ItemInfo addNewItem(int position, int index) {
        ItemInfo ii = new ItemInfo();
        ii.position = position;
        ii.object = mAdapter.instantiateItem(this, position);
        ii.heightFactor = mAdapter.getPageWidth(position);
        if (index < 0 || index >= mItems.size()) {
            mItems.add(ii);
        } else {
            mItems.add(index, ii);
        }
        return ii;
    }
Like {@link View#scrollBy}, but scroll smoothly instead of immediately.

@param x        the number of pixels to scroll by on the X axis
@param y        the number of pixels to scroll by on the Y axis
@param velocity the velocity associated with a fling, if applicable. (0 otherwise)
dkplayer-sample/src/main/java/xyz/doikki/dkplayer/widget/VerticalViewPager.java