Block的本质

Block本质是个OC对象,它内部也有个isa指针,是封装了函数调用以及函数调用环境的OC对象.

        int age = 10;
        void (^block)(int,int) = ^(int a,int b){
            NSLog(@"this is a block ----%d",age);
        };
        
        block(10,10);

终端运行以下命令

xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m

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


struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;
    //构造函数 类似于oc的init 返回结构体对象
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
      impl.isa = &_NSConcreteStackBlock;//block类型
    impl.Flags = flags;
    impl.FuncPtr = fp;//__main_block_func_0
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {
  int age = __cself->age; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_865327_mi_0,age);
        }

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, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        int age = 10;
        //定义block变量
        void (*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));
        
        //执行block代码 block->FuncPtr(block)
        ((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 10, 10);
    }
    return 0;
}

block其实就是一个__main_block_impl_0的结构体,其内部有个同名的构造函数,其传入的参数一个是该block实现的函数地址,一个是该block的信息.block的调用其实质就是通过结构体的指针拿到函数的地址并将自身作为参数传入调用.

变量捕获

        auto  int age = 10; //离开作用域就销毁 值传递
        static int age2 = 30;
        void (^block)(int,int) = ^(int a,int b){
            NSLog(@"this is a block ----%d ----%d",a,b);
            NSLog(@"age is %d---- age2 is %d",age,age2);
        };
        age = 20;
        age2 = 40;
        
        block(10,10);

输出 age = 10 age2 = 40;

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;
  int *age2;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int *_age2, int flags=0) : age(_age), age2(_age2) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {
  int age = __cself->age; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_2b925f_mi_0,a,b);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_2b925f_mi_1,age);
        }

    auto int age = 10;
        static int age2 = 30;
        void (*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age, &age2));
        age = 20;
        age2 = 40;
}

因为自动变量离开作用域就销毁的原因,block需要记录的是这个值,而不是地址.因为这块地址这时候可能另作他用.而静态局部变量不同一直存在内存.
接下来看看全局变量

int age_ = 10;
static int height_ = 10;

int main(int argc, const char * argv[]) {
    @autoreleasepool {
  
        void (^block)(void) = ^{

            NSLog(@"age is %d---- height_ is %d",age_,height_);
        };
        age_ = 20;
        height_ = 20;
        block();
        
    }
    return 0;
}

40.807363+0800 Block[34211:1225423] age is 20---- height_ is 20

查看Cpp文件

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) {


            NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_f716de_mi_0,age_,height_);
        }

可以发现block并没有捕获进去,直接就是传入的全局变量.

image.png

Block类型

先来看看block的class

        void (^block)(void) = ^{

            NSLog(@"age is %d---- height_ is %d",age_,height_);
        };
        age_ = 20;
        height_ = 20;
        block();
        
        NSLog(@"%@",[block class]);
        NSLog(@"%@",[[block class] superclass]);
        NSLog(@"%@",[[[block class] superclass]superclass]);
        NSLog(@"%@",[[[[block class] 
superclass]superclass]superclass]);

Block[34708:1251319] __NSGlobalBlock__
Block[34708:1251319] __NSGlobalBlock
Block[34708:1251319] NSBlock
Block[34708:1251319] NSObject

可以发现其是继承与NSBlock,而NSBlock又是继承于 NSObject的.

        
        void (^block)(void) = ^{
             NSLog(@"hello");
         };
        
        int age = 10;
        void (^block2)(void) = ^{
            NSLog(@"hello--age %d",age);
        };
        
        NSLog(@"\n%@ \n %@ \n %@",[block class],[block2 class],[^{
            NSLog(@"hell3-age%d",age);
        } class]);
输出:        
 __NSGlobalBlock__ 
 __NSMallocBlock__ 
 __ NSStackBlock__

image.png

• NSGlobalBlock 没有访问auto变量
• NSMallocBlock (ARC 访问auto变量 栈上自动copy到堆)
• NSStackBlock (非ARC 访问auto 变量 栈上会自动销毁)

   xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 main.m

__block

再次编译,__block int age 被转成如下结构:

 __attribute__((__blocks__(byref))) __Block_byref_age_0 age = {
(void*)0,
(__Block_byref_age_0 *)&age,
 0,
 sizeof(__Block_byref_age_0), 
10};

再来看__Block_byref_age_0

struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;
 int __flags;
 int __size;
 int age;
};

这里可以看出__block 修饰的变量被包装成一个对象,__forwarding指向的就是自身所在的地址.
在看block的结构:

  TestBlock block = ((void (*)())&__main_block_impl_0((void*)__main_block_func_0,
 &__main_block_desc_0_DATA, 
(__Block_byref_age_0 *)&age,
 570425344));

block内部有个指针指向__Block_byref_age_0结构体,这个结构体内部又有个成员存储age这个变量的值.

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_age_0 *age = __cself->age; // bound by ref

            (age->__forwarding->age) = 20;
        }

可以看出block内部是通过拿到__Block_byref_age_0在拿到__forwarding,因为这个是指向自己的,再拿到age.