因?yàn)閎lock一般都在對(duì)象內(nèi)部聲明.. 如果在block內(nèi)部使用了當(dāng)前對(duì)象的屬性,就會(huì)造成循環(huán)引用(block擁有當(dāng)前對(duì)象的地址,而當(dāng)前對(duì)象擁有block的地址),而引起內(nèi)存泄露,block和當(dāng)前對(duì)象都無(wú)法釋放.

@weakify 將當(dāng)前對(duì)象聲明為weak.. 這樣block內(nèi)部引用當(dāng)前對(duì)象,就不會(huì)造成引用計(jì)數(shù)+1可以破解循環(huán)引用

@strongify 相當(dāng)于聲明一個(gè)局部的strong對(duì)象,等于當(dāng)前對(duì)象.可以保證block調(diào)用的時(shí)候,內(nèi)部的對(duì)象不會(huì)釋放

AFN ，SDWebImage

__weak __typeof(self)weakSelf = self;
block = ^(){
    __strong __typeof(weakSelf)strongSelf = weakSelf;
    // strongSelf.property
};

YYKit庫(kù)里用到

/**
 Synthsize a weak or strong reference.
 
 Example:
    @weakify(self)
    [self doSomething^{
        @strongify(self)
        if (!self) return;
        ...
    }];

 */
#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