* Sets the Z order which stands for the drawing order, and reorder this node in its parent's children array.
*
* The Z order of node is relative to its "brothers": children of the same parent.
* It's nothing to do with OpenGL's z vertex. This one only affects the draw order of nodes in cocos2d.
* The larger number it is, the later this node will be drawn in each message loop.
* Please refer to setVertexZ(float) for the difference.
*
* @param nZOrder Z order of this node.
*/
Z order用於表示node節點被描繪的順序和記錄它在父節點中的子節點數組中的位置。
node節點的Z order只是和當前的父節點的所有子節點的進行比較和起作用,具有局部性。
Z order跟OperGL的頂點無關,它只是cocos2d的節點的畫圖特性;
Z order越大,則表示節點在消息循環中被描繪的順序越靠後;
可以使用setVertexZ對節點的Z order進行設置,node的nZOrder代表了node的Z order值;
二、setZOrder和_setZOrder之間的區別:_setZOrder僅僅是設置m_nZOrder的成員變量值;setZOrder先是首先設置m_nZOrder的值,然後記錄該節點在父節點的子節點數組終端的位置---m_uOrderOfArrival成員變量;