附加属性

WPF的 PasswordBox 的 Password 属性不是依赖属性，不能绑定。为了实现这个控件的MVVM模式，可以利用附加属性实现 PasswordBox 控件的绑定使用。其主要的思想是，设计一个PasswordBoxHelper 类型，并在其中定义一个附加属性，这个属性的名称也叫 Password，将来作为 PasswordBox 控件的附加属性。

    /// <summary>
    /// PasswordBox扩展属性
    /// </summary>
    public class PasswordBoxHelper
    {
        public static string GetPwd(DependencyObject obj)
        {
            return (string)obj.GetValue(PasswordProperty);
        }

        public static void SetPwd(DependencyObject obj, string value)
        {
            obj.SetValue(PasswordProperty, value);
        }

         public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.RegisterAttached(
                "Pwd",
                typeof(string),
                typeof(PasswordBoxHelper),
                new PropertyMetadata("", OnPasswordPropertyChangedCallback)
            );

        /// <summary>
        /// 自定义附加属性发生变化，改变Password属性
        /// </summary>
        /// <param name="d"></param>
        /// <param name="e"></param>
        private static void OnPasswordPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox = (PasswordBox)d;
            string newPwd = (string)e.NewValue;
            if (passwordBox != null && !passwordBox.Password.Equals(newPwd))
            {
                passwordBox.PasswordChanged -= PasswordBox_PasswordChanged;
　　　　　　　　　 passwordBox.Password = e.NewValue?.ToString() ?? string.Empty;
                passwordBox.PasswordChanged += PasswordBox_PasswordChanged;
            }
        }

        private static void PasswordBox_PasswordChanged(object sender, RoutedEventArgs e)
        {
            if (sender is PasswordBox passwordBox)
            {
                SetPwd(passwordBox, passwordBox.Password);
            }
        }
    }

 

  <PasswordBox
      helper:PasswordBoxHelper.Pwd="{Binding Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
      md:HintAssist.Hint="请输入密码"/>