关闭警告：
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
[myObj performSelector:mySelector withObject:name];
#pragma clang diagnostic pop
忽略没用的变量
#pragma unused (foo)
明确定义错误和警告
#error Whoa, buddy, you need to check for zero here!
#warning Dude, don't compare floating point numbers like this!
