首先#pragma在本质上是声明，常用的功能就是注释，尤其是给Code分段注释；而且它还有另一个强大的功能是处理编译器警告，但却没有上一个功能用的那么多。
clang diagnostic 是#pragma 第一个常用命令：
#pragmaclang diagnostic push#pragmaclang diagnostic ignored"-相关命令"// 你自己的代码#pragmaclang diagnostic pop
常见的用法
1. 方法弃用告警
#pragmaclang diagnostic push#pragmaclang diagnostic ignored"-Wdeprecated-declarations"[TestFlight setDeviceIdentifier:[[UIDevice currentDevice] uniqueIdentifier]];#pragmaclang diagnostic pop
2. 不兼容指针类型
#pragmaclang diagnostic push#pragmaclang diagnostic ignored"-Wincompatible-pointer-types"// #pragmaclang diagnostic pop
3. 循环引用
// completionBlock is manually nilled out in AFURLConnectionOperation to break the retain cycle. #pragmaclang diagnostic push#pragmaclang diagnostic ignored"-Warc-retain-cycles"self.completionBlock = ^ { ... };#pragmaclang diagnostic pop
4. 未使用变量
#pragmaclang diagnostic push#pragmaclang diagnostic ignored"-Wunused-variable"inta;#pragmaclang diagnostic pop
#pargma 用法详情：Which Clang Warning Is Generating This Message?
