Question

I’m developing on the iPhone and I have seen in the AppStore that the images are progressively downloaded in the UITableView… How can I implement that?
My idea was to detect the showed content (which cells are shown) and download these images. My problem is that I don’t know how to detect the showed cells!
Is it the good way to do it?

Best

Answer 1

The way this was demonstrated at the iPhone Developer Tech Talk was to use NSOperation and NSOperationQueue.

The idea is to wrap up your image download request (using NSURLRequest) into an NSOperation.

You can set your cell as the receiver of a call back sent by your operation when it’s complete so that you can attach the image to the cell (draw it manually, or add it to an image view).

Then basically, in your cellForRowAtIndexPath method, tell the cell to start the image download, and have the cell create an NSOperation and add it to the operation queue managed by your table view controller or something.

The queue will start executing the operations and call back to each cell when they’re done.

If you want to, you can cancel the operation if the cell moves offscreen, so you don’t waste resources downloading an image that will be thrown away because the cell won’t be visible to display the image.