自定义WPF的PasswordBox控件，解决MVVM绑定问题

    最近在用MVVM框架做一个用户登录模块，涉及到密码框绑定的问题遇到阻碍。WPF中有现成的密码输入框控件PasswordBox，通过Password属性获取密码，但是Password属性不能进行数据绑定，原因是Password属性不是依赖属性。

    网上常见的解决思路是“将密码框的密码和某一个缓冲区进行同步, 缓冲区在和后台进行绑定. 其中密码框与缓冲区之间的同步可采用事件进行通知, 并将缓冲区打造成依赖属性, 然后缓冲区就支持绑定了, 并给后台提供正确的密码”，这个方法确实可以达到绑定的功能，但是有一个问题“在更改了密码框的密码后, 需要手动更新密码框插入符(CaretIndex)的位置”，虽然也有相对应的解决方案，但效果不好。本思路的解决方案请参考周银辉的《[WPF]实现密码框的密码绑定》http://www.cnblogs.com/zhouyinhui/archive/2009/08/27/1554943.html

    另外一种思路是重写PasswordBox，在此参考，该作者已经用TextBox作为基础重写了类似PasswordBox的控件：

《Passwords in WPF: How to find yours and how to prevent it being found》

http://www.codeproject.com/Articles/30976/Passwords-in-WPF-How-to-find-yours-and-how-to-prev

为了实现数据绑定，笔者将该控件作了相应修改，实现如下所示：

    /// <summary>
    /// More secure PasswordBox based on TextBox control
    /// </summary>
    public class SecurePasswordBox : TextBox
    {
        // Fake char to display in Visual Tree
        private char PWD_CHAR = '●';

        /// <summary>
        /// Only copy of real password
        /// </summary>
        /// <remarks>For more security use System.Security.SecureString type instead</remarks>
        private string _password = string.Empty;

        /// <summary>
        /// TextChanged event handler for secure storing of password into Visual Tree,
        /// text is replaced with PWD_CHAR chars, clean text is keept into
        /// Text property (CLR property not snoopable without mod)   
        /// </summary>
        protected override void OnTextChanged(TextChangedEventArgs e)
        {
            if (dirtyBaseText == true)
                return;

            string currentText = this.BaseText;

            int selStart = this.SelectionStart;
            if (currentText.Length < _password.Length)
            {
                // Remove deleted chars          
                _password = _password.Remove(selStart, _password.Length - currentText.Length);
                SetRealText(this, _password);
            }
            if (!string.IsNullOrEmpty(currentText))
            {
                for (int i = 0; i < currentText.Length; i++)
                {
                    if (currentText[i] != PWD_CHAR)
                    {
                        // Replace or insert char
                        if (this.BaseText.Length == _password.Length)
                        {
                            _password = _password.Remove(i, 1).Insert(i, currentText[i].ToString());
                        }
                        else
                        {
                            _password = _password.Insert(i, currentText[i].ToString());
                            
                        }
                    }
                }
　　               SetRealText(this, _password);

                this.BaseText = new string(PWD_CHAR, _password.Length);
                this.SelectionStart = selStart;
            }
            base.OnTextChanged(e);
        }

        // flag used to bypass OnTextChanged
        private bool dirtyBaseText;

        /// <summary>
        /// Provide access to base.Text without call OnTextChanged 
        /// </summary>
        private string BaseText
        {
            get { return base.Text; }
            set
            {
                dirtyBaseText = true;
                base.Text = value;
                dirtyBaseText = false;
            }
        }

        /// <summary>
        /// Clean Password
        /// </summary>
        public new string Text
        {
            get { return _password; }
            set
            {
                _password = value;
                this.BaseText = new string(PWD_CHAR, value.Length);
            }
        }


        //数据绑定用的附加属性RealText  
        
        public static string GetRealText(DependencyObject obj)
        {
            return (string)obj.GetValue(RealTextProperty);
        }

        public static void SetRealText(DependencyObject obj, string value)
        {
            obj.SetValue(RealTextProperty, value);
        }

        // Using a DependencyProperty as the backing store for RealText.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RealTextProperty =
            DependencyProperty.RegisterAttached("RealText", typeof(string), typeof(SecurePasswordBox), new UIPropertyMetadata(""));

    }

数据绑定如下：【注意：附加属性的绑定默认是单向的】

<local:SecurePasswordBox  local:SecurePasswordBox.RealText="{Binding Password,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Text="{Binding Password,Mode=OneWay,UpdateSourceTrigger=PropertyChanged}"/>