《Quartz 2D编程指南》之【graphics context】图形上下文的作用、分类、状态的保持、恢复、 上下文的矩阵操作（修改上下文的形变）

引言

• The Graphics Context：数据类型为CGContextRef

You can think of a graphics context as a drawing destination. A graphics context encapsulates all of the information needed to draw to an underlying canvas,including the current drawing attributes and a device-specific representation of the digital paint on the