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

以下情形，iOS會自動進行離屏渲染：

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).

我們可以通過兩種方法發(fā)現(xiàn)自己app哪些界面發(fā)生了離屏渲染：

1、通過Xcode自帶工具Instruments（Xcode->Open Developer Tool->Instruments),打開Core Animation模板，如下圖片：

點擊底部工具欄Debug Options，從彈出菜單中選擇Color Offscreen-Rendered Yellow

2、比較簡單，用模擬器調(diào)試的時候，打開菜單Debug > Color Offscreen-Rendered選項。