【Core Graphics Layer Drawing】
CGLayer objects (CGLayerRef data type) allow your application to use layers for drawing.
Layers are suited for the following:
CGLayer objects and transparency layers are parallel to CGPath objects and paths created by CGContext functions. In the case of a CGLayer or CGPath object, you paint to an abstract destination and can then later draw the complete painting to another destination, such as a display or a PDF. When you paint to a transparency layer or use the CGContext functions that draw paths, you draw directly to the destination represented by a graphics context. There is no intermediate abstract destination for assembling the painting.
You need to perform the tasks described in the following section to draw using a CGLayer object:
Layer就是层,一个层是一个单独可绘制的Canvas,层可被绘制到Graphics Contex上。除了层,Bitmap,Image,Primitives都可被绘制到Graphics Context上。
原文:http://www.cnblogs.com/tekkaman/p/3567787.html