IOS Blocks详解-Block的实质

1.Block 实质

通过clang(LLVM编译器)我们可以把block转换为我们想要的可读代码，该代码是c++源码，但实际上也就用了struct的结构，本质为C语言

clang -rewrite-objc  main.m   //无uikit库调用
clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m //含uikit库调用

为了进一步弄清Block在底层的代码逻辑，我们先将函数简化如下并运行上面的命令得到main.cpp文件

void (^blk)(void) = ^{
        printf("Block function");
      };
    blk();

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;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

        printf("Block function");
      }

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)};
int main(int argc, char * argv[]) {

    void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

    return 0;
}

与C++的this和OC的self相同，__cself是__main_block_impl_0的结构体指针，该结构体有两个成员变量 一个是impl 一个是Desc指针，便于理解我们先将如下代码简化

void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

struct __main_block_impl_0 tmp = __main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA);
struct __main_block_impl_0 *blk = &tmp;

不难看出，将Block语法生成的Block赋给Block类型变量blk，等同于将__main_block_impl_0的结构体实例的指针赋值给变量blk，
再来看blk部分

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

(*blk->impl.FuncPtr)(blk);

这就是简单的使用函数指针调用函数，由代码看出Block真是作为参数进行了传递

关于impl 中impl.isa = &_NSConcreteStackBlock; _NSConcreteStackBlock可以看做class_t的结构体实例，将Block作为OC对象处理时，该类信息放在_NSConcreteStackBlock中，isa 对象指向类 对象的isa指向本类 类的isa指向元类 元类的isa指向根元类 根元类的isa指向自己