Typewriting effect in swift

TypingLabel.swift

//
//  TypingLabel.swift
//
//  Created by Samuel Beek on 09/12/15.
//  Copyright © 2015 Samuel Beek. All rights reserved.
//

import UIKit
import SwiftyTimer

class TypingLabel : UILabel {
    
    var timer : NSTimer?
    var interval = 0.05
    var waitInterval = 1.3
    
    func sequence(strings: [String]) {
        for (index, string) in strings.enumerate() {
            NSTimer.after(waitForIndex(index, strings: strings),  {
                self.addAndRemoveText(string)
            })
        }
    }
    
    func loop(strings: [String]) {
        sequence(strings)
        
        let wait = waitForIndex(strings.count, strings: strings)

        NSTimer.after(wait, {
            self.loop(strings)
        })
        
    }
    
    func waitForIndex(index: Int, strings: [String]) -> NSTimeInterval {
        var wait = 0.0
        
        for (i, string) in strings.enumerate() {
            if i < index {
                wait = wait + timeFor(text: string) * 2
            }
        }
        
        return wait
    }
    
    func addAndRemoveText(string: String) {
        
        addText(string)
        NSTimer.after(timeFor(text: string), {
            self.emptyText()
        })
    }
    
    func timeFor(text text: String) -> NSTimeInterval {
        return (Double(text.characters.count)*interval) + interval + waitInterval
    }
    
    func emptyText() {
        timer = NSTimer.after(interval, {
            self.removeLetter()
        })
    }
    
    func removeLetter() {
        if let text = self.text where text.characters.count > 0 {
            self.text = String(text.characters.dropLast())
            timer?.invalidate()
            timer = NSTimer.after(interval, {
                self.removeLetter()
            })
            
        }
    }
    
    func addText(text: String) {
        
        if text.characters.count > 0 {
            timer = NSTimer.after(interval, {
                self.addLetter(text[text.startIndex])
                self.timer?.invalidate()
                self.addText(String(text.characters.dropFirst()))
            })
        }
    }
    
    func addLetter(letter: Character) {
        self.text?.append(letter)
    }
    
    
}