Adding Mask to a TextField | Swift 4, GCD | + Static Prefix if need.

UITextField+Mask.swift

/// Usage: phoneTextField.formatPattern(pattern: "(***) ***-**-**", replacementChar: "*", staticPrefixString: "+7 ", allowText: false, allowNumbers: true)

extension UITextField {
    
    /// Структура с перечислениями не хранимых полей
    private struct FieldMaskProperties {
        
        static var pattern            = "pattern"
        static var replacementChar    = "replacementChar"
        static var staticPrefixString = "staticPrefixString"
        static var allowNumbers       = "allowNumbers"
        static var allowText          = "allowText"
    }
    
    /// Pattern you want to use Example: (***)**-**-**
    private var pattern: String {
        if let result = objc_getAssociatedObject(self, &FieldMaskProperties.pattern) as? String {
            return result
        }
        return ""
    }
    
    /// Symbol to change in pattern (ex: "*" or "X")
    private var replacementChar: String {
        if let result = objc_getAssociatedObject(self, &FieldMaskProperties.replacementChar) as? String {
            return result
        }
        return ""
    }
    
    /// Is text allowed or not 
    private var allowText: Bool {
        if let result = objc_getAssociatedObject(self, &FieldMaskProperties.allowText) as? Bool {
            return result
        }
        return true
    }
    
    /// Is numbers allowed or not
    private var allowNumbers: Bool {
        if let result = objc_getAssociatedObject(self, &FieldMaskProperties.allowNumbers) as? Bool {
            return result
        }
        
        return true
    }
    
    /// Unchangable prefix in pattern (прим.: "+0")
    private var staticPrefixString: String {
        if let result = objc_getAssociatedObject(self, &FieldMaskProperties.staticPrefixString) as? String {
            return result
        }
        return ""
    }
    
    /// Create pattern format for text
    ///
    /// - Warning: Use deinit for stop receiving notifications from field 
    func formatPattern(pattern: String? = nil, replacementChar: String? = nil, staticPrefixString: String? = nil, allowText: Bool = true, allowNumbers: Bool = true)
    {
        objc_setAssociatedObject(self, &FieldMaskProperties.pattern,            pattern ?? "",           .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FieldMaskProperties.replacementChar,    replacementChar ?? "*",  .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FieldMaskProperties.staticPrefixString,  staticPrefixString ?? "", .OBJC_ASSOCIATION_RETAIN)
        
        objc_setAssociatedObject(self, &FieldMaskProperties.allowNumbers,       allowNumbers,    .OBJC_ASSOCIATION_RETAIN)
        objc_setAssociatedObject(self, &FieldMaskProperties.allowText,          allowText,       .OBJC_ASSOCIATION_RETAIN)
        
        registerNotifications()
    }
    
    private func registerNotifications() {
        NotificationCenter.default.addObserver(self, selector: #selector(textDidChange), name: NSNotification.Name(rawValue: "UITextFieldTextDidChangeNotification"), object: self)
    }
    
    func removeNotifications() {
        NotificationCenter.default.removeObserver(self)
    }
    
    private func prepareString(_ string: String) -> String {
        var charSet: CharacterSet!
        
        if allowText && allowNumbers {
            charSet = CharacterSet.alphanumerics.inverted
        }
        else if allowText {
            charSet = CharacterSet.letters.inverted
        }
        else if allowNumbers {
            charSet = CharacterSet.decimalDigits.inverted
        }
        
        var tempString = string
        
        if !staticPrefixString.isEmpty {
            tempString = String(tempString[staticPrefixString.endIndex...])
        }
        
        let result = tempString.components(separatedBy: charSet)
        
        return staticPrefixString + result.joined(separator: "")
    }
    
    @objc func textDidChange() {
        
        guard let text = self.text else {
            return
        }
        
        guard !text.isEmpty, !pattern.isEmpty else { return }
        
        var finalText   = ""
        var stop        = false
        let tempString  = prepareString(text)
        
        var formatIndex = pattern.startIndex
        var tempIndex   = staticPrefixString.isEmpty ? tempString.startIndex : staticPrefixString.endIndex
        
        DispatchQueue.global(qos: .userInteractive).async { [weak self] in
            
            guard let strongSelf = self else { return }
            
            while !stop
            {
                let formattingPatternRange = formatIndex ..< strongSelf.pattern.index(formatIndex, offsetBy: 1)
                
                if strongSelf.pattern[formattingPatternRange.lowerBound..<formattingPatternRange.upperBound] != String(strongSelf.replacementChar) {
                    
                    finalText = finalText.appending(strongSelf.pattern[formattingPatternRange.lowerBound..<formattingPatternRange.upperBound])
                    
                } else if tempString.count > 0 {
                    
                    let pureStringRange = tempIndex ..< tempString.index(tempIndex, offsetBy: 1)
                    finalText = finalText.appending(tempString[pureStringRange.lowerBound..<pureStringRange.upperBound])
                    tempIndex = tempString.index(tempIndex, offsetBy: 1)
                }
                
                formatIndex = strongSelf.pattern.index(formatIndex, offsetBy: 1)
                
                if formatIndex >= strongSelf.pattern.endIndex || tempIndex >= tempString.endIndex {
                    stop = true
                }
                
                DispatchQueue.main.async { [weak self] in
                    self?.text = finalText
                }
            }
        }
    }
}
}
}
}