Singleton object becomes null after app is resumed

Question

For my Android project, I need global singleton Cache object to access data about a user through the app.

A problem occurs when an app goes into the background and after some time of using other apps I try to open app variables in Cache objects are null. Everything is okay when I kill the app and open it again. I'm using dependency injection to access Cache object. Why doesn't app start again if that happened? Is there some annotation to keep cache variable even in low memory conditions?

This is my Cache class

class Cache {
    var categories : Array<BaseResponse.Category>? = null
    var user : BaseResponse.User? = null
    var options : BaseResponse.OptionsMap? = null
    var order: MenuOrderDataModel? = null
}

This is Storage module for DI

@Module class StorageModule {

    @Singleton @Provides fun getSharedPrefs(context: Context): SharedPreferences {
        return PreferenceManager.getDefaultSharedPreferences(context)
    }


    @Singleton @Provides fun getCache() : Cache = Cache()
}

I inject object @Inject lateinit var cache: Cache and then populate with user data in splash screen.

Edit - added code snippets from Application and launch activity

class MyApp : Application() {
    val component: ApplicationComponent by lazy {
        DaggerApplicationComponent
                .builder()
                .appModule(AppModule(this))
                .build()
    }

    companion object {
        @JvmStatic lateinit var myapp: MyApp 
    }

    override fun onCreate() {
        super.onCreate()
        myapp= this
        Fabric.with(this, Crashlytics())
    }
}

Splash activity:

class SplashActivity : AppCompatActivity(), View.OnClickListener {

    @Inject lateinit var viewModel : ISplashViewModel
    private lateinit var disposable : Disposable

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_splash)

        MyApp.myapp.component.inject(this)
}

Answer 1

I would like to expand on @EpicPandaForce answer with implementation details using savedInstanceState and SavedStateHandle.


Official documentation: https://developer.android.com/topic/libraries/architecture/viewmodel/viewmodel-savedstate

Note that I’m using Koin for dependency injection.

Inject your viewModel inside your Activity as stateViewModel:

val viewModel: MyActivityViewModel by stateViewModel(state = { Bundle() })

That way you can inject SavedStateHandle inside ViewModel and handle storing and restoring data inside it.

Override onPause method in your Activity that will be opened after the process is killed so you can storeState at that moment.

override fun onPause() {
    super.onPause()
    viewModel.storeState()
}

Store all properties you don’t want to become null after the process is killed. An object has to be parcelable.

savedStateHandle.set(STATE_KEY, yourProperty)

In the onCreate method, you will check if savedInstanceState is not null so you can restore the state.

override fun onCreate(savedInstanceState: Bundle?) {
    setTheme(R.style.AppTheme)
    super.onCreate(savedInstanceState)

    if (savedInstanceState != null) {
        viewModel.restoreState()
    }
}

Reinit your property and remove that value from savedStateHandle

fun restoreState() {
    yourProperty = savedStateHandle.get<Any>(STATE_KEY)
    savedStateHandle.remove<Any>(STATE_KEY)
}

Answer 2

You're getting crashes because you initialize those variables in one Activity, and expect it to be set always, in the other Activity.

But Android doesn't work like that, you can easily end up crashing because after low memory condition happens, only the current Activity gets recreated at first, previous Activity is recreated on back navigation, and all static variables are nulled out (because the process is technically restarted).

Try it out:

• put your application in background with HOME button

• click the TERMINATE button on Logcat tab

• then re-launch the app from the launcher.

You'll experience this phenomenon.

In Android Studio 4.0, after you use Run from Android Studio instead of starting the app from launcher, the Terminate button SOMETIMES works a bit differently and MIGHT force-stop your app making it forget your task state on your attempt. In that case, just try again, launching the app from launcher. It will work on your second try.

You can also trigger the "Terminate Application" behavior from the terminal, as per https://codelabs.developers.google.com/codelabs/android-lifecycles/#6, it looks like this:

$ adb shell am kill your.app.package.name

---

In the new Logcat (Android Studio Electric Eel and above), the "terminate process" button is removed, and was moved into the Device Monitor tab ("Kill Process").

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Solution: check for nulls and re-initialize things in a base activity (or in LiveData.onActive), or use onSaveInstanceState.