public void addOnPageChangeListener(OnPageChangeListener listener) {
        if (!mOnPageChangeListeners.contains(listener)) {
            mOnPageChangeListeners.add(listener);
        }
    }
Add a listener that will be invoked whenever the page changes or is incrementally
scrolled. See {@link OnPageChangeListener}.

@param listener Listener to add
library/src/main/java/com/daimajia/slider/library/Tricks/ViewPagerEx.java