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：
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-相关命令"
#pragma clang diagnostic pop
常见的用法
1. 方法弃用告警
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
[TestFlight setDeviceIdentifier:[[UIDevice currentDevice] uniqueIdentifier]];
#pragma clang diagnostic pop
2. 不兼容指针类型
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wincompatible-pointer-types"
#pragma clang diagnostic pop
3. 循环引用
// completionBlock is manually nilled out
in AFURLConnectionOperation
to break the retain cycle.
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"
self.completionBlock = ^ {
...
}
#pragma clang diagnostic pop
4. 未使用变量
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-variable"
int a;
#pragma clang diagnostic pop
#pargma 用法详情：
http://nshipster.cn/pragma/http://nshipster.com/clang-diagnostics/http://fuckingclangwarnings.com/http://clang.llvm.org/docs/UsersManual.[html](http://www.2cto.com/kf/qianduan/css/)#diagnostics_pragmas
