@Override
    protected void drawableStateChanged() {
        super.drawableStateChanged();
        final Drawable d = mMarginDrawable;
        if (d != null && d.isStateful()) {
            d.setState(getDrawableState());
        }
    }
Set a drawable that will be used to fill the margin between pages.

@param resId Resource ID of a drawable to display between pages
library/src/main/java/com/daimajia/slider/library/Tricks/ViewPagerEx.java