block本质和原理,网上资料早已被写的烂大街了,但是仅仅阅读网上博客,对于原理的理解是不够的,绝知此事要躬行.

先写一个最简单的block

我这里新建的的是iOS项目

int main(int argc, char * argv[]) {
    @autoreleasepool {
        NSLog(@"======1=======");
        void(^myBlock)(void) = ^(){
            NSLog(@"testBlock");
        };
        myBlock();
        NSLog(@"======2=======");
        
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

当然新建一个Mac console项目也行

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSLog(@"======1=======");
        void(^myBlock)(void) = ^(){
            NSLog(@"testBlock");
        };
        myBlock();
        NSLog(@"======2=======");
    }
    return 0;
}

cd 命令切换到main.m目录,执行下面命令,把OC代码编译成C++代码,得到mian.cpp文件

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

上面代码编译成的C++代码如下:

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_vh_q79lg2jd2zz2by83j2rfsn_r0000gp_T_main_6cfc2b_mi_0);
        
        void(*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
        ((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
        
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_vh_q79lg2jd2zz2by83j2rfsn_r0000gp_T_main_6cfc2b_mi_2);
    }
    return 0;
}

所以block定义和调用,分别变成如下

void(*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

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

接着进一步简化代码为

void(*myBlock)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA);
     
(__block_impl *) myBlock->FuncPtr(myBlock);

所以mian函数最终可以简化为:

int main(int argc, const char * argv[]) {

    void(*myBlock)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA);
    
    (__block_impl *) myBlock->FuncPtr(myBlock);
    
    return 0;
}

完整代码为:

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
    NSLog(@"testBlock");
}

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)};


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;
  __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;
  }
};


int main(int argc, const char * argv[]) {

    void(*myBlock)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA);
    
    (__block_impl *) myBlock->FuncPtr(myBlock);
    
    return 0;
}

下面开始分析代码:
由上面代码可以看出,block最终被编译成了一个结构体,这个结构体内部又有两个结构体成员impl和desc;其中impl结构体内部又包含isa指针和FuncPtr函数指针;
由于由于impl和block结构体是同一个内存地址,所以可以直接使用block的结构体指针myBlock访问函数指针方式调用函数,即myBlock->FuncPtr()

所以blcok本质可以简写如下:

static struct __block_desc_0 {
    size_t reserved;
    size_t Block_size;
} __block_desc_0_DATA = { 0, sizeof(struct __block_impl_0)};


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

struct __block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
    
  __block_impl_0(void *fp, struct __block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};