedacross
processdeath?
•Navigation state is already managed by the
system on Android out of thebox
–Empty ctor + using intent extras / fragment arguments
•Screen state is partially managed by the system
–Views with IDs have their state persisted
–Complex state (f.ex. RecyclerView selection) are not
persisted automatically
–Dynamically added views should be recreatable(!)

17. What SHOULDN’T be persisted?
•Data
–Bundle has a size limit
–Data should be fetched asynchronously, off the UI
thread
•Transient state
–„Loading” state: computed from progress of side-
effect („something is happening”, but is it really?)

18. Example for saving/restoringstate
(the„old way”)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
if (savedInstanceState != null) {
selectedSportId = savedInstanceState.getLong( "selectedSportId")
selectedPosition = savedInstanceState.getInt( "selectedPosition")
selectedTags.clear()
selectedTags.addAll(
savedInstanceState.getStringArrayList( "selectedTags"))
}
}
override fun onSaveInstanceState(outState: Bundle) {
super.onSaveInstanceState(outState)
outState.putLong("selectedSportId", selectedSportId)
outState.putInt("selectedPosition", selectedPosition)
outState.putStringArrayList( "selectedTags", ArrayList(selectedTags))
}

19. Loading data
•Asynchronous loading should either begin on
initialization, or when observed
•Data can be loaded via a transformationchain
fromtheobservablesourcethatstores the
state –changes trigger new dataload
(switchMap, flatMapLatest)

20. HowcanJetpackhelp?

21. •Survivingconfigurationchanges
–ViewModel
•Lifecycle-awaredataloading
–Lifecycle(livedata/ lifecycle-ktx)
•Persistentstorage(observablequeries)
–Room
–DataStore(beta)
–*Paging(beta)
•Backgroundjobprocessing
–WorkManager

22. •Staterestorationacrossprocessdeath
–SavedStateRegistry(savedstate)
–SavedStateHandle(viewmodel-savedstate)
•ConnectingUI tostatemodel
–Databinding
–Compose(beta)
•Scopingdata/statebetweenscreens+ argument
passing
–Navigation
•Piecetogetherallthethings
–Hilt

23. ViewModel
•Storedacrossconfigurationchangesin a
ViewModelStore (ComponentActivity, Fragment,
and NavBackStackEntryareViewModelStoreOwner)
•Theyhavetheirownlifecycleforwhenthe
ViewModelStoreis destroyed(onCleared(),
viewModelScope)
•Stateand asynchronousoperationsgo here

24. ViewModel
•Originally:
valviewModel= ViewModelProvider(
viewModelStoreOwner,
viewModelProviderFactory
).get(MyViewModel::class.java)
•KTX (fragment-ktx):
privatevalviewModelbyviewModels<MyViewModel>(
ownerProducer= { viewModelStoreOwner },
factoryProducer = { viewModelProviderFactory },
)

25. Lifecycle-ktx
•LiveData/ MutableLiveData/
MediatorLiveData
•liveData(coroutineContext, timeoutMs) {coroutine
builder(CoroutineLiveData)
•liveData.asFlow() (FlowLiveData)

26. AlsoLifecycle-ktx
•LifecycleOwner.lifecycleScope
(fragment.viewLifecycleOwner.lifecycle
Scope)
•ViewModel.viewModelScope
•LifecycleCoroutineScope.launchWhenStarted ,
Lifecycle.whenStateAtLeast
•LifecycleOwner.addRepeatingJob ,
Lifecycle.repeatOnLifecycle (alpha)

28. Room
•Exposesobservablequeries
–ifthedatabaseis writtento, thenthequeriesare
re-evaluated
•Integrations:
–LiveData(ComputableLiveData)
–Flowable/Observable(RxJava2.x)
–Flow (kotlinx.coroutines)

29. WorkManager
•Schedule and eventuallyexecutebackgroundjobs
–Constraintsupport: executewhennetworkis available, etc
–OneTimeWorkRequest, uniquework
–Canbe usedtodownloaddataforoffline support
•Workertypes:
–ListenableWorker
–CoroutineWorker
–RxWorker

30. SavedState
•SavedStateRegistryallowssaving stateintoa Bundle
–Notcommonlyuseddirectly, butheavilyusedbyViewModel-
SavedState(and JetpackCompose’s
saveableStateHolder.SaveableStateProvider )
•Importantmethods:
–registerSavedStateProvider (String, SavedStateProvider)
–unregisterSavedStateProvider (String)
–consumeRestoredStateForKey (String): Bundle

31. ViewModel-SavedState
•SavedStateHandleallowssaving ViewModel’sstateintoa
Bundle
–Providesget(), set(), and most importantly
getLiveData() (SavingStateLiveData)
–Customtypescanbe savedwithsetSavedStateProvider ()
–Initializedwithargumentsbydefault
–OriginallycreatedbyAbstractSavedStateViewModelFactory

32. Databinding
•(PreferViewBindingifpossible)
•Allowsdirectlybindingagainststateviabinding
expressions
–one-waybindings(=”@{}”) vstwo-waybindings(=”@={}”)
–supportsObservableField / MutableLiveData /
MutableStateFlow
–Canbe usefulforforms
•Bindingadapterstosupportcustomobservable
propertiesfromXML in customviews

33. (JetpackCompose)
•(CompletelynewUI framework, beta)
•(Renderinghappensbyinvoking@Composable functions)
•(Changesin functionargumentsof @Composablefunctions
aretrackedbyCompose’sKotlincompilerplugin)
•(Whenfunctionargumentschange, thefunctionsthatdepend
onsaidstategetre-evaluatedand thereforere-rendered,
„recomposition”)

34. (JetpackCompose)

35. Navigation
•Createdwiththeintentiontosimplifyusing1 Activityforthe
wholeapp
•Theoretically, thesingleActivitycouldbe
classMainActivity: AppCompatActivity(R.layout.activity_main )
and no furthercode(seeNavHostFragment)
•Tracknavigationstate, handleargumentpassing
•AllowdefiningscopessharedbetweenFragments(see
NavBackStackEntry)

36. Hilt
•Dependencyinjectionframeworkwrittenontop of
Dagger
•Simplifyinjectionof Android componentsviaglobal
configurationand „automatic” injection
•IntegrationwithViewModel(+ SavedState) and
Navigation(byhiltNavGraphViewModels)

37. @HiltAndroidApp
classCustomApplication: Application()
@AndroidEntryPoint
classMainActivity: AppCompatActivity(R.layout.main_activity )
<?xmlversion="1.0" encoding="utf-8"?>
<FrameLayoutxmlns:android="http://schemas.android.com/ apk/res/android"
xmlns:app="http://schemas.android.com/ apk/res-auto"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent">
<androidx.fragment.app.FragmentContainerView
android:id="@+id/nav_host"
android:name="androidx.navigation.fragment.NavHostFragment "
android:layout_width="match_parent"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:navGraph="@navigation/navigation_graph"/>
</FrameLayout>

38. @AndroidEntryPoint
classMyFragment: Fragment(R.layout.my_fragment) {
privatevalviewModelbyviewModels<MainViewModel>()
overridefunonViewCreated(view: View, savedInstanceState: Bundle) {
super.onViewCreated(view, savedInstanceState)
valbinding= MyFragmentBinding.bind (view)
valviewModel= viewModel
viewLifecycleOwner.lifecycleScope.launchWhenStarted {
viewModel.sport.collectLatest { sport ->
// bindviews
}
}
}
}
@HiltViewModel
classMyViewModel@Inject constructor(
privatevalsportDao: SportDao,
privatevalhandle: SavedStateHandle
): ViewModel() {
privatevalselectedSportId: MutableLiveData<String> =
handle.getLiveData("selectedSportId")
valsport: Flow<Sport> = selectedSportId.asFlow ().flatMapLatest{ id->
sportDao.getSport(id)
}.stateIn(viewModelScope, SharingStarted.WhileSubscribed (), null)
}

39. ReactiveStateManagement

40. ReactiveStateManagement
•Goal: Minimizingmovingparts
–No mutationcanhappenwithoutbeingnotified
of it
•Data classesshouldn’thavevar! (onlyval)
•Observabledataholdershouldn’thaveeithermutable
classesormutablecollections!
–Statesynchronizationis idempotent
•observeover a dataholdershouldnotexecuteone-off
actionsthatcausea differenteffectonre-execution

41. Don’ts
MutableLiveData<ArrayList<T>>
Flow<ArrayList<T>>
publicfunsomeMethod(): ArrayList<T>
@Parcelize dataclassSomeClass(
var prop: String
): Parcelable

42. Do
MutableLiveData<List<T>>,
Flow<List<T>>
publicList<T> doSomething() {
ArrayList<T> copy= newArrayList<T>(original);
…
returnCollections.unmodifiableList (copy);
}
@Parcelize dataclassSomeClass(
valprop: String
): Parcelable

43. Then
Imaginea spreadsheetwithcellsof data,
and formulastocombinethem

44. And
privatevalb1 = savedStateHandle.getLiveData ("b1", 1)
privatevalb2 = savedStateHandle.getLiveData ("b2", 2)
privatevalb3 = savedStateHandle.getLiveData ("b3", 3)
privatevalb4 = savedStateHandle.getLiveData ("b4", 4)
privatevalb5 = savedStateHandle.getLiveData ("b5", 5)
privatevalc3 = combineTuple(
b1.asFlow(),
b2.asFlow(),
b3.asFlow(),
b4.asFlow(),
b5.asFlow(),
).map { (b1, b2, b3, b4, b5) ->
b1 * b2 * b3 * b4 * b5
}.stateIn(…)

45. Nowyoucando
// validation
valusername: MutableLiveData<String> =
savedStateHandle.getLiveData ("username", "")
valpassword: MutableLiveData<String> =
savedStateHandle.getLiveData ("password", "")
valisRegisterAndLoginEnabled : LiveData<Boolean> =
validateBy(
username.map{ it.isNotBlank() },
password.map{ it.isNotBlank() },
)

46. Or
// statemanagement
privatevalcurrentQuery= savedStateHandle.getLiveData ("currentQuery", "")
privatevalisLoading= MutableStateFlow(false)
privatevalresults: Flow<List<Sport>> = currentQuery.asFlow()
.distinctUntilChanged ()
.debounce(125L)
.onEach{ query->
if(query.isNotEmpty()) {
isLoading.value = true
}
}
.flatMapLatest{ query->
when{
query.isEmpty() -> emptyFlow()
else-> sportDao.getSports(query)
}
}.onEach{
isLoading.value = false
}.stateIn(viewModelScope, SharingStarted.WhileSubscribed (), listOf())
valstate: Flow<ViewState> = combineTuple(
currentQuery,
isLoading,
results,
).map { (query, isLoading, results) ->
ViewState(query, isLoading, results) // no copyingneeded!
}.stateIn(viewModelScope, SharingStarted.WhileSubscribed (), null)

47. But

48. Trytoavoid
abstractclassBaseViewModel<S: BaseViewModel.State>( // donottrust
initialState: S
): ViewModel() {
abstractclassState
privatevalflowState: MutableStateFlow<S> = // no
MutableStateFlow(initialState)
// or
privatevalliveDataState: MutableLiveData<S> = // no
MutableLiveData(initialState)
abstractvalstate: StateFlow<S> // canbe ok withgetter
// (butcomplicatedand notneeded)
}
classMyViewModel: BaseViewModel<MyViewModel.State>() {
@Parcelize
dataclassState( // no
valallData: List<T> = emptyList(), // no
valallFields: String= "",
valtransientStates: Boolean = false, // no
): BaseViewModel.State()
// state.value= state.value.copy(someState= it.someState.copy(…))
}

49. Somereactivehelpersyoucanuse
•Rx-CombineTuple-Kt
•Rx-ValidateBy-Kt
•LiveData-CombineTuple-Kt
•LiveData-ValidateBy-Kt
•Flow-CombineTuple-Kt
•Flow-ValidateBy-Kt
•LiveData-CombineUtil-Java

50. Otherresources
•Understand Kotlin Coroutines on Android (Google
I/O'19)
•LiveDatawith Coroutines and Flow (Android Dev
Summit '19)
•Android Coroutines: How to manage async tasks in
Kotlin -Manuel Vivo
•Building Reactive UIs with LiveDataand
SavedStateHandle(or equivalent approaches like Rx)

51. Thank you for your attention!
Q/A?