隐藏在Swift TextField中的部分文本

Question

我使用（555）555-5555这样的遮罩，但我也希望它看起来像（555）5 **-****。输入文本时，我该怎么做才能掩盖用户？

掩码函数：

func applyPatternOnNumbers(pattern: String, replacmentCharacter: Character) -> String {
    var pureNumber = self.replacingOccurrences( of: "[^0-9]", with: "", options: .regularExpression)
    for index in 0 ..< pattern.count {
        guard index < pureNumber.count else { return pureNumber }
        let stringIndex = String.Index(encodedOffset: index)
        let patternCharacter = pattern[stringIndex]
        guard patternCharacter != replacmentCharacter else { continue }
        pureNumber.insert(patternCharacter, at: stringIndex)
    }
    return pureNumber
}

用法：

sender.text = textString.applyPatternOnNumbers(pattern: "(###) ###-####", replacmentCharacter: "#")