如何通过行为清除绑定到 ViewModel 的 WPF PasswordBox？

Question

我编写 WPF MVVM Prism 6.2 应用程序。在登录窗口（即 PrismUserControl）的视图中，我有一个 PaswordBox 绑定（通过行为）到视图模型中的“密码”属性。 每次在应用程序运行时调用登录窗口时，PasswordBox 必须为空。 （例如，在用户关闭当前会话后，他或她必须只看到空的 Shell 和 Shell 上方的登录窗口.) 我的问题是上述密码框仅在应用程序加载后第一次显示为空。如果密码框在第二次或第三次等中显示，则它不为空。请看下图：

如您所见，密码不为空，但在这种情况下必须为空。下面是来自登录窗口标记的 XAML sn-p，其中 PaswordBox 是：

. . . . . . . . . . . . . .
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
. . . . . . . . . . . . . .
<PasswordBox Grid.Row="1" Grid.Column="1" Height="30" Margin="0 10 5 0" AutomationProperties.AutomationId="UserPasswordBox">
        <i:Interaction.Behaviors>
            <behavior:PasswordBoxBindingBehavior Password="{Binding Password}"/>
        </i:Interaction.Behaviors>
</PasswordBox>
. . . . . . . . . . . . . . . .

下面是 XAML 中也涉及的行为类别，如您在上面看到的：

public class PasswordBoxBindingBehavior : Behavior<PasswordBox>
{
    protected override void OnAttached()
    {
        AssociatedObject.PasswordChanged += OnPasswordBoxValueChanged;
    }

    public SecureString Password
    {
        get { return (SecureString)GetValue(PasswordProperty); }
        set { SetValue(PasswordProperty, value); }
    }

    public static readonly DependencyProperty PasswordProperty =
        DependencyProperty.Register("Password", typeof(SecureString), typeof(PasswordBoxBindingBehavior), new PropertyMetadata(null));

    private void OnPasswordBoxValueChanged(object sender, RoutedEventArgs e)
    {
        var binding = BindingOperations.GetBindingExpression(this, PasswordProperty);
        if (binding != null)
        {
            PropertyInfo property = binding.DataItem.GetType().GetProperty(binding.ParentBinding.Path.Path);
            if (property != null)
                property.SetValue(binding.DataItem, AssociatedObject.SecurePassword, null);
        }
    }
}

下面是视图模型中的“密码”属性。 PasswordBox 通过 PasswordBoxBindingBehavior 绑定到该属性：

public SecureString Password
{
    get { return this._password; }
    set { this.SetProperty(ref this._password, value); }
}

在应用程序工作期间，每次显示登录窗口时，我都需要 PasswordBox 为空。我试图以编程方式清除视图模型中的“密码”属性，但没有帮助。我该怎么做？请帮忙。

Answer 1

您可以为您的行为的Password 依赖属性连接一个PropertyChangedCallback，当视图模型的Password 源属性设置时，将PasswordBox 的Password 属性设置为空字符串给null：

public class PasswordBoxBindingBehavior : Behavior<PasswordBox>
{
    protected override void OnAttached()
    {
        AssociatedObject.PasswordChanged += OnPasswordBoxValueChanged;
    }

    public SecureString Password
    {
        get { return (SecureString)GetValue(PasswordProperty); }
        set { SetValue(PasswordProperty, value); }
    }

    public static readonly DependencyProperty PasswordProperty =
        DependencyProperty.Register("Password", typeof(SecureString), typeof(PasswordBoxBindingBehavior), new PropertyMetadata(OnSourcePropertyChanged));

    private static void OnSourcePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if(e.NewValue == null)
        {
            PasswordBoxBindingBehavior behavior = d as PasswordBoxBindingBehavior;
            behavior.AssociatedObject.PasswordChanged -= OnPasswordBoxValueChanged;
            behavior.AssociatedObject.Password = string.Empty;
            behavior.AssociatedObject.PasswordChanged += OnPasswordBoxValueChanged;
        }
    }

    private static void OnPasswordBoxValueChanged(object sender, RoutedEventArgs e)
    {
        PasswordBox passwordBox = sender as PasswordBox;
        var behavior = Interaction.GetBehaviors(passwordBox).OfType<PasswordBoxBindingBehavior>().FirstOrDefault();
        if(behavior != null)
        {
            var binding = BindingOperations.GetBindingExpression(behavior, PasswordProperty);
            if (binding != null)
            {
                PropertyInfo property = binding.DataItem.GetType().GetProperty(binding.ParentBinding.Path.Path);
                if (property != null)
                    property.SetValue(binding.DataItem, passwordBox.SecurePassword, null);
            }
        }
    }
}

然后，您只需在视图模型中将 Password 源属性设置为 null 即可清除 PasswordBox。

Answer 2

一种更简单的方法。每次屏幕（PasswordBox 元素所在的 Grid）关闭时，这都会清除 passwordBox 文本框。

<Grid  Grid.LostFocus="event_method" ...

<PasswordBox x:Name="passwordBox" ...

在后面的代码中：

public event_method()
{
    passwordBox.Clear();
}