Grand Central Dispatch

Grand Central Dispatch (GCD) is a powerful multitasking technology in iOS. I don't pretend to know all the ins and outs about GCD, and haven't really use it much. Mostly, I tend to use it for just one thing: to prevent the main thread from blocking due to time consuming activities, such as downloading things from the network or serious image processing.

The code typically looks something like:
dispatch_queue_t nameOfQueue = dispatch_queue_create("SOME_NAME", NULL);
    dispatch_async(nameOfQueue, ^{
        /* codes to be performed in the other queue */
        /* eg. downloading something  */
        dispatch_async(dispatch_get_main_queue(), ^{
            /* codes to be performed in main thread, usually UIKit stuff */
            /* e.g. update the UI based on the data you have downloaded */
        });
    });

Note:
- the NULL in dispatch_queue_create means this is a serial queue, as opposed to a concurrent queue
- dispatch_get_main_queue() is used to get the main queue
- dispatch_get_current_queue() is used to get the current queue

It is important to dispatch back to the main queue if you want to run UIKit stuff as they are mostly not thread safe. This is what the second dispatch_async() is doing above. A few exceptions are: UIImage, UIFont, UIBezierPath, drawing things using Core Graphics. (Not sure if there are others.)

See the Apple Documentation.