iOS里的视图其实可以理解为ppt或photoshop里的图层，可以通过设置为最顶层或最底层来达到想要的效果
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
UIView *view1 = [[UIView alloc] initWithFrame:CGRectMake(10, 50, 100, 50)];
view1.backgroundColor = [UIColor blueColor];
[self.view addSubview:view1];
UIView *view2 = [[UIView alloc] initWithFrame:CGRectMake(15, 55, 100, 50)];
view2.backgroundColor = [UIColor grayColor];
[self.view addSubview:view2];
//如果将下面两行代码都注释掉 view1 会在下面 view2会在上面
// 下面这行代码能够将view2 调整到父视图的最下面
// [self.view sendSubviewToBack:view2];
//将view调整到父视图的最上面
[self.view bringSubviewToFront:view1];
} 