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Block系列2-block auto和static值捕获源码分析

Posted on Edited on In Disqus:

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int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        static int staticValues = 10;
        auto int autoValues = 20;
        void(^block)(int, int) = ^(int a, int b){
            NSLog(@"This is block %d", staticValues);
            NSLog(@"This is block %d", autoValues);
            NSLog(@"This is block %d", a);
            NSLog(@"This is block %d", b);
        };
        block(50, 60);
        
        staticValues = 30;
        autoValues = 40;
    }
    return 0;
}
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xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o arm64.cpp

编译以后可以看到Block被转换成了如下代码其中__main_block_impl_0应该就是block的实现

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struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *staticValues;   //捕获到static变量是指针
  int autoValues;      //捕获到auto类型是值拷贝， 也就是说和之前的变量已经脱离关系。
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_staticValues, int _autoValues, int flags=0) : staticValues(_staticValues), autoValues(_autoValues) {
    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 *staticValues = __cself->staticValues; // bound by copy
  int autoValues = __cself->autoValues; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_fca68f_mi_0, (*staticValues));
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_fca68f_mi_1, autoValues);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_fca68f_mi_2, a);
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_fca68f_mi_3, b);
        }

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; 

        static int staticValues = 10;
        auto int autoValues = 20;
        void(*block)(int, int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &staticValues, autoValues));
        ((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 50, 60);

        staticValues = 30;
        autoValues = 40;
    }
    return 0;
}

本质

__main_block_impl_0中第一个参数是一个__block_impl的结构体，而__block_impl第一个参数是isa指针，所以Block的本质就是一个封装了运行环境的OC对象。而且也可以看到外部引用的参数age，也被分装到Block里面了，这也是为什么我们在block里面修改的参数，并不会影响外面的参数。

这里需要注意一点就是static和auto两种修饰符号，block捕获的方式不一样。static的变量，Block是通过指针捕获，因为在作用域结束以后，变量不会销毁。但是auto的变量在作用域结束以后，就会被销毁，所以Block的捕获方式是直接拷贝到结构体实现中。

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struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *staticValues;
  int autoValues;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_staticValues, int _autoValues, int flags=0) : staticValues(_staticValues), autoValues(_autoValues) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

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

调用

从block最终的实现可以看到结构体__main_block_impl_0初始化的时候传入了__main_block_func_0的参数，调用的时候实质上是调用了__block_impl的FuncPtr方法。

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static int staticValues = 10;
auto int autoValues = 20;
    
//block实现
void(*block)(int, int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &staticValues, autoValues));
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 50, 60);