Adding Mask to a TextField

UITextField+Mask.swift

extension UITextField {
    
    private struct FKFieldPatterns {
        static var pattern          = "pattern"
        static var replacementChar  = "replacementChar"
        static var allowNumbers     = "allowNumbers"
        static var allowText        = "allowText"
    }
    
    private var pattern : String {
        if let result = objc_getAssociatedObject(self, &FKFieldPatterns.pattern) as? String {
            return result
        }
        
        return ""
    }
    
    private var replacementChar : String {
        if let result = objc_getAssociatedObject(self, &FKFieldPatterns.replacementChar) as? String {
            return result
        }
        
        return ""
    }
    
    private var allowText : Bool {
        if let result = objc_getAssociatedObject(self, &FKFieldPatterns.allowText) as? Bool {
            return result
        }
        
        return true
    }
    
    private var allowNumbers : Bool {
        if let result = objc_getAssociatedObject(self, &FKFieldPatterns.allowNumbers) as? Bool {
            return result
        }
        
        return true
    }
    
    
    func formatPattern(pattern:String?=nil, replacementChar:String?=nil,  allowText:Bool=true, allowNumbers:Bool=true)
    {
        objc_setAssociatedObject(self, &FKFieldPatterns.pattern,            pattern ?? "",         .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FKFieldPatterns.replacementChar,    replacementChar ?? "*", .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FKFieldPatterns.allowNumbers,       allowNumbers,    .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FKFieldPatterns.allowText,          allowText,       .OBJC_ASSOCIATION_RETAIN)
        registerNotifications()
    }
    
    private func registerNotifications() {
        NotificationCenter.default.addObserver(self, selector: #selector(textDidChange), name: NSNotification.Name(rawValue: "UITextFieldTextDidChangeNotification"), object: self)
    }
    
    private func removeNotifications() {
        NotificationCenter.default.removeObserver(self)
    }
    
    func prepareString(string:String) -> String
    {
        var charSet : CharacterSet!
        
        if allowText && allowNumbers {
            charSet = CharacterSet.alphanumerics.inverted as CharacterSet
        }
        else if allowText {
            charSet = CharacterSet.letters.inverted as CharacterSet
        }
        else if allowNumbers {
            charSet = CharacterSet.decimalDigits.inverted as CharacterSet
        }
        
        let result = string.components(separatedBy: charSet)
        return result.joined(separator: "")
    }
    
    func textDidChange(notification : NSNotification) {
        
        guard let text = self.text else {
            return
        }
        
        if text.characters.count > 0 && pattern.characters.count > 0
        {
            
            var finalText   = ""
            var stop        = false
            let tempString  = prepareString(string: text)
            
            var formatIndex = pattern.startIndex
            var tempIndex   = tempString.startIndex
            
            while !stop
            {
                let formattingPatternRange = formatIndex ..< pattern.index(formatIndex, offsetBy: 1)
                
                if pattern.substring(with: formattingPatternRange) != String(replacementChar) {
                    
                    finalText = finalText.appending(pattern.substring(with: formattingPatternRange))
                    
                } else if tempString.characters.count > 0 {
                    
                    let pureStringRange = tempIndex ..< tempString.index(tempIndex, offsetBy: 1)
                    finalText = finalText.appending(tempString.substring(with: pureStringRange))
                    tempIndex = tempString.index(tempIndex, offsetBy: 1)
                }
                
                formatIndex = pattern.index(formatIndex, offsetBy: 1)
                
                if formatIndex >= pattern.endIndex || tempIndex >= tempString.endIndex {
                    stop = true
                }
                
                self.text = finalText
                
            }
        }
    }
}