Core Graphics Layer Drawing

【Core Graphics Layer Drawing】

CGLayer objects (CGLayerRef data type) allow your application to use layers for drawing.

Layers are suited for the following:

• High-quality offscreen rendering of drawing that you plan to reuse. 
• Repeated drawing. 
• Buffering. Although you can use layers for this purpose, you shouldn’t need to because the Quartz Compositor makes buffering on your part unnecessary. If you must draw to a buffer, use a layer instead of a bitmap graphics context.

　　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:

1. “Create a CGLayer Object Initialized with an Existing Graphics Context”: CGLayerCreateWithContext
2. “Get a Graphics Context for the Layer”: CGLayerGetContext
3. “Draw to the CGLayer Graphics Context”
4. “Draw the Layer to the Destination Graphics Context”:  CGContextDrawLayerInRect,  CGContextDrawLayerAtPoint,

　　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。