Block的本质和类型

Block的本质

- (void)typeOfBlcok {
  void(^block)(void) = ^{
    
  };
  NSLog(@"%@",[block superclass]);
  NSLog(@"%@",[[block superclass] superclass]);
  NSLog(@"%@",[[[block superclass] superclass] superclass]);
  block();
}

下图是打印结果：

block类型.jpg

从打印结果可以看出block本质上也是继承自NSObject的OC对象；

将上面OC代码通过clang转换成的C++代码

/*
  对应上面代码声明的block,是个C++的结构体，这个结构体有两个成员变量impl和Desc
  impl是这种结构体类型
  struct __block_impl {
  void *isa;//block含有isa指针
  int Flags;
  int Reserved;
  void *FuncPtr;//保存block块要执行的代码
};
  __ViewController__typeOfBlcok_block_impl_0为结构体的初始化方法，接收三个参数fp、desc、flags，并在初始化时将参数分别赋值给第一个成员变量impl对应的成员；
*/ 
struct __ViewController__typeOfBlcok_block_impl_0 {
struct __block_impl impl; 
struct __ViewController__typeOfBlcok_block_desc_0* Desc;
__ViewController__typeOfBlcok_block_impl_0(void *fp, struct __ViewController__typeOfBlcok_block_desc_0 *desc, int flags=0) {
  impl.isa = &_NSConcreteStackBlock;
  impl.Flags = flags;
  impl.FuncPtr = fp;
  Desc = desc;
  }
};

/*
这个函数对应block内部要执行的代码，block初始化时将这个函数作为第一个参数传了进去，也即是fp
*/
static void __ViewController__typeOfBlcok_block_func_0(struct __ViewController__typeOfBlcok_block_impl_0 *__cself) {

}

/*
 这个结构体就是block第二个成员变量的类型，描述block，block初始化时将这个结构体的地址作为第二个参数传了进去
*/
static struct __ViewController__typeOfBlcok_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __ViewController__typeOfBlcok_block_desc_0_DATA = { 0, sizeof(struct __ViewController__typeOfBlcok_block_impl_0)};

static void _I_ViewController_typeOfBlcok(ViewController * self, SEL _cmd) {

void(*block)(void) = ((void (*)())&__ViewController__typeOfBlcok_block_impl_0((void *)__ViewController__typeOfBlcok_block_func_0, &__ViewController__typeOfBlcok_block_desc_0_DATA));
  
  NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_06e180_mi_13,((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("superclass")));
 
  NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_06e180_mi_14,((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("superclass")), sel_registerName("superclass")));
  
  NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_06e180_mi_15,((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("superclass")), sel_registerName("superclass")), sel_registerName("superclass")));
  
  ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);  
  // block调用，直接取block的第一个成员变量impl调用它的funcPtr函数，因为block地址就是首个成员变量的地址。
}

Block的类型和变量捕获

block块会保存一段代码延后执行，所以为了保证代码能正常访问外部变量，block块在访问外部变量时可能会捕获外部变量，我们先看MRC环境下Block访问外部变量的表现,以下都是MRC环境的代码。

继续下面代码前，需要先说下关键字auto(自动变量)，一般声明局部变量，默认就是自动变量，自动变量的作用域为变量所在方法，方法执行完毕自动变量自动释放；以下描述摘自维基百科：

在计算机编程领域，自动变量（Automatic Variable）指的是局部作用域变量，具体来说即是在控制流进入变量作用域时系统自动为其分配存储空间，并在离开作用域时释放空间的一类变量。在许多程序语言中，自动变量与术语“局部变量”（Local Variable）所指的变量实际上是同一种变量，所以通常情况下“自动变量”与“局部变量”是同义的。

首先先给出两个结论：

• block的变量捕获

capture.png

• block类型

block类型	访问变量情况(MRC)	存储区域
NSStackBlock	访问了auto变量	栈
NSGlobalBlock	没有访问auto变量	数据区域
NSMallocBlock	NSStackBlock调用copy	堆

不论访问的auto变量是基本数据还是对象类型，结论都是一样的。在ARC环境下，编译器会根据情况将栈上的block拷贝到堆上，所以在ARC环境下打印访问了auto变量的block类型经常显示为NSMallocBlock，具体什么时候会被拷贝到堆上，在下一篇文章中详细说明。

下面通过代码证明上述结论

• 访问auto变量

  - (void)captureAutoAndStaticVariable {
    int a = 10;
    static int b = 10;

    void(^block)(void) = ^{
        NSLog(@"autoAndStatic:%d----%d",a,b);//打印结果：autoAndStatic: 10 ----20
    };
    a = 11;
    b = 20;
    block();
   
    NSLog(@"autoAndStatic:%@",[block class]);//打印结果：autoAndStatic:__NSStackBlock__

    NSLog(@"autoAndStatic_copy:%@",[[block copy] class]);//打印结果：autoAndStatic:__NSMallocBlock__
 }

 // 将上面OC代码通过clang转换成的C++代码
struct __ViewController__captureAutoAndStaticVariable_block_impl_0 {
  struct __block_impl impl;
  struct __ViewController__captureAutoAndStaticVariable_block_desc_0*  Desc;
  int a;
  int *b;
  __ViewController__captureAutoAndStaticVariable_block_impl_0(void *fp, struct __ViewController__captureAutoAndStaticVariable_block_desc_0 *desc, int _a, int *_b, int flags=0) : a(_a), b(_b) {
   impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __ViewController__captureAutoAndStaticVariable_block_func_0(struct __ViewController__captureAutoAndStaticVariable_block_impl_0 *__cself) {
  int a = __cself->a; // bound by copy
  int *b = __cself->b; // bound by copy
  NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_3,a,(*b));
}

static struct __ViewController__captureAutoAndStaticVariable_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __ViewController__captureAutoAndStaticVariable_block_desc_0_DATA = { 0, sizeof(struct __ViewController__captureAutoAndStaticVariable_block_impl_0)};

static void _I_ViewController_captureAutoAndStaticVariable(ViewController * self, SEL _cmd) {
  int a = 10;
  static int b = 10;
  void(*block)(void) = ((void (*)())&__ViewController__captureAutoAndStaticVariable_block_impl_0((void *)__ViewController__captureAutoAndStaticVariable_block_func_0, &__ViewController__captureAutoAndStaticVariable_block_desc_0_DATA, a, &b));
   a = 11;
   b = 20;
   ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
    
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_4,((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")));
   
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_5,((Class (*)(id, SEL))(void *)objc_msgSend)((id)((id (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("copy")), sel_registerName("class")));
}

从代码中我们可以看出这里的block有四个成员变量。除了跟第一段代码相同的impl和desc外，还多了int类型的a和int *类型的b，并且初始化时将外部的a的值赋值给了这个a，将b的地址赋值给了int *指针b。也即是block内部新增加了成员变量a，存储了外部变量a的值，新增了int *指针b存储了静态变量b的地址。在执行block代码块时，即在__ViewController__captureAutoAndStaticVariable_block_func_0函数中取到block的成员变亮a打印结果，所有打印的是内部捕获到的值10，而不是11。外部变量a的值虽然改变了，但block内部的值仍是10；取出b地址里的值打印，所有打印结果是20；印证了我们上面的结论，当block访问局部变量时，捕获auto类型变量的值，捕获static类型变量的地址。这是因为auto类型变量超过其作用域后内存会自动销毁，如果捕获其地址，block执行时访问到值是未知的。而static变量在程序运行过程中是不会被销毁的，不存在这个问题。

• 访问全局变量

int globalA = 10;
@implementation ViewController

- (void)captureGlobalVariable {
  void(^block)(void) = ^{
       NSLog(@"globalVariable:%d",globalA);//打印结果是 11
  };
  globalA = 11;
  block();
  NSLog(@"globalVariable:%@",[block class]);//打印结果：globalVariable:__NSGlobalBlock__
  NSLog(@"globalVariable_copy:%@",[[block copy] class]);//打印结果：globalVariable:__NSGlobalBlock__
}


// 将上面OC代码通过clang转换成的C++代码
struct __ViewController__captureGlobalVariable_block_impl_0 {
  struct __block_impl impl;
  struct __ViewController__captureGlobalVariable_block_desc_0* Desc;
  __ViewController__captureGlobalVariable_block_impl_0(void *fp, struct __ViewController__captureGlobalVariable_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __ViewController__captureGlobalVariable_block_func_0(struct __ViewController__captureGlobalVariable_block_impl_0 *__cself) {

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_6,globalA);
}

static struct __ViewController__captureGlobalVariable_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __ViewController__captureGlobalVariable_block_desc_0_DATA = { 0, sizeof(struct __ViewController__captureGlobalVariable_block_impl_0)};

static void _I_ViewController_captureGlobalVariable(ViewController * self, SEL _cmd) {

    void(*block)(void) = ((void (*)())&__ViewController__captureGlobalVariable_block_impl_0((void *)__ViewController__captureGlobalVariable_block_func_0, &__ViewController__captureGlobalVariable_block_desc_0_DATA));

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

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_7,((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")));

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_zx_b2snvmns3v14tgx8jk3ysvrm0000gn_T_ViewController_64edfa_mi_8,((Class (*)(id, SEL))(void *)objc_msgSend)((id)((id (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("copy")), sel_registerName("class")));
}

从打印结果可以看出，GlobalBlock类型的block，copy后仍是GlobalBlock类型。且block内部没有新增成员变量接受globalA的值或地址，执行block块时直接取出globalA的值打印，所以打印的是最新值。