onActivityCreated is deprecated, how to properly use LifecycleObserver?

Question

Google deprecate fragment’s onActivityCreated() on Android and recommend to use LifeCycleObserver:

 To get a callback specifically when a Fragment activity's
     * {@link Activity#onCreate(Bundle)} is called, register a
     * {@link androidx.lifecycle.LifecycleObserver} on the Activity's
     * {@link Lifecycle} in {@link #onAttach(Context)}, removing it when it receives the
     * {@link Lifecycle.State#CREATED} callback.

So I try to make it in recommended way, but only state I can observe in Logcat is just State: INITIALIZED.

 private lateinit var lifecycleObserver: LifecycleObserver

 override fun onAttach(context: Context) {
    super.onAttach(context)

    hostActivity = context as HostActivity

    lifecycleObserver = object : LifecycleObserver {

        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        fun onCreate() {
            Logger.tag("SOME-TAG")d("State: ${lifecycle.currentState}")

            if(lifecycle.currentState.isAtLeast(Lifecycle.State.CREATED)) {
                Logger.tag("SOME-TAG").d("CREATED")
                hostActivity.lifecycle.removeObserver(lifecycleObserver)
            }
        }
    }

    hostActivity.lifecycle.addObserver(lifecycleObserver)
}

What is wrong in code above?

UPDATE 1: Looks like I forgot to use hostActivity.lifecycle.currentState and checked fragment's lifecycle instead of Activities lifecycle.

UPDATE 2: Suggested by Google approach not worked for 1 Host activity and 2 fragments when you click back button from one to another, cause onAttach never called, but onActivityCreated called.

Answer 1

As per the changelog here

The onActivityCreated() method is now deprecated. Code touching the fragment's view should be done in onViewCreated() (which is called immediately before onActivityCreated()) and other initialization code should be in onCreate(). To receive a callback specifically when the activity's onCreate() is complete, a LifeCycleObserver should be registered on the activity's Lifecycle in onAttach(), and removed once the onCreate() callback is received.

You can do something like this in your fragment class:

class MyFragment : Fragment(), LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreated() {
        // ... Your Logic goes here ...
    }

    override fun onAttach(context: Context) {
        super.onAttach(context)
        activity?.lifecycle?.addObserver(this)
    }

    override fun onDetach() {
        activity?.lifecycle?.removeObserver(this)
        super.onDetach()
    }
}

Answer 2

All I needed was onActivityCreated(...), hence I did implement an observer that:

• Automatically removes itself (using .removeObserver(...)).
• Then calls passed callback (update()).

I did it in next way:

class MyActivityObserver(
    private val update: () -> Unit
) : DefaultLifecycleObserver {

    override fun onCreate(owner: LifecycleOwner) {
        super.onCreate(owner)
        owner.lifecycle.removeObserver(this)
        update()
    }
}

and use it in fragments onAttach (or another lifecycle method) like:

myActivity.lifecycle.addObserver(MyActivityObserver {
    myOnActivityCreated()
})