Best multithreading approach in Objective C?

Question

I'm developing an iPad-app and I'm currently struggling with finding the best approach to multithreading. Let me illustrate this with a simplified example:
I have a view with 2 subviews, a directory picker and a gallery with thumbnails of all the images in the selected directory. Since 'downloading' and generating these thumbnails can take quite a while I need multithreading so the interaction and updating of the view doesn't get blocked.

This is what I already tried:
[self performSelectorInBackground:@selector(displayThumbnails:) withObject:currentFolder];
This worked fine because the users interactions didn't get blocked, however it miserably fails when the user taps on another folder while the first folder is still loading. Two threads are trying to access the same view and variables which results in messing up each others proper execution. When the users taps another folder, the displayThumbnails of the currently loading folder should get aborted. I didn't find any way to do this..

NSThreads
I tried this but struggled with almost the same problems as with the first method, I didn't find a (easy) way to cancel the ongoing method. (Yes, I know about [aThread cancel] but didn't find a way to 'resume' the thread). Maybe I should subclass NSThread and implement my own isRunning etc methods? But isn't there any better way or a third (or even fourth and fifth) option I'm overlooking?

I think this is a fairly simple example and I think there is perhaps a better solution without subclassing NSThread. So, what would you do? Your opinions please!

Answer 1

NSOperationQueue should work well for this task.

Another option would be plain GCD, however, if you've never worked with it, NSOperationQueue is probably the better choice since it pretty much automatically guides you to implementing things "the right way", has obvious ways for cancellation, etc.

Answer 2

You want to use Concurrent NSOperations to download and process images in the background. These would be managed by an NSOperationsQueue. Essentially these operations would be configured to fetch one image per operation, process it, save it in the file system, then message back to the main app in the main thread that the image is available.

There are several projects on github that you can look at that show how to do this - just search github using "Concurrent" or "NSOperation".

iOS has a really nice facility for doing background work. Grand Central Dispatch (GCD) and Blocks, but those don't let you have an object using delegate callbacks - thus NSOperation.

So you need to read up on blocks, GCD, and then look at some open source Concurrent NSOperations code. Using Concurrent NSOperations is not as simple as using blocks.