IOS之UIView的tag學習

tag是UIView的一個屬性，而且要求tag值唯一。父視圖可以通過tag來找到一個子視圖

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];

下麵來看下幾種需要註意的錯誤示例

由於示例代碼有點多，怕麻煩的童鞋可以跳到最後的結論部分(最好把錯誤示例4的代碼看一下)

一，view下有tag值相同的兩個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]];

結果是yellowView的背景被改變了，說明這種情況下會取得父視圖載入的第一個tag是1000的子視圖。

二，父視圖取得子視圖的子視圖。

 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]];

結果是greenView背景被改變了，說明這個是可行的。

三，取存在但不是自己子視圖的視圖

將示例2中的倒數第二句代碼改為   

UIView *getView = [redView viewWithTag:1003];

結果是greenView的背景沒有被改變，說明這是行不通的。

四，別的視圖中的子視圖和本視圖的子視圖有相同的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]];

結果來看還可以

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]];

結果是greenView的背景色被改變了。

最後說一下結論

由以上的代碼可以推出，父視圖通過tag取得子視圖的順序是深度優先，也就是先查看自己的第一個子視圖，然後查看第一個子視圖的所有子視圖的

tag值，直到第一個子視圖下的所有子視圖搜索完，再搜索自己第二個子視圖，直到找到為止。找不到就返回nil。

當然，最穩妥的方法還是確保tag值的唯一。