ios开发中view.layer.shouldRasterize = YES 的使用说明

在做一个NavigationController push 子页面时，发现push和pop时很卡，研究了一大阵子后，发现在子页面里影响UI流畅的只有UIImageView的圆角设置；然后我就关闭了圆角，重新运行果然流畅多了。但是产品的需求必须加圆角，没办法，去stackoverflow找方案，发现方法都大同小异，只不过是绘制上做一些优化。后来查看layer的头文件，最后找到了一个牛B的属性：

/* 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;

当shouldRasterize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。

额外收获：如果在滚动tableView时，每次都执行圆角设置，肯定会阻塞UI，设置这个将会使滑动更加流畅。