- 代码块：
精简代码, 返回最后一句的值，这个方法有一个优点，所有的变量都在代码块中，也就是只在代码块的区域中有效，这意味着可以减少对其他作用域的命名污染。
NSURL *url = ({ NSString *urlString = [NSString stringWithFormat:@"%@/%@", baseURLString, endpoint];
[NSURL URLWithString:urlString];
});
- 关于编译器：
- 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!
- block
下面的方法可以避免循环引用，但是执行多行时，可能变成Nil;
__weak typeof(self) weakSelf = self;
dispatch_block_t block = ^{ [weakSelf doSomething]; // weakSelf != nil
// preemption, weakSelf turned nil
[weakSelf doSomethingElse]; // weakSelf == nil
};
最好这样调用：
__weak typeof(self) weakSelf = self;
myObj.myBlock = ^{
__strong typeof(self) strongSelf = weakSelf;
if (strongSelf) { [strongSelf doSomething]; // strongSelf != nil
// preemption, strongSelf still not nil（抢占的时候，strongSelf 还是非 nil 的)
[strongSelf doSomethingElse]; // strongSelf != nil }
else { // Probably nothing... return;
}
};
