UIView animateWithDuration每隔一段时间使用.alpha参数

Question

我试图为我的一个观点设置.alpha参数的动画。下面是一段代码：

- (IBAction)closeImg:(id)sender
{
    [UIView animateWithDuration:0.25 animations:^ {

          imgViewList.alpha = 0.0;  

          [self.navigationController setToolbarHidden:NO animated:NO];

          self.navigationController.toolbar.barTintColor = nil;
          self.navigationController.toolbar.clipsToBounds = NO;

          NSMutableArray *toolbarButtons = [self.toolbarItems mutableCopy];

          [toolbarButtons removeObject:self.editButton];
          [toolbarButtons removeObject:self.spaceBar];
          [toolbarButtons removeObject:self.deleteImgButton];
          [toolbarButtons removeObject:self.closeButton];
          [toolbarButtons removeObject:self.shareBtn];
          [toolbarButtons removeObject:self.dismiss2];
          [toolbarButtons removeObject:self.upload2];

          [toolbarButtons addObject:self.backBtn];
          if (![[[NSUserDefaults standardUserDefaults] valueForKey:@"imageToDisplay"] isEqualToString:@"newsImg"]) {
              [toolbarButtons addObject:self.spaceBar];
              [toolbarButtons addObject:self.menuBtn];
          }

          [self setToolbarItems:toolbarButtons animated:YES];
          self.view.window.windowLevel = UIWindowLevelNormal;

          [imgUpload removeAllObjects];


          pgScroll.contentSize = CGSizeMake(pgScroll.frame.size.width, imgList.frame.origin.y+imgList.frame.size.height+20);
          pgScroll.contentInset = contentInsets;
          pgScroll.scrollIndicatorInsets = contentInsets;
    }
}

它每隔一段时间（每隔4或5次）就会起作用，但它并不适用。 imgViewList并没有消失。 偶尔会发生什么？因为它通常会一直有效，或者它没有。或者该代码的其他部分可能产生负面影响？