How to implement UIImageView-like delayed loading in UITableView

Question

Hi all

Background: I have a UITableView showing an image in each cell. These images are all part of a big pdf-file. So what I am doing is actually rendering little pdf-parts in those UITableViewCells, each cell displaying just one piece. Therefore I add a UIView to the contentview of the cell and render the view on demand. Rendering these pdf-parts is expensive and each takes about 0.2 seconds (only drawing the Pdf-part), which slows the scrolling of the table terrible down.

Idea: Now, I know there may be a solution when I look how UIImage renders URL-based images. If you create a UIImage based on a url, these images are rendered somehow delayed. E.g have a look at the iTunes-App. Smooth scrolling is possible, each image is displayed unrendered and after rendering is finished it appears smoothly.

Problem: How can I render an expensive rendering in a UITableViewCell like described above, by somehow showing the cell delayed? Has anybody an idea, how Apple solved it within UIImageView?

Thanks in advance

Daniel

Answer 1

Hi,

this is (mostly) duplicate of http://stackoverflow.com/questions/531482/lazy-load-images-in-uitableviewcell. Check the links and examples there.

In short, what you need is spawning a separate thread for loading the PDF thumbnails, and that thread then signaling back to the main thread with the generated images. This is easily done by using performSelectorInBackground:withObject: and performSelectorOnMainThread:withObject:waitUntilDone: methods.