IOS Performance之CALayer shadow Sucks!

核心動畫在設計的時候就考慮了效能。它首先是層層級的呈現，並且設計運行在小型的裝置上（iphone和itouch），這些裝置記憶體有限，並且cpu和gpu不如案頭電腦上的強大，核心動畫是被設計的比較高效的，但是並不意味著你就可以在代碼中隨便用。

陰影也是代價很高的。因為他們屬性部分透明的層，它需要大量的計算，來決定每個像素（因為每個像素都需要計算，直到有不透明的層遇到。如果陰影重疊的話，就增加了消耗。考慮限制只有最外層的陰影，並允許內層不產生任何陰影

QuartzCore layer.shadow吸效能。他們需要重新渲染，每次有新的變化，都會造成投影片，Lag，Lag。

http://stackoverflow.com/questions/10133109/fastest-way-to-do-shadows-on-ios/10133182#10133182

Adding a shadowPath should give you a huge performance boost. The following example assumes you only want the shadow on the sides of your view

CGPathRef path = [UIBezierPath bezierPathWithRect:view.bounds].CGPath;[view.layer setShadowPath:path];

EDIT: On default a CALayer draws a shadow during animations, the following code allows you to cache the shadow as a bitmap and reuse it instead of
redrawing it:

self.view.layer.shouldRasterize = YES;// Don't forget the rasterization scale// I spent days trying to figure out why retina display assets weren't working as expectedself.view.layer.rasterizationScale = [UIScreen mainScreen].scale;

還有一個答案是：

You can greatly improve the performance of a CALayer’s shadow by using its shadowPath property—this
allows it to draw the shadow without having to recalculate the alpha mask of the layer. For a rectangular view, you’d use it like this:

theView.layer.shadowPath = [UIBezierPath bezierPathWithRect:theView.bounds].CGPath;

or, if its corners are rounded,

theView.layer.shadowPath = [UIBezierPath bezierPathWithRoundedRect:theView.bounds cornerRadius:theView.layer.cornerRadius].CGPath;

根據實際情況，提升效能！