Objective C - Single Background Thread

Question

I want to run a single background thread for an iPhone application which is available in background all the time and gets executed when specific event fires and go to wait for specific event to fire to start its execution again. During the execution of thread if specific event is fired again then thread should restart its work.

I am working on a custom map application. On TouchesMoved event, I need to load the map image tiles according to the positions moved in a background thread. The problem is that when I move the map with speed the touchesMoved event is fired the previous thread has not finished its work and new thread is started. It causes thread safety issue and my application is crashed.

So I am thinking of a solution to have a single thread all the time available and starts its work when touchesMoved is fired if touchesMoved is fired again it should restart its work instead of starting a new thread. I think it will prevent the thread safety issue.

Please help

Answer 1

Put a run loop in your background compute thread. Then use an NSOperation queue to manage sending messages to it. The queue plus the run loop will serialize all the work requests for you.

Answer 2

This sounds like an ideal job for an NSOperationQueue. Have a read of the operation queue section of the concurrency guide.

Essentially, you create an NSOperation object for each map tile load and place them on a queue that only allows them to execute one at a time.

Answer 3

Firstly I'd echo the use of NSOperation and NSOperationQueue. You could fall-back to using NSThread directly, but the point of NSOperation is that it hides threading from you, leaving you to concentrate on the processing you need to do. Try firing NSOperation requests as and when required, see what the performance is like in your use-case; even if these operations get data in an async manner, it should provide you with a cleaner solution with good performance, plus future proof.

I've successfully used NSInvocationOperation to fire requests as often as required, and it sounds like the sort-of requirements and behaviour you're after. I would suggest more generally that you experiment with these in a test project; here you can test performance.

The following weblog's helped me start playing with NSOperation:

http://www.dribin.org/dave/blog/archives/2009/09/13/snowy_concurrent_operations/

http://www.cimgf.com/2008/02/16/cocoa-tutorial-nsoperation-and-nsoperationqueue/

As always, the Apple Threading Programming Guide is a key read, to figure out which way to go depending on needs.