tag是UIView的一个属性，而且要求tag值唯一。父视图可以通过tag来找到一个子视图
1 UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 2 redView.backgroundColor = [UIColor redColor]; 3 redView.tag = 1000; 4 [self.window addSubview:redView]; 5 6 UIView *getView = [self.window viewWithTag:1000];
下面来看下几种需要注意的错误示例
一，view下有tag值相同的两个subview
1 UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 2 redView.backgroundColor = [UIColor redColor]; 3 redView.tag = 1000; 4 5 UIView *yellowView = [[UIView alloc]initWithFrame:CGRectMake(0, CGRectGetHeight(self.window.frame)/2, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 6 yellowView.backgroundColor = [UIColor yellowColor]; 7 yellowView.tag = 1000; 8 9 [self.window addSubview:yellowView]; 10 [self.window addSubview:redView]; 11 12 UIView *getView = [self.window viewWithTag:1000]; 13 [getView setBackgroundColor:[UIColor whiteColor]];
结果是yellowView的背景被改变了，说明这种情况下会取得父视图加载的第一个tag是1000的子视图。
二，父视图取得子视图的子视图。
1 UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 2 redView.backgroundColor = [UIColor redColor]; 3 redView.tag = 1000; 4 5 UIView *yellowView = [[UIView alloc]initWithFrame:CGRectMake(0, CGRectGetHeight(self.window.frame)/2, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 6 yellowView.backgroundColor = [UIColor yellowColor]; 7 yellowView.tag = 1001; 8 9 UIView *blueView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 10 blueView.backgroundColor = [UIColor blueColor]; 11 blueView.tag = 1002; 12 13 UIView *greenView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 14 greenView.backgroundColor = [UIColor greenColor]; 15 greenView.tag = 1003; 16 17 [redView addSubview:blueView]; 18 [yellowView addSubview:greenView]; 19 20 21 [self.window addSubview:yellowView]; 22 [self.window addSubview:redView]; 23 24 UIView *getView = [self.window viewWithTag:1003]; 25 [getView setBackgroundColor:[UIColor whiteColor]];
结果是greenView背景被改变了，说明这个是可行的。
三，取存在但不是自己子视图的视图
将示例2中的倒数第二句代码改为
UIView *getView = [redView viewWithTag:1003];
结果是greenView的背景没有被改变，说明这是行不通的。
四，别的视图中的子视图和本视图的子视图有相同的tag值
1 UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 2 redView.backgroundColor = [UIColor redColor]; 3 redView.tag = 1000; 4 5 UIView *yellowView = [[UIView alloc]initWithFrame:CGRectMake(0, CGRectGetHeight(self.window.frame)/2, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 6 yellowView.backgroundColor = [UIColor yellowColor]; 7 yellowView.tag = 1001; 8 9 UIView *blueView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 10 blueView.backgroundColor = [UIColor blueColor]; 11 blueView.tag = 1002; 12 13 UIView *greenView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 14 greenView.backgroundColor = [UIColor greenColor]; 15 greenView.tag = 1002; 16 17 [redView addSubview:blueView]; 18 [yellowView addSubview:greenView]; 19 20 21 [self.window addSubview:yellowView]; 22 [self.window addSubview:redView]; 23 24 UIView *getView = [redView viewWithTag:1002]; 25 [getView setBackgroundColor:[UIColor whiteColor]];
结果来看还可以
五，greenView和redView的tag值相同
1 UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 2 redView.backgroundColor = [UIColor redColor]; 3 redView.tag = 1000; 4 5 UIView *yellowView = [[UIView alloc]initWithFrame:CGRectMake(0, CGRectGetHeight(self.window.frame)/2, CGRectGetWidth(self.window.frame), CGRectGetHeight(self.window.frame)/2)]; 6 yellowView.backgroundColor = [UIColor yellowColor]; 7 yellowView.tag = 1001; 8 9 UIView *blueView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 10 blueView.backgroundColor = [UIColor blueColor]; 11 blueView.tag = 1002; 12 13 UIView *greenView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)]; 14 greenView.backgroundColor = [UIColor greenColor]; 15 greenView.tag = 1000; 16 17 [redView addSubview:blueView]; 18 [yellowView addSubview:greenView]; 19 20 21 [self.window addSubview:yellowView]; 22 [self.window addSubview:redView]; 23 24 UIView *getView = [self.window viewWithTag:1000]; 25 [getView setBackgroundColor:[UIColor whiteColor]];
结果是greenView的背景色被改变了。
由以上的代码可以推出，父视图通过tag取得子视图的顺序是深度优先，也就是先查看自己的第一个子视图，然后查看第一个子视图的所有子视图的
tag值，直到第一个子视图下的所有子视图搜索完，再搜索自己第二个子视图，直到找到为止。找不到就返回nil。
当然，最稳妥的方法还是确保tag值的唯一。
原文：http://www.cnblogs.com/ieso-ios/p/5114416.html