UIScrollView setContentOffset will add a subview to the scrollview?

Question

I happened to find that setContentOffset to a UIScrollView will cause the scrollView append a new view to its subviews. here is the code:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake((320 - 261) / 2, 50, 261, 67)];
    scrollView.pagingEnabled = YES;
    scrollView.showsHorizontalScrollIndicator = NO;
    scrollView.bounces = NO;
    scrollView.backgroundColor = [UIColor darkGrayColor];
    for (int i = 0; i < 5; i ++) {
       UIView *view = [[UIView alloc] initWithFrame:CGRectMake(i * 87, 1, 87, 65)];
       view.tag = i;
       UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(10, 10, 50, 30)];
       label.backgroundColor = [UIColor clearColor];
       label.textColor = [UIColor whiteColor];
       label.text = [NSString stringWithFormat:@"%d", i];
       [view addSubview:label];
       view.backgroundColor = [UIColor greenColor];
       [scrollView addSubview:view];
   }
   scrollView.contentSize = CGSizeMake(87 * 5, 67);
   NSLog(@"before scroll: count of subview is %d", scrollView.subviews.count);
   CGPoint offset = CGPointMake(87, 0);
   [scrollView setContentOffset:offset animated:YES];
   NSLog(@"after scroll: count of subview is %d", scrollView.subviews.count);
   [self.view addSubview:scrollView];
}

Before calling setContentOffset:offset , the number of subviews of the scrollView is 5. This is what I expect. After that, the number turns to be 6. Is it working as designed? How to avoid the new subview appended?

Answer 1

The extra subview is the scroll indicator. If you check again in a later method, it will have gone away. Don't worry.

You can confirm this by hiding the scroll indicators ( showsVerticalScrollIndicator and showsHorizontalScrollIndicator properties).

Don't try to assume things about the view hierarchy of UIKit classes. UIKit can, and does, add its own views to several things - see tableviews and their cells, a navigation controller's view and so on.