Formatter validation in Swift for Cocoa

ViewController.swift

//
//  ViewController.swift
//
//  Created by Robert Ryan on 7/4/16.
//  Copyright © 2016 Robert Ryan. All rights reserved.
//

import Cocoa

class TwoDigitFormatter: NumberFormatter {
    
    let maxLength = 2
    let wrongCharacterSet = CharacterSet(charactersIn: "0123456789").inverted
    
    override func isPartialStringValid(_ partialString: String, newEditingString newString: AutoreleasingUnsafeMutablePointer<NSString?>?, errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool {
        if partialString.characters.count > maxLength {
            return false
        }
        
        if partialString.rangeOfCharacter(from: wrongCharacterSet) != nil {
            return false
        }
        
        return true
    }
    
    override func attributedString(for obj: AnyObject, withDefaultAttributes attrs: [String : AnyObject]? = [:]) -> AttributedString? {
        let stringVal = string(for: obj)
        
        guard let string = stringVal else { return nil }
        
        return AttributedString(string: string, attributes: attributes)
    }
    
    var attributes: [String: AnyObject] {
        let style = NSMutableParagraphStyle()
        style.minimumLineHeight = 100
        style.maximumLineHeight = 100
        style.paragraphSpacingBefore = 0
        style.paragraphSpacing = 0
        style.alignment = .center
        style.lineHeightMultiple = 1.0
        style.lineBreakMode = .byTruncatingTail
        let droidSansMono = NSFont(name: "DroidSansMono", size: 70)!
        return [NSParagraphStyleAttributeName: style, NSFontAttributeName: droidSansMono, NSBaselineOffsetAttributeName: -60]
    }
}

class ViewController: NSViewController, NSTextFieldDelegate {
    
    @IBOutlet weak var textField: NSTextField!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        textField.formatter = TwoDigitFormatter()
    }
    
}

Re http://stackoverflow.com/questions/38190958/how-to-use-a-nsnumberformatter-with-an-attributedstring