1. __block可以用于解决block内部无法修改auto变量值的问题

2. __block不能修饰全局变量、静态变量(static)

3. __block修饰变量时，编译器会把该变量包装成一个对象

// OC代码
__block int age = 10;
__block NSObject *obj = [[NSObject alloc] init];
void (^block1)(void) = ^{
   age = 20;
   obj = nil;
   NSLog(@"block a = %d",age);
};

//修饰基本数据类型，外部变量用__block修饰之后的结构体
struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;
 int __flags;
 int __size;
 int age;
};
// 修饰类
struct __Block_byref_obj_1 {
  void *__isa;
__Block_byref_obj_1 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSObject *__strong obj;
};

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};

// 捕获__block修饰的外部变量之后的 Block结构体
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_age_0 *age; // by ref
  __Block_byref_obj_1 *obj; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_age_0 *_age, __Block_byref_obj_1 *_obj, int flags=0) : age(_age->__forwarding), obj(_obj->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};