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iOS block的值捕获与指针捕获详解,

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iOS block的值捕获与指针捕获详解,


目录
  • 指针与指针变量
  • block捕获变量方式
    • 值捕获
    • 指针捕获
    • __block修饰的变量
  • 关于block延伸的知识点
    • 总结

      指针与指针变量

      通俗的理解:

      指针:内存地址
      指针变量:存放内存地址的变量
      指针变量的指针:指针变量自身的内存地址

      Person *p = [Person new]

      右边isa为:对象的内存地址 - 指针

      p为:指针变量

      左边isa为:指针变量的内存地址 - 指针变量的指针

      block捕获变量方式

      对局部变量捕获有两种形式:1、值捕获(局部自动变量) 2、指针捕获(局部静态变量);全局变量无需捕获,可直接进行访问。

      clang -rewrite-objc **.m -o **.cpp 不同场景下转换成C++代码结果如下(嫌代码长不想看的直接看代码下面的结论)

      值捕获

      指针变量的捕获

      block内部用一个新的指针变量来接收原指针变量。接收后,两个指针变量里面存储的值都是对象的内存地址,所以也可以说是值的捕获。

      局部自动变量:

      int main(int argc, const char * argv[]) {
          @autoreleasepool {
              Person *p = [Person new];
              void (^block)(void) = ^{
                  NSLog(@"%@",p);
              };
              block();
          }
          return 0;
      }
      struct Person_IMPL {
          struct NSObject_IMPL NSObject_IVARS;
      };
      
      struct __main_block_impl_0 {
        struct __block_impl impl;
        struct __main_block_desc_0* Desc;
        Person *p;
        __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, Person *_p, int flags=0) : p(_p) {
          impl.isa = &_NSConcreteStackBlock;
          impl.Flags = flags;
          impl.FuncPtr = fp;
          Desc = desc;
        }
      };
      
      static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
      
        Person *p = __cself->p; // bound by copy
                  NSLog((NSString *)&__NSConstantStringImpl__var_folders_7w_wgxxl_655s9g6tms_7z44s6w0000gn_T_main_f76e59_mi_0,p);
      }
      
      static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->p, (void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);}
      
      static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);}
      
      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};
      
      int main(int argc, const char * argv[]) {
          /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
              Person *p = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("new"));
              
              void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, p, 570425344));
              
              ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
      
          }
          return 0;
      }

      代码分析,生成的__main_block_impl_0结构体里面创建了一个指针变量p,main函数里面的__main_block_impl_0初始化时,传入的也是指针变量p。所以block对局部自动变量采用的捕获方式是指针变量的捕获,也就是值捕获。

      指针捕获

      对指针变量自身指针的捕获

      block内部用一个新的指针来接收(指向)原指针变量自身的地址。

      局部静态变量:

      int main(int argc, const char * argv[]) {
          @autoreleasepool {
              static Person *p = nil;
              p = [Person new];
              void (^block)(void) = ^{
                  NSLog(@"%@",p);
              };
              block();
          }
          return 0;
      }
      struct Person_IMPL {
          struct NSObject_IMPL NSObject_IVARS;
      };
      
      struct __main_block_impl_0 {
      
        struct __block_impl impl;
        struct __main_block_desc_0* Desc;
        Person **p;
        
        __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, Person **_p, int flags=0) : p(_p) {
          impl.isa = &_NSConcreteStackBlock;
          impl.Flags = flags;
          impl.FuncPtr = fp;
          Desc = desc;
        }
      };
      
      static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
      
        Person **p = __cself->p; // bound by copy
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_7w_wgxxl_655s9g6tms_7z44s6w0000gn_T_main_bd39c2_mi_0,(*p));
      }
      
      static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->p, (void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);}
      
      static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);}
      
      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};
      
      int main(int argc, const char * argv[]) {
      
          /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
      
              static Person *p = __null;
              p = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("new"));
      
              void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &p, 570425344));
              ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
          }
          return 0;
      }

      代码分析,生成的__main_block_impl_0结构体里面创建了一个指针*p,main函数里面的__main_block_impl_0初始化时,传入的是指针变量p的地址&p。所以block对局部静态变量采用的捕获方式是指针变量自身地址的捕获,也就是指针捕获。

      __block修饰的变量

      int main(int argc, const char * argv[]) {
          @autoreleasepool {
              __block Person *p = [Person new];
              void (^block)(void) = ^{
                  NSLog(@"%@",p);
              };
              block();
          }
          return 0;
      }
      struct Person_IMPL {
          struct NSObject_IMPL NSObject_IVARS;
      };
      
      struct __Block_byref_p_0 {
        void *__isa;
      __Block_byref_p_0 *__forwarding;
       int __flags;
       int __size;
       void (*__Block_byref_id_object_copy)(void*, void*);
       void (*__Block_byref_id_object_dispose)(void*);
       Person *p;
      };
      
      struct __main_block_impl_0 {
        struct __block_impl impl;
        struct __main_block_desc_0* Desc;
        
        __Block_byref_p_0 *p; // by ref
        
        __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_p_0 *_p, int flags=0) : p(_p->__forwarding) {
          impl.isa = &_NSConcreteStackBlock;
          impl.Flags = flags;
          impl.FuncPtr = fp;
          Desc = desc;
        }
      };
      
      static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
        __Block_byref_p_0 *p = __cself->p; // bound by ref
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_7w_wgxxl_655s9g6tms_7z44s6w0000gn_T_main_6c171f_mi_0,(p->__forwarding->p));
      }
      
      static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->p, (void*)src->p, 8/*BLOCK_FIELD_IS_BYREF*/);}
      
      static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->p, 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};
      
      int main(int argc, const char * argv[]) {
      
          /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
              __attribute__((__blocks__(byref))) __Block_byref_p_0 p = {(void*)0,(__Block_byref_p_0 *)&p, 33554432, sizeof(__Block_byref_p_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("new"))};
      
              void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_p_0 *)&p, 570425344));
      
              ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
      
          }
          return 0;
      }

      代码分析,使用__block修饰的指针变量p,会被转换为__Block_byref_p_0的结构体,结构体内持有p。main函数里面初始化__main_block_impl_0时传入的是__Block_byref_p_0的地址,访问p时,通过__Block_byref_p_0的__forwarding指针进行访问。其实就相当于block内部捕获了__Block_byref_p_0的指针,通过指针去访问__Block_byref_p_0持有的p。所以__block修饰的变量本质上也相当于是一种指针捕获,只不过不是直接捕获指针变量p的自身地址。

      值捕获能否重新赋值? 进行值拷贝时,block内部同名指针变量如果执行重新赋值操作,相当于使内部的指针变量指向了一个新的对象,再对此对象进行任何操作都与原指针变量指向的原对象无关,所以不能进行重新赋值。

      指针捕获能否重新赋值? block内部将block外部的指针变量的指针赋值给一个新的指针,block内部、外部的指针都指向的是同一个指针变量。如果进行赋值操作,操作的是同一个指针变量,所以可以进行重新赋值。

      关于block延伸的知识点

      如果文章看到了这里,相信对值捕获和指针捕获已经有了一个清晰的认识,那么可以自行思考以下几个问题,看是否真的理解了block,文章没有的答案放在评论区

      • 值捕获能否在block内被重新赋值?如果是静态变量呢?(文中已有)
      • 经__block修饰变量生成的持有变量的结构体里面__forwarding的意义在于什么?
      • 使用block有什么需要注意的点,如何去解决?

      总结

      到此这篇关于iOS block的值捕获与指针捕获的文章就介绍到这了,更多相关iOS block值捕获与指针捕获内容请搜索3672js教程以前的文章或继续浏览下面的相关文章希望大家以后多多支持3672js教程!

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