iOS使用shouldRasterize优化性能

/* When true, the layer is rendered as a bitmap in its local coordinate 
 * space ("rasterized"), then the bitmap is composited into the 
 * destination (with the minificationFilter and magnificationFilter 
 * properties of the layer applied if the bitmap needs scaling). 
 * Rasterization occurs after the layer's filters and shadow effects 
 * are applied, but before the opacity modulation. As an implementation 
 * detail the rendering engine may attempt to cache and reuse the 
 * bitmap from one frame to the next. (Whether it does or not will have 
 * no affect on the rendered output.) 
 * 
 * When false the layer is composited directly into the destination 
 * whenever possible (however, certain features of the compositing 
 * model may force rasterization, e.g. adding filters). 
 * 
 * Defaults to NO. Animatable. */  
  
@property BOOL shouldRasterize;  
  
/* The scale at which the layer will be rasterized (when the 
 * shouldRasterize property has been set to YES) relative to the 
 * coordinate space of the layer. Defaults to one. Animatable. */  
  
@property CGFloat rasterizationScale;

如果在滚动tableView/collectionView时，每次都在cell中执行圆角设置，肯定会阻塞 UI，设置这个将会使滑动更加流畅。

当 shouldRasterize（光栅化）设成 true 时，layer被渲染成一个 bitmap，并缓存起来，等下次使用时不会再重新去渲染了。

实现圆角本身就是在做颜色混合（blending），如果每次页面出来时都blending，消耗太大，这时layer.shouldRasterize = YES，下次就只是简单的从渲染引擎的 cache 里读取那张 bitmap，节约系统资源。

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