OC中的Block（一）

Block的本质

block本质上也是一个OC对象，它内部也有个isa指针

void (^block)(void) = ^{
    NSLog(@"I am a block");
  };

block();

以上OC代码生成的C++代码为：

//block的定义声明
 void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
//block的调用
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

其中的生成的C++结构体分别是：

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;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};

代码块生成为函数

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

    NSLog(...);
}

• block最后生成的是结构体__main_block_impl_0，
• block是封装了函数调用以及函数调用环境的OC对象
• 底层结构如下图所示：

Blcok_Layout	备注
(void *) isa	isa指针
(int) flags
int Reserved
void *FuncPtr	函数地址
struct __main_block_desc_0* Desc	block对象信息

__main_block_desc_0	备注
reserved
Block_size	block结构体大小