@Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int count = getChildCount();
        int width = r - l;
        int height = b - t;
        int paddingLeft = getPaddingLeft();
        int paddingTop = getPaddingTop();
        int paddingRight = getPaddingRight();
        int paddingBottom = getPaddingBottom();
        final int scrollY = getScrollY();

        int decorCount = 0;

        // First pass - decor views. We need to do this in two passes so that
        // we have the proper offsets for non-decor views later.
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                int childLeft = 0;
                int childTop = 0;
                if (lp.isDecor) {
                    final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                    switch (hgrav) {
                        default:
                            childLeft = paddingLeft;
                            break;
                        case Gravity.LEFT:
                            childLeft = paddingLeft;
                            paddingLeft += child.getMeasuredWidth();
                            break;
                        case Gravity.CENTER_HORIZONTAL:
                            childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                    paddingLeft);
                            break;
                        case Gravity.RIGHT:
                            childLeft = width - paddingRight - child.getMeasuredWidth();
                            paddingRight += child.getMeasuredWidth();
                            break;
                    }
                    switch (vgrav) {
                        default:
                            childTop = paddingTop;
                            break;
                        case Gravity.TOP:
                            childTop = paddingTop;
                            paddingTop += child.getMeasuredHeight();
                            break;
                        case Gravity.CENTER_VERTICAL:
                            childTop = Math.max((height - child.getMeasuredHeight()) / 2,
                                    paddingTop);
                            break;
                        case Gravity.BOTTOM:
                            childTop = height - paddingBottom - child.getMeasuredHeight();
                            paddingBottom += child.getMeasuredHeight();
                            break;
                    }
                    childTop += scrollY;
                    child.layout(childLeft, childTop,
                            childLeft + child.getMeasuredWidth(),
                            childTop + child.getMeasuredHeight());
                    decorCount++;
                }
            }
        }

        final int childHeight = height - paddingTop - paddingBottom;
        // Page views. Do this once we have the right padding offsets from above.
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                ItemInfo ii;
                if (!lp.isDecor && (ii = infoForChild(child)) != null) {
                    int toff = (int) (childHeight * ii.offset);
                    int childLeft = paddingLeft;
                    int childTop = paddingTop + toff;
                    if (lp.needsMeasure) {
                        // This was added during layout and needs measurement.
                        // Do it now that we know what we're working with.
                        lp.needsMeasure = false;
                        final int widthSpec = MeasureSpec.makeMeasureSpec(
                                (int) (width - paddingLeft - paddingRight),
                                MeasureSpec.EXACTLY);
                        final int heightSpec = MeasureSpec.makeMeasureSpec(
                                (int) (childHeight * lp.heightFactor),
                                MeasureSpec.EXACTLY);
                        child.measure(widthSpec, heightSpec);
                    }
                    if (DEBUG) Log.v(TAG, "Positioning #" + i + " " + child + " f=" + ii.object
                            + ":" + childLeft + "," + childTop + " " + child.getMeasuredWidth()
                            + "x" + child.getMeasuredHeight());
                    child.layout(childLeft, childTop,
                            childLeft + child.getMeasuredWidth(),
                            childTop + child.getMeasuredHeight());
                }
            }
        }
        mLeftPageBounds = paddingLeft;
        mRightPageBounds = width - paddingRight;
        mDecorChildCount = decorCount;

        if (mFirstLayout) {
            scrollToItem(mCurItem, false, 0, false);
        }
        mFirstLayout = false;
    }
This is the persistent state that is saved by ViewPager.  Only needed
if you are creating a sublass of ViewPager that must save its own
state, in which case it should implement a subclass of this which
contains that state.
dkplayer-sample/src/main/java/xyz/doikki/dkplayer/widget/VerticalViewPager.java