在WPF的MVVM框架中实现对密码框的密码绑定（转）

正如绑定TextBox控件的Text属性一样, 我们希望能够将PasswordBox空间的Password属性进行绑定, 比如在MVVM模式中,这似乎是必须的, 但可惜的是, Password属性是不支持绑定的(不是依赖属性, 也没有实现INotifyPropertyChanged).
这应该是出于安全性的考虑. 但在我们的系统为了实现View层密码框中的密码与后台其它层之间的密码属性之间的绑定, 可以采取如下思路: 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.
缓冲区可以是哈希表或其他字典结构, 以便将密码框和缓冲区中的密码一 一对应起来, 也可以使AttachProperty(附加属性), 其实附加属性的机制也就是对缓存了的一个大字典进行操作。

public static class PasswordBoxBindingHelper
{
public static bool GetIsPasswordBindingEnabled(DependencyObject obj)
{
return (bool)obj.GetValue(IsPasswordBindingEnabledProperty);
}

public static void SetIsPasswordBindingEnabled(DependencyObject obj, bool value)
{
obj.SetValue(IsPasswordBindingEnabledProperty, value);
}

public static readonly DependencyProperty IsPasswordBindingEnabledProperty =
DependencyProperty.RegisterAttached("IsPasswordBindingEnabled", typeof(bool),
typeof(PasswordBoxBindingHelper),
new UIPropertyMetadata(false, OnIsPasswordBindingEnabledChanged));

private static void OnIsPasswordBindingEnabledChanged(DependencyObject obj,
DependencyPropertyChangedEventArgs e)
{
var passwordBox = obj as PasswordBox;

if(passwordBox != null)
{
passwordBox.PasswordChanged -= PasswordBoxPasswordChanged;

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

}
}

//when the passwordBox's password changed, update the buffer
static void PasswordBoxPasswordChanged(object sender, RoutedEventArgs e)
{
var passwordBox = (PasswordBox) sender;

if (!String.Equals(GetBindedPassword(passwordBox),passwordBox.Password))
{
SetBindedPassword(passwordBox, passwordBox.Password);
}
}

public static string GetBindedPassword(DependencyObject obj)
{
return (string)obj.GetValue(BindedPasswordProperty);
}

public static void SetBindedPassword(DependencyObject obj, string value)
{
obj.SetValue(BindedPasswordProperty, value);
}

public static readonly DependencyProperty BindedPasswordProperty =
DependencyProperty.RegisterAttached("BindedPassword", typeof(string),
typeof(PasswordBoxBindingHelper),
new UIPropertyMetadata(string.Empty, OnBindedPasswordChanged));

//when the buffer changed, upate the passwordBox's password
private static void OnBindedPasswordChanged(DependencyObject obj,
DependencyPropertyChangedEventArgs e)
{
var passwordBox = obj as PasswordBox;
if (passwordBox != null)
{

passwordBox.Password = e.NewValue == null ? string.Empty : e.NewValue.ToString();
}
}

}
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在View层, 如下使用便可以了:

<PasswordBox Helpers:PasswordBoxBindingHelper.IsPasswordBindingEnabled="True"
Helpers:PasswordBoxBindingHelper.BindedPassword=
"{Binding Path=Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

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另外, 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, 可惜的是, 密码框并没有给我们提供这样的属性或方法(TextBox有, PasswordBox没有), 可以采用下面的方法来设置:

private static void SetPasswordBoxSelection(PasswordBox passwordBox, int start, int length)
{
var select = passwordBox.GetType().GetMethod("Select",
BindingFlags.Instance | BindingFlags.NonPublic);

select.Invoke(passwordBox, new object[] { start, length });
}
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