How to tell activity that fragments are finished with network calls

I have this situation:

1. I got main activity that user will open when he start an app an inside that activity is a Viewpager alongside tabs in top bar so user can navigate between 4 fragments.

2. Every fragment is using retrofit to do some network request like load news data, load about section etc.

I am adding fragments to pager in classic way:

private void setupViewPager(ViewPager viewPager) {
        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
        adapter.addFrag(new WelcomeFragment(), "Home");
        adapter.addFrag(new NewsFragment(), "News");
        adapter.addFrag(new AboutFragment(), "About");
        adapter.addFrag(new Locators(), "Locations");


        viewPager.setAdapter(adapter);
    }

Network calls in fragments are done by Retrofit(Example)

NetworkSDK.getInstance().getNews(size, offset, new Callback<List<News>>() {
        @Override
        public void onResponse(Call<List<News>> call, Response<List<News>> response) {
        }

        @Override
        public void onFailure(Call<List<News>> call, Throwable t) {

        }
    });

Now i want to implement some kind of progressbar (loader) that will spin on main activity until all 4 fragments are finished with network calls. How can i notify main activity that fragments network calls are finished ? I