1.CALayer包含在QuartzCore框架中，这是一个跨平台的框架，既可以用在iOS中又可以用在Mac OS X中。
2.在使用Core Animation开发动画的本质就是将CALayer中的内容转化为位图从而供硬件操作
3.当利用drawRect:方法绘图的本质就是绘制到了UIView的layer（属性）中
4.anchorPoint:和中心点position重合的一个点，称为“锚点”，锚点的描述是相对于x、y位置比例而言的默认在图像中心点(0.5,0.5)的位置
5.图层绘图有两种方法，不管使用哪种方法绘制完必须调用图层的setNeedDisplay方法（注意是图层的方法，不是UIView的方法，前面我们介绍过UIView也有此方法）
通过图层代理drawLayer: inContext:方法绘制
通过自定义图层drawInContext:方法绘制
6.注释代码块
/**< <# method name #> */
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输入expr @import UIKit 在控制台打印输出from
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在苹果文档里面看到这段话，涨知识了
During the instantiation process, each object in the archive is unarchived and then initialized with the method befitting its type. Objects that conform to the NSCoding protocol (including all subclasses of UIView and UIViewController) are initialized using their initWithCoder: method. All objects that do not conform to the NSCoding protocol are initialized using their init method. After all objects have been instantiated and initialized, the nib-loading code reestablishes the outlet and action connections for all of those objects. It then calls the awakeFromNib method of the objects. For more detailed information about the steps followed during the nib-loading process, see Nib Files in Resource Programming Guide.
