Question

I’m working on an an app that updates both a UITableView and a UIScrollView. I like how UITableView updating works. As I add, remove, or update items, I call insertRowsAtIndexPaths:withRowAnimation:, reloadRowsAtIndexPaths:withRowAnimation:, and deleteRowsAtIndexPaths:withRowAnimation: as appropriate. However, updating the UIScrollView has turned out to be more of a challenge.

So how can I do it? My UIScrollView contains images, and I want to be able to insert, update, and remove individual images — with animation — efficiently and smoothly, and perhaps in random order. How does one do this sort of thing?

As an example, I have code like this in my UIScrollViewDelegate implementation for adding a new image:

if (newImageindex == currentImageIndex) {
    [scrollView setNeedsDisplay];
    [scrollView setContentOffset:portalView.contentOffset animated:YES];
} else if (newImageIndex < currentImageIndex) {
    currentImageIndex++;
    CGPoint offset = portalView.contentOffset;
    offset.x += portalView.frame.size.width;
    portalView.contentOffset = offset;
}

This is close, I think, but not quite right. I end up with the images added, but the UIScrollView seems to scroll to a position before the first image in the view. If I start scrolling it by hand, the first image appears. It’s like it’s scrolled to position -1 in my images.

The example may not help to highlight my problem much, but surely it’s a common need to dynamically rejigger the images appearing in a UIScrollView. What’s the state of the art on this?

Answer 1

A UIScrollView is not so specialized and structured as a UITableView. The former is a generic view that scrolls and zooms anything you put in it, and you can put any subviews into it anywhere, whereas the latter is made especially to display lists of cells stacked on top of each other.

So the answer is: you need to animate the subviews in the scrollview yourself. There are various libraries like Three20 which provide frameworks for creating more advanced views, you’ll need to use a suitable component from one of those or roll your own.