State management in android applications

State Management
in Android Applications
Gabor Varadi
@zhuinden

What is the problem, and why are we
solving it?

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.

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

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

Expecting Singletons / statics to survive
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
fab.setOnClickListener { view ->
ObjectHolder.myObject = MyObject("someName", 27)
startActivity(intentFor<SecondActivity>())
}
}
}
class SecondActivity: AppCompatActivity() {
setContentView(R.layout.activity_second)
inputName.text = ObjectHolder.myObject.name
}
}

05-08 14:05:39.845 3601-3601/com.zhuinden.processdeathexample D/AndroidRuntime:
Shutting down VM
05-08 14:05:39.845 3601-3601/com.zhuinden.processdeathexample E/AndroidRuntime: FATAL
EXCEPTION: main
Process: com.zhuinden.processdeathexample, PID: 3601
java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.zhuinden.processdeathexample/com.zhuinden.processdeathexample.SecondA
ctivity}: kotlin.UninitializedPropertyAccessException: lateinit property myObject has
not been initialized
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
Caused by: kotlin.UninitializedPropertyAccessException: lateinit property myObject has
not been initialized
at com.zhuinden.processdeathexample.ObjectHolder.getMyObject(ObjectHolder.kt:4)
at
com.zhuinden.processdeathexample.SecondActivity.onCreate(SecondActivity.kt:12)
Expecting Singletons / statics to survive

„savedInstanceState == null for
the first launch”?
super.onCreate(savedInstanceState);
// „don’t refetch data after rotation”
if (savedInstanceState == null) {
loadData();
}
// ...
}

the first launch”-- nope

the first launch” -- nope
HOWEVER:
• Fragments ARE recreated by super.onCreate()
• The first fragment MUST be added in a
if(savedInstanceState == null) check

FragmentPagerAdapter.getItem()?
private fun initFragment() {
homeFragment = HomeScreenFragment.newInstance()
incidentFragment = IncidentFragment.newInstance()
weatherFragment = WeatherFragment.newInstance()
val fm = supportFragmentManager
viewPager.adapter = object: FragmentPagerAdapter(fm) {
override fun getItem(position: Int): Fragment =
when (position) {
0 -> homeFragment
1 -> incidentFragment
2 -> weatherFragment
}
}
}

Should probably be „createFragment”
Process: XXXXXX, PID: 3192 kotlin.UninitializedPropertyAccessException:
lateinit property has not been initialized
at xxx.ui.IncidentScreenFragment.showData(IncidentScreenFragment.kt:78)
at xxx.ui.home.HomeActivity.filterData(HomeActivity.kt:110)

Should probably be „createFragment”
Fixed version:
viewPager.adapter = object: FragmentPagerAdapter(fm) {
override fun getItem(position: Int): Fragment =
when (position) {
0 -> HomeFragment.newInstance()
1 -> IncidentFragment.newInstance()
2 -> WeatherFragment.newInstance()
}
}

Don’t trust savedInstanceState after
Fragment.onCreate
• The savedInstanceState inside
onCreateView and after can be out of date
• onSaveInstanceState to Bundle is not called
when a Fragment is detach()/attach()ed

What needs to be persisted?
• Navigation state is already managed by the
system on Android out of the box*
– 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

Example for saving/restoring state
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))
}

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?)

Single object state!
@Parcelize
data class PersonViewState(
// loading should come from elsewhere
val personFilter: String = "",
val unhandledError: ErrorType? = null // enum or sealed class
): Parcelable
...but is this really necessary?

Loading data
• Asynchronous loading should either begin on
initialization, or when observed
• Jetpack: store LiveData inside ViewModel, and expose
it to observers – fetch begins when observed
• Data can be loaded via a Transformation chain from a
MutableLiveData that stores the state – changes trigger
new data load
• Note: LiveData is analogous with BehaviorRelay

Saving state of a ViewModel
class MainViewModel: ViewModel() {
private val _selectedPosition = MutableLiveData<Int>()
fun saveState(bundle: Bundle) {
bundle.putInt("selectedPosition", _selectedPosition.value)
}
fun restoreState(bundle: Bundle) {
_selectedPosition.value = bundle.getInt("selectedPosition")
}
}

class MainActivity : AppCompatActivity() {
private lateinit var viewModel: MainViewModel
setContentView(R.layout.activity_main)
viewModel = ViewModelProviders.of(this, object: ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T =
MainViewModel().let { viewModel ->
savedInstanceState?.getBundle("viewModelState")?.let { bundle ->
viewModel.restoreState(bundle)
}
} as T
}).get(MainViewModel::class.java)
...
}
override fun onSaveInstanceState(outState: Bundle) {
super.onSaveInstanceState(outState)
val viewModelState = Bundle().also { bundle ->
viewModel.saveState(bundle)
}
outState.putBundle("viewModelState", viewModelState)
}
}

Jetpack: SavedState
private val mainViewModel by viewModels { ... }
class MainViewModel(
private val sportDao: SportDao
private val handle: SavedStateHandle
): ViewModel() {
private val _selectedSportId = handle.getLiveData("selectedSportId")
private val _selectedPosition = handle.getLiveData("selectedPosition")
val sport = _selectedSportId.switchMap { sportDao.getSport(it) }
val selectedPosition: LiveData<Int>
get() = _selectedPosition
}

Jetpack: SavedState
class MainViewModelFactory(
private val sportDao: SportDao,
owner: SavedStateRegistryOwner,
defaultState: Bundle?
): AbstractSavedStateVMFactory(owner, defaultState) {
override fun <T: ViewModel?> create(
key: String, modelClass: Class<T>, handle: SavedStateHandle
): T = MainViewModel(sportDao, handle) as T
}

Talks to watch
Fred Porciúncula: A Dagger Journey
https://www.youtube.com/watch?v=9fn5s8_CYJI
Fun with LiveData (Android Dev Summit '18)
https://www.youtube.com/watch?v=2rO4r-JOQtA
Google I/O Day 2: What’s new in Architecture
Components
https://www.youtube.com/watch?v=Qxj2eBmXLHg&list=PLOU2XLYxms
ILVTiOlMJdo7RQS55jYhsMi&index=29

Thank you for your attention!
Q/A?

State management in android applications

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