【Canvas】
1、Canvas
The Canvas is the area that all UI elements should be inside. The Canvas is a Game Object with a Canvas component on it, and all UI elements must be children of such a Canvas.
Canvas uses the EventSystem object to help the Messaging System.
2、绘制顺序
UI elements in the Canvas are drawn in the same order they appear in the Hierarchy. The first child is drawn first, the second child next, and so on. If two UI elements overlap, the later one will appear on top of the earlier one.
SetAsFirstSibling、SetAsLastSibling、SetSiblingIndex 可以控制绘制顺序。
3、
原文：http://www.cnblogs.com/tekkaman/p/7617702.html