重点：实现了NestedScrollingParent2
默认支持标题栏的平滑收缩。

onLayout

  private void layoutChild(View child, int layoutDirection) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        final Rect parent = acquireTempRect();
        parent.set(getPaddingLeft() + lp.leftMargin,
                getPaddingTop() + lp.topMargin,
                getWidth() - getPaddingRight() - lp.rightMargin,
                getHeight() - getPaddingBottom() - lp.bottomMargin);

        if (mLastInsets != null && ViewCompat.getFitsSystemWindows(this)
                && !ViewCompat.getFitsSystemWindows(child)) {
            // If we're set to handle insets but this child isn't, then it has been measured as
            // if there are no insets. We need to lay it out to match.
            parent.left += mLastInsets.getSystemWindowInsetLeft();
            parent.top += mLastInsets.getSystemWindowInsetTop();
            parent.right -= mLastInsets.getSystemWindowInsetRight();
            parent.bottom -= mLastInsets.getSystemWindowInsetBottom();
        }

        final Rect out = acquireTempRect();
        GravityCompat.apply(resolveGravity(lp.gravity), child.getMeasuredWidth(),
                child.getMeasuredHeight(), parent, out, layoutDirection);
        child.layout(out.left, out.top, out.right, out.bottom);

        releaseTempRect(parent);
        releaseTempRect(out);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int layoutDirection = ViewCompat.getLayoutDirection(this);
        final int childCount = mDependencySortedChildren.size();
        for (int i = 0; i < childCount; i++) {
            final View child = mDependencySortedChildren.get(i);
            if (child.getVisibility() == GONE) {
                // If the child is GONE, skip...
                continue;
            }

            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            final Behavior behavior = lp.getBehavior();

            if (behavior == null || !behavior.onLayoutChild(this, child, layoutDirection)) {
                onLayoutChild(child, layoutDirection);
            }
        }
    }