Letter by letter animation for UILabel?

Update for 2018, Swift 4.1:

extension UILabel {

    func animate(newText: String, characterDelay: TimeInterval) {

        DispatchQueue.main.async {

            self.text = ""

            for (index, character) in newText.enumerated() {
                DispatchQueue.main.asyncAfter(deadline: .now() + characterDelay * Double(index)) {
                    self.text?.append(character)
                }
            }
        }
    }

}

calling it is simple and thread safe:

myLabel.animate(newText: myLabel.text ?? "May the source be with you", characterDelay: 0.3)

@objC, 2012:

Try this prototype function:

- (void)animateLabelShowText:(NSString*)newText characterDelay:(NSTimeInterval)delay
{    
    [self.myLabel setText:@""];

    for (int i=0; i<newText.length; i++)
    {
        dispatch_async(dispatch_get_main_queue(),
        ^{
            [self.myLabel setText:[NSString stringWithFormat:@"%@%C", self.myLabel.text, [newText characterAtIndex:i]]];
        });

        [NSThread sleepForTimeInterval:delay];
    }
}

and call it in this fashion:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0),
^{
    [self animateLabelShowText:@"Hello Vignesh Kumar!" characterDelay:0.5];
});