iOS Block __block說明符

系列文章：
iOS Block概念、語法及基本使用
 iOS Block實現(xiàn)原理
 iOS Block存儲域及循環(huán)引用

上一節(jié)我們已經明白Block的實質了，語法奇特蚂会，最終也是轉換為C語言來處理的园细。那么這一節(jié)來看看 __block修飾符到底干了啥，為什么使用了 __block修飾符昔馋，在Block內部就可以改變變量值了呢筹吐，帶著疑問來看下編譯后的源碼。

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        __block int val = 10;
        void (^blk)(void) = ^{
            val = 2;
            printf("%d\n",val);
        };
        blk();
    }
    return 0;
}

編譯后绒极，
一步一步來看：

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

上述__block_impl結構體沒有變化骏令。

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
};

Desc結構體多了兩個函數(shù)，copy dispose垄提，貌似從字面上可以猜測到是用來干嘛的榔袋，持有和銷毀周拐。

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->val, (void*)src->val, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->val, 8/*BLOCK_FIELD_IS_BYREF*/);}

這兩個函數(shù)貌似不就是對應__main_block_desc_0結構體copy和dispose函數(shù)嘛，現(xiàn)在只是猜想凰兑，接著往下看妥粟。

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_val_0 *val; // by ref 
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_val_0 *_val, int flags=0) : val(_val->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

來看看Block變量編譯后的Block結構體。多了 __Block_byref_val_0 *val結構體指針變量吏够，這個東西是什么呢勾给，帶有val，val不是我們的變量名稱嗎（int val = 10）锅知，構造函數(shù)參數(shù)也有相應增加播急。

struct __Block_byref_val_0 {
  void *__isa;
__Block_byref_val_0 *__forwarding;
 int __flags;
 int __size;
 int val;
};

臥槽看到了吧，原來把我們的變量轉成了結構體售睹。__block int val = 10;這是源代碼桩警，如果不加
__block說明符，val會變成__main_block_impl_0的一個變量（上節(jié)我們已經得知）昌妹，加上了__block說明符后捶枢，這個變量就變成了結構體了，神不神奇飞崖。

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_val_0 *val = __cself->val; // bound by ref 上下文拿到__cself烂叔，也就是__main_block_impl_0結構體實例

            (val->__forwarding->val) = 2;
            printf("%d\n",(val->__forwarding->val));
        }

該函數(shù)賦值給結構體__block_impl內的FuncPtr函數(shù)指針。

int main(int argc, const char * argv[]) {
    /* @autoreleasepool*/ { __AtAutoreleasePool __autoreleasepool; 
    
        __attribute__((__blocks__(byref))) __Block_byref_val_0 val = {//初始化
            (void*)0,
            (__Block_byref_val_0 *)&val,
            0,
            sizeof(__Block_byref_val_0),
            10
        };
        //block結構體實例初始化及賦值
        void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_val_0 *)&val, 570425344));
        //block調用
        ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);
    }
    return 0;
}

來看下main函數(shù)固歪，把__block修飾的變量初始化為_Block_byref_val_0結構體變量蒜鸡，并把默認值10帶進去。Block的__main_block_impl_0結構體實例持有指向 __block 變量的__Block_byref_val_0結構體實例的指針昼牛。

__Block_byref_val_0結構體實例的成員變量_forwarding指針指向自己术瓮。通過__forwarding訪問成員變量val（成員變量val是該實例自己持有的變量，它相當于原自動變量贰健，也就是 int val= 10）胞四。

_forwarding指針.png

下面看看 single APP 使用__block說明符編譯后的源代碼：

源代碼
- (void)viewDidLoad {
    [super viewDidLoad];
    __block NSArray *arr = nil;
    void (^blk)(void) = ^{
        arr = [NSArray array];
    };
    blk();
}

編譯后：

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

static struct __ViewController__viewDidLoad_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __ViewController__viewDidLoad_block_impl_0*, struct __ViewController__viewDidLoad_block_impl_0*);
  void (*dispose)(struct __ViewController__viewDidLoad_block_impl_0*);
} __ViewController__viewDidLoad_block_desc_0_DATA = {
    0,
    sizeof(struct __ViewController__viewDidLoad_block_impl_0),
    __ViewController__viewDidLoad_block_copy_0,
    __ViewController__viewDidLoad_block_dispose_0
};

//變量結構體
struct __Block_byref_arr_0 {
  void *__isa;
__Block_byref_arr_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 NSArray *arr;
};

//賦值給__block_impl結構體實例內的FuncPtr函數(shù)指針 
static void __ViewController__viewDidLoad_block_func_0(struct __ViewController__viewDidLoad_block_impl_0 *__cself) {
  __Block_byref_arr_0 *arr = __cself->arr; // bound by ref

        (arr->__forwarding->arr) = ((NSArray * _Nonnull (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSArray"), sel_registerName("array"));
    }

//copy dispose
static void __ViewController__viewDidLoad_block_copy_0(struct __ViewController__viewDidLoad_block_impl_0*dst, struct __ViewController__viewDidLoad_block_impl_0*src) {_Block_object_assign((void*)&dst->arr, (void*)src->arr, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __ViewController__viewDidLoad_block_dispose_0(struct __ViewController__viewDidLoad_block_impl_0*src) {_Block_object_dispose((void*)src->arr, 8/*BLOCK_FIELD_IS_BYREF*/);}

//block結構體實例
struct __ViewController__viewDidLoad_block_impl_0 {
  struct __block_impl impl;
  struct __ViewController__viewDidLoad_block_desc_0* Desc;
  __Block_byref_arr_0 *arr; // by ref
  __ViewController__viewDidLoad_block_impl_0(void *fp, struct __ViewController__viewDidLoad_block_desc_0 *desc, __Block_byref_arr_0 *_arr, int flags=0) : arr(_arr->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

//viewDidLoad函數(shù)（后邊為OC語言的默認參數(shù)：id self, SEL _cmd）
static void _I_ViewController_viewDidLoad(ViewController * self, SEL _cmd) {
    ((void (*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("ViewController"))}, sel_registerName("viewDidLoad"));
    
    __attribute__((__blocks__(byref))) __Block_byref_arr_0 arr = {
        (void*)0,
        (__Block_byref_arr_0 *)&arr,
        33554432,
        sizeof(__Block_byref_arr_0),
        __Block_byref_id_object_copy_131,
        __Block_byref_id_object_dispose_131,
        __null
    };
    
    //block結構體實例初始化并賦值
    void (*blk)(void) = ((void (*)())&__ViewController__viewDidLoad_block_impl_0((void *)__ViewController__viewDidLoad_block_func_0, &__ViewController__viewDidLoad_block_desc_0_DATA, (__Block_byref_arr_0 *)&arr, 570425344));
    
    //block調用
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);
}

以上就是__block說明符（具體說是存儲域類說明符）所做的事情。

當然肯定會有疑問的伶椿，copy dispose 函數(shù)指針只是提了一下辜伟，它們到底做了哪些事情沒有具體講，下節(jié)會講到脊另。

Block的種類及存儲域
__block變量存儲域
Block循環(huán)引用