關(guān)于OC中Block的實現(xiàn)原理，百度可以搜到一堆。但沒有一篇文章說明__weak修飾的變量是如何被捕獲的為何沒影響到原對象的引用計數(shù)。以下是對這個丟失的部分的補充。

PS:閱讀以下內(nèi)容之前，應(yīng)了解Block和weak的實現(xiàn)原理。

1. 首先，main.m有以下代碼

typedef void(^Block)();

Block block;

int main(int argc, char * argv[]) {
    
    NSObject *obj = [NSObject new];
    
    NSLog(@"%@",[obj valueForKey:@"retainCount"]);
    
    __weak id weakObj = obj;
    
    block = ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_global_queue(0, 0), ^{
            NSLog(@"weakObj -- %@",[weakObj valueForKey:@"retainCount"]);
        });
    };
    
    block();
    
    NSLog(@"%@",[obj valueForKey:@"retainCount"]);
    
    sleep(10);
}

2. cd到工程目錄后，clang重寫

clang -rewrite-objc -fobjc-arc -stdlib=libc++ -mmacosx-version-min=10.7 -fobjc-runtime=macosx-10.7 -Wno-deprecated-declarations main.m

3. 打開main.cpp并觀察重寫后的block結(jié)構(gòu)體

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __weak id weakObj;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __weak id _weakObj, int flags=0) : weakObj(_weakObj) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

4. 結(jié)論

如果你也注意到了__weak id weakObj;想必已經(jīng)明了。

完。