Save & restore Fragment's UI state when pushed and popped out of the backstack

Question

Background:

I have a main Activity, it wraps a main Fragment that can be changed, and in order to keep a backstack I use FragmentManager's backstack.

The main difference from keeping an activity stack is that when a fragment is pushed to the backstack and get replaced it will call it's onDestroyView() but not it's onDestroy(), and when it get back it's view will be re-created with onCreateView(). (however onCreate() is not called as the fragment object is not disposed)

In an activity stack it won't happen and the views remain.

This has a positive effect on low-end devices as the Android OS can free some memory and you don't have to keep the views right (in my app messages from the server might change the view in any time) so one can save precious bandwidth as well.

The Actual Problem:

Let's say I have a fragment and the user click on something and it's view is changed, e.g. a list is expanded.

If the user then go to another screen (i.e. fragment) the previous fragment will be pushed to the backstack and it's view will be destroyed.

When the user is going back, the fragment will be re-created and will not "remember" the changes the user had made, e.g. the list would not be expanded as it should

so how can I save the state and restore it without making special cases for every view?

Undesired Answers:

• keep the view alive: doing something to keep the view would break the fragment efficiency
• using onSaveInstanceState(): it will not get called when the fragment is pushed to the backstack as the activity is not destroyed and that's not a configuration change.
• special object: prefer not to do it if there is a way the system can do it for you.

Interfluent

possible duplicate of How to save and restore the state of an ExpandableListView in Android? – Jennet

Tnx, but that is not the same question, here I'm talking about a general way to save the enitre fragment state, In the other question they talk about one kind of a view and how to save a special feature of it. – Interfluent

I don't think you can save the whole of fragment's state, but you can save things like scroll position of a ListView, button/checkbox state, etc. I believe it will be what you want to do majority of your time. If there's a reason for that, mention that and I hope there are other ways to it. – Jennet

@Jennet Yea, I don't think so either, I didn't want a special handling for every view in a fragment but I guess I have no other choice..., It is strange that android have a mechanism exactly for it (onSaveInstanceState()) which re-creates activities, but not for fragments... – Interfluent

Are you calling .replace() when opening new fragment (with addToBackstack)? Maybe instead of .replace(), you could call .add() (with addToBackstack) and the state of the fragment that you will return to, will be kept. – Semite

@Interfluent restoring an Activity state is not much different than that of Fragment, only difference is that you restore state of Fragment in onActivityCreated(). – Jennet