*Note: This is a re-print from the Mac Developer Network.*
I tend to frequent the StackOverflow website and watch for questions about Core Data. Generally if it is a question I can answer I will and if it is a question I can’t answer then I really dig in for a fun session of exploration and attempt to find the answer.
I know I mentioned we would talk about customizing the fetch requests, however, I have been working on some code related to the Active Record Fetching project, which I am renaming to MagicalRecord, that is also just as useful as fetching–threading.
Whenever most cocoa developers mention threading and Core Data in the same sentence, the reaction I see most often is that of mysticism and disbelief. For one, multithreaded programming in general is hard–hard to design correctly, hard to write correctly, and debugging threads is just asking for trouble. Introducing Core Data into that mix can seem like the straw that broke the camel’s back. However, by following a few simple rules and guidelines, and codifying them into a super simple pattern, one that may be familiar to you, we can achieve safer Core Data threading without the common headaches.
In my first post I wrote about using NSOperation to grab an image of the current frame of a QuickTime movie while it was playing and save it out to the filesystem. Considering the excitement that is surrounding Core Animation these days, I thought it might be interesting to take that project and change it to instead grab the current frame and animate it across the screen using a Core Animation layer (CALayer). I had to employ a few tricks to get it to work, but the end result, while completely useless, is quite excellent.
Forget Mandelbrot sets (Apple coding headstarts) and Expression Trees. NSOperation is really not that hard.
In his post, Marcus introduced how to use NSOperation to greatly simplify multi-threading tasks in your application. I am now going to provide a step-by-step walk-through sample application that uses NSOperation, NSOperationQueue, and QTKit.
While looking around the Internet, I noticed that the only examples of using NSOperation available were related to scientific applications. I wanted something that I could relate to a little better and since I’ve been working with QTKit a lot lately, I figured it would be a good framework to build from. This application simply grabs images of a movie while it is playing back and saves them out to a file. It’s pretty simple, but it shows how to do something fairly practical.
Threading is hard in any language. And what is worse, when it goes wrong, it usually goes wrong in a very bad way. Because of this, programmers either avoid threading completely (and refer to it as the spawn of the devil) or spend a LOT of time making sure that everything is perfect.
Fortunately, Apple has made a lot of progress in OS X 10.5 Leopard. NSThread itself has received a number of very useful new methods that make threading easier. In addition, they have introduced two new objects: NSOperation and NSOperationQueue.