在AndroidX中不推荐使用onRetainCustomNonConfigurationInstance

Question

onRetainCustomNonConfigurationInstance was deprecated in AndroidX when the first version got released back in 2018! As the AndroidX Activity library says in the release notes:

onRetainCustomNonConfigurationInstance has been deprecated. Use a ViewModel for storing objects that need to survive configuration changes.

I just want a single object to survive configuration changes, this was the purpose of onRetainCustomNonConfigurationInstance !

For my use case, I want an instance of a Dagger graph to survive configuration changes in an Activity but using AAC ViewModel doesn't feel right to me for that use case. Dagger injects my ViewModels, I don't want to wrap my graph in another ViewModel just for the sake of making it survive configuration changes.

Is there any other way I can make an object survive configuration changes?

Answer 1

Using ViewModel is the most accurate and recommended way to make an object survive configuration changes, you should use it. You could've used onSaveInstanceState but that'd force all the objects needing to support Parcelable , that's not only reasonable, it's sometimes impossible.

To replace onRetainCustomNonConfigurationInstance , you can use the same APIs that support ViewModel in a way that handles all the complexity for you.

You can use this implementation of LongLastingElement API (code here) that uses ViewModel under the hood and removes all the boilerplate code to make an object survive configuration changes.

For your example, to make the Dagger graph survive configuration changes with this API, the code would look like this:

class LoginActivity : AppCompatActivity() {

    private lateinit var loginComponent: LoginComponent

    ...

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

        loginComponent = LongLastingElement.getInstance<LoginComponent>(this).getElement {
            (applicationContext as MyDaggerApplication).appComponent.loginComponent().create()
        }

        loginComponent.inject(this)
    }
}

LongLastingElement has a static method called getInstance where you specify the type of the object that needs to be stored and you pass a lifecycle owner in. Then, the getElement method is called, it receives a lambda as a parameter that needs to create an instance of the object you want to store.

Since the Lifecycle owner is used to obtain the LongLastingElement instance, the lambda that getElement takes as a parameter will be only called once. It'll create an instance of the object the very first time is called and the same instance will be reused after configuration changes and subsequent calls to getElement . This works for any lifecycle owner such as Activities and Fragments.