iOS，使用ib在界面上做了一个label和一个slider，然后代码写了一个button，slider的值显示在label上，button让slider向下移动。当slider移动之后，拖动slider的手柄，slider会回到初始的位置？why？

#import "ViewController.h" @interface ViewController () //@property (nonatomic, retain) UISlider *slider; @property (nonatomic, retain) UIButton *button; @property (weak, nonatomic) IBOutlet UILabel *label; @property (weak, nonatomic) IBOutlet UISlider *slider; @end @implementation ViewController - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. //设置一个button来移动slider _button = [[UIButton alloc] initWithFrame:CGRectMake(20, 500, 40, 40)]; _button.backgroundColor = [UIColor blackColor]; [self.view addSubview:_button]; [_button addTarget:self action:@selector(checkButton) forControlEvents:UIControlEventTouchUpInside]; } -(void)checkButton { CGRect rect = _slider.frame; rect.origin.y += 20; _slider.frame = rect; } - (IBAction)changValue:(id)sender { _label.text = \[NSString stringWithFormat:@"%f",_slider.value\]; } - (void)didReceiveMemoryWarning { \[super didReceiveMemoryWarning\]; // Dispose of any resources that can be recreated. } @end