Determine if fragment restart is due to screen rotation or Viewpager swipe?

Question

I have an Activity with ViewPager. the ViewPager have a lot of pages, Just like a book. Each fragment has RecyclerViews with a lot of content. The following is my use case

1 - When I swipe page, RecyclerView must start from the beginning of the list. 2. If I rotate my device, It should read from exact last position of where I left before rotation.

If I don't use any logic, 2nd scenario works perfectly. ie, rotation. But not the first scenario.

If I do some logic like below. Helps to achieve first scenario, But not the second scenario.

  @Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setHasOptionsMenu(true);
    if (savedInstanceState==null) {
        mIsFirstTime = true;
    }
}

@Override
public void onResume() {
    super.onResume();
    try {
        getActivity().invalidateOptionsMenu();
            if (!mIsFirstTime) {
                mListView.scrollToPosition(0);                
            }
    } catch (Exception e) {
        e.printStackTrace();
    }

}

So My question is

How can I determine a fragment restart is due to screen rotation or Viewpager swipe?

I already tried onConfigurationChanged(Configuration newConfig) . But it doesn't help me. Could you please help me

Answer 1

From a comment of OP:

My question is how I know that onSaveInstanceState() is called due to screen orientation change and not fragment restart

You can do that via Activity#isChangingConfigurations() API.

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        if (isChangingConfigurations()) {
            // this callback is executed as a result of a configuration change (e.g. orientation change)
        } else {
            // no configuration change happens (e.g. a click on a home button would end up here)
        }
    }

Now you can save a boolean value into Bundle outState and appropriate setup UI after recreation:

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putBoolean("isConfigurationChange", isChangingConfigurations());
    }

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // initialization logics here

        if (savedInstanceState != null) {
            boolean isConfigurationChange = savedInstanceState.getBoolean("isConfigurationChange");

            // if `onCreate` is called as a result of configuration change
            if (isConfigurationChange) {
                // view hierarchy is not yet laid out, have to post an action 
                listView.post(new Runnable() {
                    @Override
                    public void run() {
                        // we are guaranteed, that `listView` is already laid out
                        listView.scrollToPosition(0);
                    }
                });
            }
        }
    }

Note, that performing boolean value checking logics inside onResume() is a bug (pressing home button and navigating back to the app would result in scrolling to the top again, which is not desired). Instead onCreate() should be chosen as a correct callback.

Answer 2

I think you can declare a variable and check the status to true on swipe action.

viewPager.addOnPageChangeListener(new OnPageChangeListener() {
public void onPageScrollStateChanged(int state) {}
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {}

public void onPageSelected(int position) {
// here assign the variable to true;
// This method will get called only after a swipe action is invoked on 
//viewpager.  
 }});

Inside your fragment check whether the variable status is true, if so it's not getting generated from orientation, then reset the variable to false.

Happy coding :)