iOS block以及变量捕获

不同类型变量的捕获方式

1、局部变量和block

使用局部变量的block

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        int age = 10;
        block = ^{
            // age的值捕获进来（capture）
            NSLog(@"age is %d, ", age);
        };
        age = 20;
        block();
    }
    return 0;              
}

clang编译后的底层代码

struct __test_block_impl_0 {
  struct __block_impl impl;
  struct __test_block_desc_0* Desc;
  int age;
  __test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

可以看到，在block的内部多了成员变量_age，并且在构造这个block时，会把age的值直接传进去，几_age=age，可以看出来这时是block捕获了局部变量，并且进行了值传递。

2、静态局部变量

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // auto：自动变量，离开作用域就销毁
        auto int age = 10;
        static int height = 10;

        void (^block)(void) = ^{
            // age的值捕获进来（capture）
            NSLog(@"age is %d, height is %d", age, height); // 10  20
        };

        age = 20;
        height = 20;

        block();

    }
    return 0;
}

编译后的结果：

struct __test_block_impl_0 {
  struct __block_impl impl;
  struct __test_block_desc_0* Desc;
  int age;
  int *height;
  __test_block_impl_0(void *fp, struct __test_block_desc_0 *desc, int _age, int *_height, int flags=0) : age(_age), height(_height) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

//定义的时候
block = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, age, &height));
//执行的时候
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {
  int age = __cself->age; // bound by copy
  int *height = __cself->height; // bound by copy


        NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, age, (*height));
    }

可以看出来age传递的是值，但是静态局部变量height传递的是指针。

3、全局变量和block

int age_ = 10;
static int height_ = 10;
void (^block)(void);
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;
}

编译后：

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  // 构造函数（类似于OC的init方法），返回结构体对象
  __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;
  }
};
//调用函数的时候
static void __test_block_func_0(struct __test_block_impl_0 *__cself) {
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, age_, height_);
    }

可以看到全局变量没有捕获到block内部，是直接访问的，所以每次访问都能获得最新值。