Block循环引用之weak&strong

在ARC环境下，我们会使用_weak修饰符来修饰一个变量防止其在block中被循环引用，特殊情况下还会在block中使用_strong修饰符来修饰这个变量如下

__weak __typeof(self)weakSelf = self;
AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
  __strong __typeof(weakSelf)strongSelf = weakSelf;
  strongSelf.networkReachabilityStatus = status;
  if (strongSelf.networkReachabilityStatusBlock) {
    strongSelf.networkReachabilityStatusBlock(status);
  }
};

这是因为在block中调用self会引起循环引用，同时在block中需要对weakSelf进行strong操作，保证self不会被释放，当block执行完会自动释放该strongSelf。但实际上strong是为了防止野指针，而block里大部分是用weakSelf访问self的属性，而不是局部变量，我们也不可能主动释放self，这样看来就没有strong的必要了当然不排除有特殊情况。
除开上面这种一般写法，我们看看YYKit中是怎么使用weak&strong：

#ifndef weakify
    #if DEBUG
        #if __has_feature(objc_arc)
        #define weakify(object) autoreleasepool{} __weak __typeof__(object) weak##_##object = object;
        #else
        #define weakify(object) autoreleasepool{} __block __typeof__(object) block##_##object = object;
        #endif
    #else
        #if __has_feature(objc_arc)
        #define weakify(object) try{} @finally{} {} __weak __typeof__(object) weak##_##object = object;
        #else
        #define weakify(object) try{} @finally{} {} __block __typeof__(object) block##_##object = object;
        #endif
    #endif
#endif

#ifndef strongify
    #if DEBUG
        #if __has_feature(objc_arc)
        #define strongify(object) autoreleasepool{} __typeof__(object) object = weak##_##object;
        #else
        #define strongify(object) autoreleasepool{} __typeof__(object) object = block##_##object;
        #endif
    #else
        #if __has_feature(objc_arc)
        #define strongify(object) try{} @finally{} __typeof__(object) object = weak##_##object;
        #else
        #define strongify(object) try{} @finally{} __typeof__(object) object = block##_##object;
        #endif
    #endif
#endif

/**
 Example:
    @weakify(self)
    [self doSomething^{
        @strongify(self)
        if (!self) return;
        ...
    }];
 */

还有来自libextobjc的这种写法：

#define weakify(...) \\
    autoreleasepool {} \\
    metamacro_foreach_cxt(rac_weakify_,, __weak, __VA_ARGS__)

#define strongify(...) \\
    try {} @finally {} \\
    _Pragma("clang diagnostic push") \\
    _Pragma("clang diagnostic ignored \\"-Wshadow\\"") \\
    metamacro_foreach(rac_strongify_,, __VA_ARGS__) \\
    _Pragma("clang diagnostic pop")

其中细节可以参考这篇文章http://www.jianshu.com/p/3d6c4416db5e