struct Block_layout {    
    void *isa;    
    int flags;    
    int reserved;   
    void (*invoke)(void *, ...);                       //Block内部可执行的部分
   struct Block_descriptor *descriptor;    //Block描述
   /* 捕获的上下文变量 */
};

struct Block_descriptor {    
    unsigned long int reserved;    
    unsigned long int size;   
    void (*copy)(void *dst, void *src);    
    void (*dispose)(void *);
};