PagerIndicator.java

public class PagerIndicator extends View {
    // The rest of the code   

    public void setPager(ViewPager pager) {
        this.pager = pager;
        pager.addOnPageChangeListener(this);
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        // Not used
    }

    @Override
    public void onPageSelected(int position) {
        invalidate();
    }

    @Override
    public void onPageScrollStateChanged(int state) {
        // Not used
    }
}