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iOS?block的值捕獲與指針捕獲詳解_IOS

作者：豌豆_射手 ? 更新時間： 2022-04-12 編程語言

指針與指針變量

通俗的理解：

指針：內存地址
指針變量：存放內存地址的變量
指針變量的指針：指針變量自身的內存地址

Person *p = [Person new]

右邊isa為：對象的內存地址 - 指針

p為：指針變量

左邊isa為：指針變量的內存地址 - 指針變量的指針

block捕獲變量方式

對局部變量捕獲有兩種形式：1、值捕獲（局部自動變量） 2、指針捕獲（局部靜態(tài)變量）；全局變量無需捕獲，可直接進行訪問。

clang -rewrite-objc **.m -o **.cpp 不同場景下轉換成C++代碼結果如下（嫌代碼長不想看的直接看代碼下面的結論）

值捕獲

指針變量的捕獲

block內部用一個新的指針變量來接收原指針變量。接收后，兩個指針變量里面存儲的值都是對象的內存地址，所以也可以說是值的捕獲。

局部自動變量：

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;
}

代碼分析，生成的__main_block_impl_0結構體里面創(chuàng)建了一個指針變量p，main函數里面的__main_block_impl_0初始化時，傳入的也是指針變量p。所以block對局部自動變量采用的捕獲方式是指針變量的捕獲，也就是值捕獲。

指針捕獲

對指針變量自身指針的捕獲

block內部用一個新的指針來接收（指向）原指針變量自身的地址。

局部靜態(tài)變量：

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;
}

代碼分析，生成的__main_block_impl_0結構體里面創(chuàng)建了一個指針*p，main函數里面的__main_block_impl_0初始化時，傳入的是指針變量p的地址&p。所以block對局部靜態(tài)變量采用的捕獲方式是指針變量自身地址的捕獲，也就是指針捕獲。

__block修飾的變量

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;
}

代碼分析，使用__block修飾的指針變量p，會被轉換為__Block_byref_p_0的結構體，結構體內持有p。main函數里面初始化__main_block_impl_0時傳入的是__Block_byref_p_0的地址，訪問p時，通過__Block_byref_p_0的__forwarding指針進行訪問。其實就相當于block內部捕獲了__Block_byref_p_0的指針，通過指針去訪問__Block_byref_p_0持有的p。所以__block修飾的變量本質上也相當于是一種指針捕獲，只不過不是直接捕獲指針變量p的自身地址。

值捕獲能否重新賦值？ 進行值拷貝時，block內部同名指針變量如果執(zhí)行重新賦值操作，相當于使內部的指針變量指向了一個新的對象，再對此對象進行任何操作都與原指針變量指向的原對象無關，所以不能進行重新賦值。

指針捕獲能否重新賦值？ block內部將block外部的指針變量的指針賦值給一個新的指針，block內部、外部的指針都指向的是同一個指針變量。如果進行賦值操作，操作的是同一個指針變量，所以可以進行重新賦值。

關于block延伸的知識點

如果文章看到了這里，相信對值捕獲和指針捕獲已經有了一個清晰的認識，那么可以自行思考以下幾個問題，看是否真的理解了block，文章沒有的答案放在評論區(qū)

  • 值捕獲能否在block內被重新賦值？如果是靜態(tài)變量呢？（文中已有）
  • 經__block修飾變量生成的持有變量的結構體里面__forwarding的意義在于什么？
  • 使用block有什么需要注意的點，如何去解決？

總結

原文鏈接：https://juejin.cn/post/7060899012806705188

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