The Concurrency Programming Guide by Apple is a nice reading. Concurrent programming is not something you might want to pick up by copying some sample code from the web and hacking until you are happy. It’s good to know the options and principles to save yourself from trouble.

Revisiting the answer after some time, nowadays you almost can’t go wrong using Grand Central Dispatch. Running a task in background looks like this:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{    [self doSomeLongTask]; // 1    dispatch_async(dispatch_get_main_queue(), ^{        [self longTaskDidFinish]; // 2    });});

The long task (1) will run on some background thread and there’s no catch that I am aware of, ie. there’s already an autorelease pool in that thread, you don’t have to care about run loops etc. After the task finishes the code calls -longTaskDidFinish on the main thread (2), so that you can update UI or whatever else. This is an often used idiom.

Maybe the best thing to do is this tutorial from Apple. I read it carefully (10-20 minutes) and “threaded” all my application! Excellent!

Swift 3

DispatchQueue.global(qos: .userInteractive).async {    // Code to run on background thread    // Switch to the main UI thread to display any results needed    DispatchQueue.main.async {        // Run code on main UI thread here    }}

The qos parameter stands for "Quality of Service". Think of it like a priority to give your background thread:

• .userInteractive (highest priority)
• .userInitiated (when you can spare a few seconds)
• .utility (when you can spare a few seconds to few minutes)
• .background (lowest priority - minutes/hours to spare)