Turn off or turn on arc

 Xcode5之后创建项目，默认会自动开启ARC自动引用计数机制，但我们在实际应用开发中考虑到种种原因，有时候不得不关闭（开启）整个应用或部分类的ARC。

The steps are:
Apple LLVM 6.0-language-objective C, objective-c Automatic Reference counting set N, Project-Build settings O To turn off the entire arc, set Yes to on.
Sometimes when we introduce a third-party class library, because the third-party class library does not support arc, you need to close the arc of the class library separately, in the following steps:
1.project-build Phases-compile Sources

2. Locate the mm file you want to set, and change the-fobjc-arc to-fno-objc-arc in the right compiler flag.

After the save is OK.

Turn off or turn on arc