code: private void checkAdapter() {
          PagerAdapter adapter = mPager.getAdapter();
          if (mAdapter != adapter) {
              if (mAdapter != null) {
                  mAdapter.unregisterDataSetObserver(mObserver);
              }
              mAdapter = adapter;
              if (mAdapter != null) {
                  mAdapter.registerDataSetObserver(mObserver);
              }
              reset();
          }
      }
docs: Check if the pager has a new adapter and switch to it if it does.