WPF---数据绑定之PasswordBox绑定

一、概述

众所周知，绑定的源既可以是依赖属性也可以是普通的CLR属性，而绑定的目标只能是依赖属性。

控件PasswordBox的Password属性不是依赖属性，不可以作为绑定的目标与后台数据进行绑定，而在MVVM模式中，前台和后台的绑定是经常需要的，为了达到这种目的，我们可以借助附加属性来实现PasswordBox的Password属性的绑定。

二、绑定思路

思路如下：

1）定义一个PasswordBoxHelper类，在类中定义PasswordProperty、AttachProperty和IsUpdatingProperty三个附加属性以及相应的属性改变事件；

2）在AttachProperty的OnAttachPropertyChanged事件中添加PasswordBox的PasswordChanged事件处理程序，这样PasswordBox控件中输入密码的时候，就会触发PasswordBoxHelper类中PasswordChanged事件处理函数；

3）PasswordChanged事件处理函数执行的时候，把控件中的信息赋值给PasswordBoxHelper类中的依赖属性PasswordProperty；

三、Demo

<Window x:Class="PasswordBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:PasswordBinding"
        mc:Ignorable="d"
        Title="MainWindow" Height="447.125" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="134*"/>
            <RowDefinition Height="101*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="60*"/>
            <ColumnDefinition Width="457*"/>
        </Grid.ColumnDefinitions>
        <TextBlock Margin="10 50" Text="密码："></TextBlock>
        <PasswordBox Grid.Row="0" Grid.Column="1" Margin="10,50,10,157" BorderBrush="Red" local:PasswordBoxHelper.Attach ="True" Name="pwd"
                     local:PasswordBoxHelper.Password ="{Binding Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        <TextBlock Margin="10,30,10,10" Text="显示：" Grid.Row="1" Grid.Column="0"></TextBlock>
        <TextBlock Grid.Row="1" Grid.Column="1" Margin="10,70,10,72" Background="AliceBlue" Foreground="#FF4EB24E" Name="tbl" />
        <!--附加属性绑定的时候，记得一定要加括号！！！-->
        <TextBlock Grid.Column="1" Margin="10,10,10,135" Background="AliceBlue" Foreground="#FF4EB24E" Grid.Row="1" Text="{Binding ElementName=pwd, Path=(local:PasswordBoxHelper.Password)}"  />
        <Button Grid.Row="1" Grid.Column="1" Margin="370,135,0,0" Content="显示密码" Click="Button_Click" ></Button>

    </Grid>
</Window>

using System.Windows;

namespace PasswordBinding
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        //private string password;

        //public string Password
        //{
        //    get { return password; }
        //    set { password = value; }
        //}

        public string Password//依赖属性具有自动通知的能力，不需要实现INotifi接口
        {
            get { return (string)GetValue(PasswordProperty); }
            set { SetValue(PasswordProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Password.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.Register("Password", typeof(string), typeof(MainWindow), new PropertyMetadata(""));


        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            //tbl.Text = password;
            tbl.Text = PasswordBoxHelper.GetPassword(pwd);
        }
    }
}

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

namespace PasswordBinding
{

    public static class PasswordBoxHelper
    {

        public static readonly DependencyProperty PasswordProperty = DependencyProperty.RegisterAttached("Password", typeof(string), typeof(PasswordBoxHelper), 
                                                                                                          new FrameworkPropertyMetadata(string.Empty, OnPasswordPropertyChanged));
        public static readonly DependencyProperty AttachProperty = DependencyProperty.RegisterAttached("Attach", typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(false, OnAttachPropertyChanged));

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


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

        private static void OnAttachPropertyChanged(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;
            SetIsUpdating(passwordBox, true);
            SetPassword(passwordBox, passwordBox.Password);
            SetIsUpdating(passwordBox, false);
        }
    }
}