【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:
- “Create a CGLayer Object Initialized with an Existing Graphics Context”: CGLayerCreateWithContext
- “Get a Graphics Context for the Layer”: CGLayerGetContext
- “Draw to the CGLayer Graphics Context”
- “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。