gistfile1.swift

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
    // do some stuff in the background
    let string = "my string"

    // remove if you don't need to call the main queue
    dispatch_async(dispatch_get_main_queue(), {
        // do some stuff in the main queue
        // mostly call the UI
    })
    // end of remove
})