我們發現我們無法繫結密碼框的密碼，PasswordBox 的 Password 不能繫結。

我們想做 MVVM ，我們需要繫結密碼，不能使用前臺 xaml.cs 監聽 密碼改變得到密碼的值，傳到 ViewModel 。

本文提供一個簡單方法來繫結 WPF 的 PasswordBox 的 Password 。這種方法不僅在 WPF 可以使用，在 UWP 也可以使用。關於 UWP 繫結密碼，可以在我部落格 win10 uwp 繫結密碼 檢視。

我在網上找的很多大神給出的可以解決繫結密碼的方法，下面是我找的一個簡單方法。

首先需要新建一個類 PasswordHelper ，他是一個靜態類，當然不是靜態也沒關係，但是一般寫靜態的可以讓我們少犯錯，因為我們所有屬性等都是需要靜態的。

    public static class PasswordHelper
    {
        public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.RegisterAttached("Password",
            typeof(string), typeof(PasswordHelper),
            new FrameworkPropertyMetadata(string.Empty, OnPasswordPropertyChanged));

        public
 
 static readonly DependencyProperty AttachProperty =
            DependencyProperty.RegisterAttached("Attach",
            typeof(bool), typeof(PasswordHelper), new PropertyMetadata(false, Attach));

        private static readonly DependencyProperty IsUpdatingProperty =
           DependencyProperty.RegisterAttached("IsUpdating"
 
, typeof(bool),
           typeof(PasswordHelper));


        public static void SetAttach(DependencyObject dp, bool value)
        {
            dp.SetValue(AttachProperty, value);
        }

        public static bool GetAttach(DependencyObject dp)
        {
            return (bool)dp.GetValue(AttachProperty);
        }

        public static string GetPassword(DependencyObject dp)
        {
            return (string)dp.GetValue(PasswordProperty);
        }

        public static void SetPassword(DependencyObject dp, string value)
        {
            dp.SetValue(PasswordProperty, value);
        }

        private static bool GetIsUpdating(DependencyObject dp)
        {
            return (bool)dp.GetValue(IsUpdatingProperty);
        }

        private static void SetIsUpdating(DependencyObject dp, bool value)
        {
            dp.SetValue(IsUpdatingProperty, value);
        }

        private static void OnPasswordPropertyChanged(DependencyObject sender,
            DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox = sender as PasswordBox;
            if (passwordBox != null)
            {
                passwordBox.PasswordChanged -= PasswordChanged;

                if (!(bool)GetIsUpdating(passwordBox))
                {
                    passwordBox.Password = (string)e.NewValue;
                }
                passwordBox.PasswordChanged += PasswordChanged;
            }
        }

        private static void Attach(DependencyObject sender,
            DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox = sender as PasswordBox;

            if (passwordBox == null)
                return;

            if ((bool)e.OldValue)
            {
                passwordBox.PasswordChanged -= PasswordChanged;
            }

            if ((bool)e.NewValue)
            {
                passwordBox.PasswordChanged += PasswordChanged;
            }
        }

        private static void PasswordChanged(object sender, RoutedEventArgs e)
        {
            PasswordBox passwordBox = sender as PasswordBox;
            if (passwordBox != null)
            {
                SetIsUpdating(passwordBox, true);
                SetPassword(passwordBox, passwordBox.Password);
                SetIsUpdating(passwordBox, false);
            }
        }
    }

寫完我們就可以使用他，使用很簡單，在我們需要密碼框的頁面的xaml 上寫兩行新的程式碼就好。

<PasswordBox local:PasswordHelper.Attach="True" 
                             local:PasswordHelper.Password="{Binding Password, Mode=TwoWay}" 
                             Width="180" Style="{DynamicResource PasswordBoxStyle}"/>

其中，Password 是 ViewModel 的PassWord，很簡單我們把PasswordBox 繫結到ViewModel。

PASSWORDPROPERTY是附加屬性，REGISTERATTACHED 就是註冊附加。

我們附加屬性是回撥，當屬性變化使用函式。

我們需要設定Attach，設定時呼叫static void Attach(DependencyObject sender, DependencyPropertyChangedEventArgs e)

在 Attach 觸發，首先要判斷設定的 sender 是不是 Password

            PasswordBox passwordBox = sender as PasswordBox;

            if (passwordBox == null)
            {
                return;
            }

判斷改變的值，Old是true還是false，如果是true，那麼之前用了事件，我們要把事件

passwordBox.PasswordChanged -= PasswordChanged;

如果之前是false，那麼沒繫結，我們不能刪除。

判斷要改變的，如果是true，我們就

passwordBox.PasswordChanged += PasswordChanged;

如果不是，我們就不使用。

我們使用了是否存在密碼修改就使用PasswordChanged函式。也就是設定了剛才的就可在密碼變化使用PasswordChanged。

我們在PasswordChanged判斷輸入是不是PasswordBox，把密碼傳進PasswordProperty。

還有一個簡單方法

using System.Windows;
using System.Windows.Controls;

namespace CustomControl
{
    public class BindablePasswordBox : Decorator
    {
        /// <summary>
        /// The password dependency property.
        /// </summary>
        public static readonly DependencyProperty PasswordProperty;

        private bool isPreventCallback;
        private RoutedEventHandler savedCallback;

        /// <summary>
        /// Static constructor to initialize the dependency properties.
        /// </summary>
        static BindablePasswordBox()
        {
            PasswordProperty = DependencyProperty.Register(
                "Password",
                typeof(string),
                typeof(BindablePasswordBox),
                new FrameworkPropertyMetadata("", FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, new PropertyChangedCallback(OnPasswordPropertyChanged))
            );
        }

        /// <summary>
        /// Saves the password changed callback and sets the child element to the password box.
        /// </summary>
        public BindablePasswordBox()
        {
            savedCallback = HandlePasswordChanged;

            PasswordBox passwordBox = new PasswordBox();
            passwordBox.PasswordChanged += savedCallback;
            Child = passwordBox;
        }

        /// <summary>
        /// The password dependency property.
        /// </summary>
        public string Password
        {
            get { return GetValue(PasswordProperty) as string; }
            set { SetValue(PasswordProperty, value); }
        }

        /// <summary>
        /// Handles changes to the password dependency property.
        /// </summary>
        /// <param name="d">the dependency object</param>
        /// <param name="eventArgs">the event args</param>
        private static void OnPasswordPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs eventArgs)
        {
            BindablePasswordBox bindablePasswordBox = (BindablePasswordBox) d;
            PasswordBox passwordBox = (PasswordBox) bindablePasswordBox.Child;

            if (bindablePasswordBox.isPreventCallback)
            {
                return;
            }

            passwordBox.PasswordChanged -= bindablePasswordBox.savedCallback;
            passwordBox.Password = (eventArgs.NewValue != null) ? eventArgs.NewValue.ToString() : "";
            passwordBox.PasswordChanged += bindablePasswordBox.savedCallback;
        }

        /// <summary>
        /// Handles the password changed event.
        /// </summary>
        /// <param name="sender">the sender</param>
        /// <param name="eventArgs">the event args</param>
        private void HandlePasswordChanged(object sender, RoutedEventArgs eventArgs)
        {
            PasswordBox passwordBox = (PasswordBox) sender;

            isPreventCallback = true;
            Password = passwordBox.Password;
            isPreventCallback = false;
        }
    }
}