Respond to a User Changing the Text Size in System Preferences

ViewController.swift

override func viewDidLoad() {
    super.viewDidLoad()
        
    NSNotificationCenter.defaultCenter().addObserver(self, selector: "preferredContentSizeChanged", name: UIContentSizeCategoryDidChangeNotification, object: nil)
}

func preferredContentSizeChanged () {
    NSLog("User wants the Font to Change")
        
    label.font = UIFont.preferredCustomFontForTextStyle(UIFontTextStyleBody)
}