IOS 避免self循环引用的方法的实例详解
示例代码：
// - weak & strong
#define myWeakify(VAR) \
try {} @finally {} \
__weak __typeof__(VAR) VAR##_myWeak_ = (VAR)
#define myStrongify(VAR) \
try {} @finally {} \
__strong __typeof__(VAR) VAR = VAR##_myWeak_
#define myStrongifyRetVIfNil(VAR, RET) \
try {} @finally {} \
__s
