How to make UIScrollView load data as scrolling happens - ios6

Question

I have an IOS application designed for ipad. In one page, I display a report to the user and the report requires 5000 UILabels to be rendered on the screen inside a UIScrollView. This causes application to crash due to being out of memory. I know that UITableView has the functionality for loading content on demand. How can I make scroll view render only certain content and as the scrolling happens remove the content that got invisible and add the content that should be visible?

Answer 1

Is there any way you can reconfigure it to use a table view with custom rows, perhaps rows that consist of several labels? That model supports load on demand and resource reuse in a very natural way...
Perhaps you can be more specific on your screen layout?

Answer 2

You should re-cycle (i.e., reuse) the labels just like a table view recycles its table cells. This is sometimes referred to as "tiling" subviews. Tiling allows you to display more than will fit into memory.
I recommend watching WWDC 2012 Session 104. This session's tutorial creates a photo app that tiles image views in a UIScrollView. Although the photo app scrolls pages of content, rather than a grid of items, I think the video could be relevant to your app.
Here's a very brief overview of the tutorial applied to your specific case:
(1.) declare iVars that keep track of your labels:
NSMutableSet *recycledLabels;
NSMutableSet *visibleLabels;
(2.) implement a method that fetches a re-usable label:
- (UILabel *)dequeueRecycledLabel;
(3.) implement a method that does the tiling:
- (void)tileLabels; // this will add/remove labels from the scroll view
(4.) set your scroll view's delegate and call tileLabels in the scrollViewDidScroll: delegate method
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
[self tileLabels];
}
The WWDC video will help you fill in the details, which I think are applicable to your case. However, you will have to do plenty of improvising in order to make it work for your specific case. This may not be easy; but its doable.
At the end of the video, tiling with CATileLayer is discussed. I'll be honest, I didn't understand that part. But I don't think it's applicable in your case because you're not displaying large images.

Answer 3

I finally implemented dynamic content loading. The method I implemented is scrollViewDidScroll.
Inside this method I determined the direction of scroll by the following
bool isUp = (currentOffset.y > lastScrollOffset);
Then I determined the visible content rectangle by the following.
CGRect visibleContentRect;
visibleContentRect.origin = scrollView.contentOffset;
visibleContentRect.size = scrollView.bounds.size;
Then I had an array of View Elements and each knew its place in the scroll view because of their frame being set. Long story short, each time scroll happened, I determined the views whose frame either intersected or contained by visible content frame. I added those views to the scroll view. I also determined the ones that disappeared and removed them from scroll view and I also set those views to nil and recreated them. Once [scrollView addSubview:view] method is called, then view gets more space in the memory because it gets visible. [view removeFromSuperView] method doesn't deallocate that space. That's why setting the view to nil and recreating it is necessary.