private boolean performDrag(float x) {
        boolean needsInvalidate = false;

        final float deltaX = mLastMotionX - x;
        mLastMotionX = x;

        float oldScrollX = getScrollX();
        float scrollX = oldScrollX + deltaX;
        final int width = getClientWidth();

        float leftBound = width * mFirstOffset;
        float rightBound = width * mLastOffset;
        boolean leftAbsolute = true;
        boolean rightAbsolute = true;

        final ItemInfo firstItem = mItems.get(0);
        final ItemInfo lastItem = mItems.get(mItems.size() - 1);
        if (firstItem.position != 0) {
            leftAbsolute = false;
            leftBound = firstItem.offset * width;
        }
        if (lastItem.position != mAdapter.getCount() - 1) {
            rightAbsolute = false;
            rightBound = lastItem.offset * width;
        }

        if (scrollX < leftBound) {
            if (leftAbsolute) {
                float over = leftBound - scrollX;
                needsInvalidate = mLeftEdge.onPull(Math.abs(over) / width);
            }
            scrollX = leftBound;
        } else if (scrollX > rightBound) {
            if (rightAbsolute) {
                float over = scrollX - rightBound;
                needsInvalidate = mRightEdge.onPull(Math.abs(over) / width);
            }
            scrollX = rightBound;
        }
        // Don't lose the rounded component
        mLastMotionX += scrollX - (int) scrollX;
        scrollTo((int) scrollX, getScrollY());
        pageScrolled((int) scrollX);

        return needsInvalidate;
    }
This method will be invoked when the current page is scrolled, either as part
of a programmatically initiated smooth scroll or a user initiated touch scroll.
If you override this method you must call through to the superclass implementation
(e.g. super.onPageScrolled(position, offset, offsetPixels)) before onPageScrolled
returns.

@param position Position index of the first page currently being displayed.
                Page position+1 will be visible if positionOffset is nonzero.
@param offset Value from [0, 1) indicating the offset from the page at position.
@param offsetPixels Value in pixels indicating the offset from position.
library/src/main/java/com/daimajia/slider/library/Tricks/ViewPagerEx.java