iOS离屏渲染

来自 https://robots.thoughtbot.com/designing-for-ios-graphics-performance

以下情形，iOS会自动进行离屏渲染：

Core Graphics (any class prefixed with CG*)

The drawRect() method, even with an empty implementation.

CALayers with a shouldRasterize property set to YES.

CALayers using masks (setMasksToBounds) and dynamic shadows (setShadow*).

Any text displayed on screen, including Core Text.

Group opacity (UIViewGroupOpacity).

我们可以通过两种方法发现自己app哪些界面发生了离屏渲染：

1、通过Xcode自带工具Instruments（Xcode->Open Developer Tool->Instruments),打开Core Animation模板，如下图片：

点击底部工具栏Debug Options，从弹出菜单中选择Color Offscreen-Rendered Yellow

2、比较简单，用模拟器调试的时候，打开菜单Debug > Color Offscreen-Rendered选项。