OnPageChangeListener setInternalPageChangeListener(OnPageChangeListener listener) {
        OnPageChangeListener oldListener = mInternalPageChangeListener;
        mInternalPageChangeListener = listener;
        return oldListener;
    }
Set a separate OnPageChangeListener for internal use by the support library.

@param listener Listener to set
@return The old listener that was set, if any.
library/src/main/java/com/daimajia/slider/library/Tricks/ViewPagerEx.java