1.设置某view到最上层
// 初始化第一个view并添加到当前控制器的view上；
UIView *first = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];
first.backgroundColor = [UIColor redColor];
[self.view addSubview:first];
// 初始化第二个view并添加到当前控制器的view上；
UIView *second = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 100, 100)];
second.backgroundColor = [UIColor greenColor];
[self.view addSubview:second];
// 设置第一个view到最上层
[self.view bringSubviewToFront:first];
2.设置某view到最下层
// 初始化第一个view并添加到当前控制器的view上；
UIView *first = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];
first.backgroundColor = [UIColor redColor];
[self.view addSubview:first];
// 初始化第二个view并添加到当前控制器的view上；
UIView *second = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 100, 100)];
second.backgroundColor = [UIColor greenColor];
[self.view addSubview:second];
// 初始化第三个view并添加到当前控制器的view上；
UIView *third = [[UIView alloc] initWithFrame:CGRectMake(70, 70, 100, 100)];
third.backgroundColor = [UIColor yellowColor];
[self.view addSubview:third];
[self.view sendSubviewToBack:second];
// 设置第二个view到最下层
[self.view sendSubviewToBack:second];
3.设置某view到指定层
// 初始化第一个view并添加到当前控制器的view上；
UIView *first = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];
first.backgroundColor = [UIColor redColor];
[self.view addSubview:first];
// 初始化第二个view并添加到当前控制器的view上；
UIView *second = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 100, 100)];
second.backgroundColor = [UIColor greenColor];
[self.view addSubview:second];
// 初始化第三个view并添加到当前控制器的view上；
UIView *third = [[UIView alloc] initWithFrame:CGRectMake(30, 70, 100, 100)];
third.backgroundColor = [UIColor yellowColor];
[self.view addSubview:third];
// 设置第一个view在第一层；第二个在第三层；第三个在第四层；第四个在第二层
first.layer.zPosition = 1; // red
second.layer.zPosition = 3; // green
third.layer.zPosition = 2; // hello
