• (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];
  }