IOS Performance之CALayer shadow Sucks!

本文主要是介绍IOS Performance之CALayer shadow Sucks!，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

核心动画在设计的时候就考虑了性能。它首先是层级别的呈现，并且设计运行在小型的设备上（iphone和itouch），这些设备内存有限，并且cpu和gpu不如桌面电脑上的强大，核心动画是被设计的比较高效的，但是并不意味着你就可以在代码中随便用。

阴影也是代价很高的。因为他们属性部分透明的层，它需要大量的计算，来决定每个像素（因为每个像素都需要计算，直到有不透明的层遇到。如果阴影重叠的话，就增加了消耗。考虑限制只有最外层的阴影，并允许内层不产生任何阴影

QuartzCore layer.shadow吸性能。他们需要重新渲染，每次有新的变化，都会造成幻灯片，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 expected
self.view.layer.rasterizationScale = [UIScreen mainScreen].scale;

还有一个答案是：

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;

根据实际情况，提升性能！

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