block修改变量

• 示例代码

        void (^block)(void);
        
        {
            __block int a = 10;
            block = ^{
                 a = 20;
                 NSLog(@"person-%d",a);
             };
            
        }
        
        block();
    }
    

• __block 会把变量包装成一个结构体

struct __Block_byref_a_0 {
  void *__isa;
__Block_byref_a_0 *__forwarding;
 int __flags;
 int __size;
 int a;
};

• block

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_a_0 *a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_a_0 *_a, int flags=0) : a(_a->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

• block 代码块/内存管理/调用环境描述

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_a_0 *a = __cself->a; // bound by ref

                 (a->__forwarding->a) = 20;
                 NSLog((NSString *)&__NSConstantStringImpl__var_folders_p8_kz1xzn596q746q_0rv80p9zc0000gn_T_main_e0776a_mi_0,(a->__forwarding->a));
             }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

• block使用

        void (*block)(void);

        {

            __attribute__((__blocks__(byref))) __Block_byref_a_0 a = {(void*)0,(__Block_byref_a_0 *)&a, 0, sizeof(__Block_byref_a_0), 10};
            block = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_a_0 *)&a, 570425344));

        }

        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

对象类型的auto变量和 __block变量

• __NSStackBlock__ 都不会强引用
• __NSMallocBlock__block从栈拷贝到堆上时，调用copy方法，copy方法调用_Block_object_assign
  1>_Block_object_assign根据__对象类型的auto变量是__strong/__weak，决定强/弱引用 （3/BLOCK_FIELD_IS_OBJECT/)
  2> _Block_object_assign对__block变量强引用（8/BLOCK_FIELD_IS_BYREF/)

__block修饰对象

• 会把对象包装成__Block_byref_person_0结构体，并对对象进行内存管理

struct __Block_byref_person_0 {
  void *__isa;
__Block_byref_person_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSPerson *__strong person;
};

• block内引用__Block_byref_person_0类型的结构体对象

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_person_0 *person; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_person_0 *_person, int flags=0) : person(_person->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

• __main_block_desc_0里面对__Block_byref_person_0进行内存管理

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_person_0 *person = __cself->person; // bound by ref


                 NSLog((NSString *)&__NSConstantStringImpl__var_folders_p8_kz1xzn596q746q_0rv80p9zc0000gn_T_main_4e2f7c_mi_0,(person->__forwarding->person));
             }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->person, (void*)src->person, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->person, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

• 创建__Block_byref_person_0时会创建一个对象传给__Block_byref_person_0，并调用__Block_byref_id_object_copy_131，__Block_byref_id_object_dispose_131对对象进行内存管理

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 


        void (*block)(void);

        {
            __attribute__((__blocks__(byref))) __Block_byref_person_0 person = {
          (void*)0,
          (__Block_byref_person_0 *)&person, 
          33554432,
          sizeof(__Block_byref_person_0), 
          __Block_byref_id_object_copy_131,
         __Block_byref_id_object_dispose_131,
         ((NSPerson *(*)(id, SEL))(void *)objc_msgSend)((id)((NSPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSPerson"), sel_registerName("alloc")), sel_registerName("init"))};



            block = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_person_0 *)&person, 570425344));

        }

        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
    }


    NSLog((NSString *)&__NSConstantStringImpl__var_folders_p8_kz1xzn596q746q_0rv80p9zc0000gn_T_main_4e2f7c_mi_1);

    return 0;
}

• __Block_byref_person_0对对象进行内存管理

static void __Block_byref_id_object_copy_131(void *dst, void *src) {
_Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);
}
static void __Block_byref_id_object_dispose_131(void *src) {
_Block_object_dispose(*(void * *) ((char*)src + 40), 131);
}

• 上面+40是指向了 NSPerson *__strong person;

struct __Block_byref_person_0 {
  void *__isa;      //8
__Block_byref_person_0 *__forwarding;  //8
 int __flags;      //4
 int __size;      //4
 void (*__Block_byref_id_object_copy)(void*, void*);  //8
 void (*__Block_byref_id_object_dispose)(void*);    //8
 NSPerson *__strong person;
};

MRC环境__block修饰对象都是弱引用