Observer pattern for notifying Fragments of a ViewPager to update their views. This will update the current Fragment, as well as the off screen Fragments that are retained.

FragmentObserver.java

public class FragmentObserver extends Observable {
    @Override
    public void notifyObservers() {
        setChanged(); // Set the changed flag to true, otherwise observers won't be notified.
        super.notifyObservers();
    }
}

MyActivity.java

public class MyActivity extends Activity {
    private MyAdapter mPagerAdapter;
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.my_activity);
        ViewPager pager = (ViewPager) findViewById(R.id.pager);
        mPagerAdapter = new MyAdapter();
        pager.setAdapter(mPagerAdapter);
    }
    
    private void updateFragments() {
        mPagerAdapter.updateFragments();
    }
}

MyAdapter.java

public class MyAdapter extends FragmentPagerAdapter {
    private Observable mObservers = new FragmentObserver();

    public MyAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        mObservers.deleteObservers(); // Clear existing observers.
        Fragment fragment = new MyFragment();
        if(fragment instanceof Observer)
            mObservers.addObserver((Observer) fragment);

        return fragment;
    }

    public void updateFragments() {
        mObservers.notifyObservers();
    }
}

MyFragment.java

public class MyFragment extends Fragment implements Observer {

    /* Fragment related stuff... */

    @Override
    public void update(Observable observable, Object data) {
        View root = getView();
        // Update your views here.
    }
}

I think if u remove mObservers.deleteObservers(); line it will work all fragments @julyzft. I did not try.

Thanks a lot!

public void update(Observable observable, Object data) {

why data is always null?

@The-RobinHood because there are 2 types of notifyObserver:

• notifyObservers() => data will be null
• notifyObservers(Object data) => data will be passed to Observer's update method

Code uses first one. You can modify it to call the second one if necessary.

public class MyFragment extends Fragment implements Observer {

/* Fragment related stuff... */

@OverRide
public void update(Observable observable, Object data) {
View root = getView();
// Update your views here.
System.out.println("Check View-->"+root);

}
}

My root is retrurning null. Kindly assist

Work fine! For notify all fragments remove mObservers.deleteObservers. Work great for me.
For notify with object change your implementation of FragmentObserver:

public class FragmentObserver extends Observable {
    @Override
    public void notifyObservers(object: Object) {
        setChanged(); // Set the changed flag to true, otherwise observers won't be notified.
        super.notifyObservers(object);
    }
}

Not updating current visible fragment proper by passing data from activity ?
If anybody can help, Thanks.

I want to update only current visible fragment in viewpager, so I'm using
mObservers.deleteObservers(); before addObserver(-)

And sending data from activity like:

            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                int p =tab.getPosition();
                viewPager.setCurrentItem(p);
                myPagerAdapter.updateFragments(p);
            }

And receiving data in fragment like:

@Override
    public void update(Observable observable, Object o) {
            Integer i = (Integer) o;
            tv.setText("TAB " + i);
            Log.i(TAG, "=== " + i); 
    }

Data receiving well, But UI not updating in some cases:
case1: On the first launch first page not show updates, after swipe and come back it shows update ?
case2: On the first launch If we start page scrolling using Tab click UI not update proper, while if we start page scrolling using finger swipe it works well and after that Tab click also works well ?

hello everyone who commented in this code can you tell me is this is really working or not because when i implementing this is not working

How can we send array list through object like here
public void updateFragments() {
mObservers.notifyObservers(); // <-- here
}
and access it in MyFragment -> update(Observable observable, Object data) ??

Thanks Man! You Save my Day

Work fine! For notify all fragments remove mObservers.deleteObservers. Work great for me.
For notify with object change your implementation of FragmentObserver:

public class FragmentObserver extends Observable {
    @Override
    public void notifyObservers(object: Object) {
        setChanged(); // Set the changed flag to true, otherwise observers won't be notified.
        super.notifyObservers(object);
    }
}

In Kotlin:

class FragmentObserver : Observable() {

override fun notifyObservers(arg: Any?) {
setChanged(); // Set the changed flag to true, otherwise observers won't be notified.
super.notifyObservers(arg)
}

}