Android ViewModel Save State (Kotlin)

NOTE:

ViewModel can survive configuration change (screen rotation)
To handle system-initiazted process death, you probably need to implement onSaveInstanceState / onRestoreInstanceState or use Saved State module for ViewModel.

NOTE: Save state will not function when the application exit via back button or kill by user via other means.

Dependencies

dependencies {
    def activity_version = "1.1.0"
    implementation "androidx.activity:activity-ktx:$activity_version"

    def fragment_version = "1.2.2"
    implementation "androidx.fragment:fragment-ktx:$fragment_version"

    def lifecycle_version = '2.2.0'
    // implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"

    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
    // Lifecycles only (without ViewModel or LiveData)
    implementation "androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version"
    // Saved state module for ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"
}

Code

class MainActivity : AppCompatActivity() {    private val viewModel by viewModels<MainViewModel>()    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.main)        setSupportActionBar(toolbar)        viewModel.getTestData().observer(this) {            textView.text = it        }        button.setOnClickListener {            viewModel.saveState()        }    }}

NOTE: SavedStateViewModelFactory (previosly known as SavedStateVMFactory) automatically configured when using Activity 1.1.0, or Fragment 1.2.0.

class MainViewModel(val state: SavedStateHandle) : ViewModel() {    fun saveState() {        state.set("test", "TEST")    }    fun getTestData(): MutableLiveData<String> {        return state.getLiveData<String>("test")    }}

Testing

In order to test if ViewModel Save State works, we need to simulate system-initialized process death.

One way of doing it is

Run app (Shift + F10) from Android Studio
Click home button on device to put into background
Click Stop (F2) from Android Studio

NOTE: There are other methods which I have yet to try.

On the device, find the app icon and launch again, the state value shall be restored.

The following method will not simulate system-initialized process death (thus state shall not be restored)

Click back button to exit app
Kill app on device via Overview button (Square icon)
Kill app via adb shell am force-stop APPLICATION_ID

References:

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