如何在Swift中自定义设置安全文本字段点

class SecureTextField: UITextField, UITextFieldDelegate {
    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }
    
    private func setup() {
        delegate = self
        isSecureTextEntry = true
        addTarget(self, action: #selector(textDidChange), for: .editingChanged)
    }
    
    @objc private func textDidChange() {
        guard let secureText = text else { return }
        let secureTextWithDots = String(repeating: "•", count: secureText.count)
        text = secureTextWithDots
    }
    
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        // 允许删除字符
        if string.isEmpty {
            return true
        }
        
        // 不允许输入其他字符
        return false
    }
}

let secureTextField = SecureTextField(frame: CGRect(x: 0, y: 0, width: 200, height: 40))
view.addSubview(secureTextField)