void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1 &&
                mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);

            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }

            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }

                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate(this);
        }

        Collections.sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) {
                    lp.heightFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }
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