如果多个视图添加到同一个父视图上，公共部分，后添加的会覆盖先添加的
//创建红色视图
UIView * redView = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 100, 100)];
redView.backgroundColor = [UIColor redColor];
redView.tag = 10;
[self.window addSubview:redView];
//创建黄色视图
UIView * yellowView = [[UIView alloc] initWithFrame:CGRectMake(80, 80, 100, 100)];
yellowView.backgroundColor = [UIColor yellowColor];
yellowView.tag = 20;
[self.window addSubview:yellowView];
//创建绿色视图
UIView * greenView = [[UIView alloc] initWithFrame:CGRectMake(110, 110, 100, 100)];
greenView.backgroundColor = [UIColor greenColor];
greenView.tag = 30;
[self.window addSubview:greenView];
1.将一个视图的层次设置成最上面
- (void)bringSubviewToFront:(UIView *)view;
[_window bringSubviewToFront:redView];
2.将某个视图放到最下面
- (void)sendSubviewToBack:(UIView *)view;
[_window sendSubviewToBack:yellowView];
3.插入视图(如果被插入的视图已经显示在界面上，那么通过插入方法只是改变它的层次关系；如果被插入的视图没有添加到界面上，通过插入方法可以将视图添加到界面上，并且改变层次关系)
- 将某个视图插入到指定的视图上面
- (void)insertSubview:(UIView *)view aboveSubview:(UIView *)siblingSubview;
[_window insertSubview:greenView aboveSubview:redView];
- 将某个视图插入到指定视图的下面
- (void)insertSubview:(UIView *)view belowSubview:(UIView *)siblingSubview;
[_window insertSubview:greenView belowSubview:yellowView];
