片段-onDestroy活动后保存/恢复模型的最佳方法？

Question

i have a fragment that is hosted inside of an activity. when user prsses the back button i need to save the model data and have it available the next time user opens the fragment/activity. But just while in the app, it does not need to be persisted to disk. So for example if user destroyed the process, then there is no need to keep the model data, it can be fetched from network again.

what i have tried: icePick and onSavedInstance calls but these dont seem to kick in when user presses the back button on the fragment. tell me if im wrong.

here is what i have implemented in my fragment:

 @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
           outState.putParcelable("myModel", Parcels.wrap(myModel));
    }

i am using the parceler library if that makes any difference. I can also convert the code to kotlin if required. when i hit the back button the fragment gets popped off the stack and the activity contain it calls onDestroy but im not getting any call back in onSaveInstanceState. Also when i check in onCreate() savedInstanceState is null. I have not overrided onSavedInstance in the activity, just in the fragment. What am i doing wrong ?

i had a though to use a database to do this, but i just need it while in memory and there should be a way to do this without a DB.

from what i learned if user hits the back button onSaveInstance is not called by the system:

If an activity is in the foreground, and the user taps the Back button, the activity transitions through the onPause(), onStop(), and onDestroy() callbacks. In addition to being destroyed, the activity is also removed from the back stack.

It is important to note that, by default, the onSaveInstanceState() callback does not fire in this case.

source: here

Answer 1

#onSaveInstanceState of fragment is strictly coupled to activity lifecycle
According to doc

Called to retrieve per-instance state from an activity before being killed

You operates only with fragments and activity is left untouched, so this method is definitely can't be used in your case and shouldn't.

My suggestion is to use some kind of persistent storage though interface. It could be in memory storage (any type of singleton, like suggested in comments. It may be scoped to app or activity or to custom case (you have to control manually cache lifecycle) and injected with dagger, for example), shared-preferences based storage, database storage. It is easy to test if you follow dependency injection patterns & use structural pattern like MVP (but it is not a point of this question)

So store the data in the repository on change or in the onPause method (because it is the last guaranteed to call when screen is being gone). And restore it in onCreate