ios - 当 View 出现时创建一个 UILabel 并在 View 消失时销毁它

Question

对于我的应用程序的教程，我正在尝试创建一个 UILabel，它会在 View 出现时在显示的屏幕上漂移，然后被销毁，这样如果用户在教程期间返回到该 View ，就会创建 UILabel一次又一次地飘过页面。这是我使用 UIPageViewController 显示的自定义 View Controller 之一的示例:

//this is just a custom UILabel with some padding
@property (nonatomic) PaddedLabel *directionsLabel;

//I have tried setting UILabel to nil or removing it from view
-(void)viewWillDisappear:(BOOL)animated
{

    NSLog(@"view is disappearing");
    //this does not remove the label
    self.directionsLabel = nil;
    //nor does this
    [self.directionsLabel removeFromSuperview];

}

- (void)viewDidAppear:(BOOL)animated
{
    [self messageTutorial];
}

- (void)messageTutorial
{

    CGFloat width = [[UIScreen mainScreen] bounds].size.width;
    CGFloat height = [[UIScreen mainScreen] bounds].size.height;
    PaddedLabel *directionsLabel = [[PaddedLabel alloc] initWithFrame:CGRectMake(_width/2, _height/2, 100, 100)];
    directionsLabel.text = @"test";

    CGRect f = directionsLabel.frame;
    f.size = size;
    directionsLabel.frame = f;
    directionsLabel.center = self.view.center;
    f = directionsLabel.frame;
    f = directionsLabel.frame;
    f.origin.y = .1*height;
    directionsLabel.frame = f;
    [self.view addSubview:directionsLabel];


   [UIView animateWithDuration:TUTORIAL_DISAPPEAR_TIME animations:^{
        directionsLabel.alpha = .5;
       CGRect f = directionsLabel.frame;
       f.origin.y = height - f.size.height*1.4;
       directionsLabel.frame = f;
       NSLog(@"animating");

    } completion:^(BOOL finished) {
        [directionsLabel removeFromSuperview];
        //this also doesn't actually remove the label

    }];


}

问题是，如果用户返回查看此 View ​​，她现在会看到一个新标签和一个旧标签，因此，如果您来回翻页，您最终会看到很多标签都说同样的事情，在屏幕上的不同进展阶段。

如何在 View 消失时删除 UILabel 并在 View 出现/重新出现时添加/创建新的？

谢谢。

Answer 1

viewWillDisappear 方法中的代码是倒退的。你需要:

- (void)viewWillDisappear:(BOOL)animated {
    NSLog(@"view is disappearing");
    [self.directionsLabel removeFromSuperview];
    self.directionsLabel = nil;
}

如您所见，在尝试删除它之前将 self.directionsLabel 设置为 nil 会导致无操作。

此外，请务必在创建标签时设置 self.directionsLabel。