Question

I have an app based on Apple’s PageControl sample. The first time the view loads, the scroll view is loaded with page 0 and page 1. Whenever a scroll is initiated, the scrollViewDidScroll method should get called by UIKit correct?

When initiating a scroll from page 0 to page 1, the app should load page-1, page and page+1, (to prevent flashes during scrolling).

My app seems to call scrollViewDidScroll 19 times and my loadScrollViewWithPage: method 19 times each with page 0 and page 1, before it finally gets to page 1 and 2, then it crashes.

These are the methods:

- (void)scrollViewDidScroll:(UIScrollView *)sender {
    NSLog(@"scrollviewdidscroll");
   // We don't want a "feedback loop" between the UIPageControl and the scroll delegate in
   // which a scroll event generated from the user hitting the page control triggers updates from
   // the delegate method. We use a boolean to disable the delegate logic when the page control is used.
   if (pageControlUsed) {
        // do nothing - the scroll was initiated from the page control, not the user dragging
       return;
   }

   // Switch the indicator when more than 50% of the previous/next page is visible
   CGFloat pageWidth = scrollView.frame.size.width;
   int page = floor((scrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
   pageControl.currentPage = page;

   // load the visible page and the page on either side of it (to avoid flashes when the user starts scrolling)
   [self loadScrollViewWithPage:page - 1];
   [self loadScrollViewWithPage:page];
   [self loadScrollViewWithPage:page + 1];

   // A possible optimization would be to unload the views+controllers which are no longer visible
}

- (void)loadScrollViewWithPage:(int)page {
    if (page < 0) return;
    if (page >= kNumberOfPages) return;

    NSLog(@"page: %i", page);

    // replace the placeholder if necessary
    KeyboardViewController *controller = [viewControllers objectAtIndex:page];
    if ((NSNull *)controller == [NSNull null]) {
        controller = [[KeyboardViewController alloc] initWithPageNumber:page];
        [viewControllers replaceObjectAtIndex:page withObject:controller];
        [controller release];
    }

    // add the controller's view to the scroll view
    CGRect frame = scrollView.frame;
    frame.origin.x = frame.size.width * page;
    frame.origin.y = 0;
    frame.size.height = scrollView.frame.size.height;
    controller.view.frame = frame;
    [scrollView setAutoresizesSubviews:YES];
    [scrollView setAutoresizingMask:UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight];
    [scrollView addSubview:controller.view];
}

Why would scrollViewDidScroll get called so many times?

Thanks

Answer 1

scrollViewDidScroll: gets called every time the scroll bounds change. This means it gets called during the scroll, as well as when it starts. You may want to try scrollViewWillBeginDragging: instead.