Block 的底层实现

    int main(int argc, const char * argv[]) {        ^{ };        return 0;    }

    struct __block_impl {        void *isa; //isa，指向所属类的指针，也就是block的类型        int Flags; //flags，标志变量，在实现block的内部操作时会用到        int Reserved; //Reserved，保留变量        void *FuncPtr; //block执行时调用的函数指针    };
    struct __main_block_impl_0 {        struct __block_impl impl;        struct __main_block_desc_0* Desc;        __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {            impl.isa = &_NSConcreteStackBlock;  //__main_block_impl_0的isa指针指向了_NSConcreteStackBlock            impl.Flags = flags;            impl.FuncPtr = fp; //从main函数中看， __main_block_impl_0的FuncPtr指向了函数__main_block_func_0            Desc = desc; //__main_block_impl_0的Desc也指向了定义__main_block_desc_0时就创建的__main_block_desc_0_DATA，其中纪录了block结构体大小等信息。        }    };
    static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
    }
    static struct __main_block_desc_0 {        size_t reserved; //保留字段        size_t Block_size; //block大小(sizeof(struct __main_block_impl_0))    } __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};    //以上代码在定义__main_block_desc_0结构体时，同时创建了__main_block_desc_0_DATA，并给它赋值，以供在main函数中对__main_block_impl_0进行初始化。
    int main(int argc, const char * argv[]) {
        ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
        return 0;    }