wpf - MVVM-如何绑定(bind)到不是 DependancyProperty 的属性？

Question

我发现了这个问题 MVVM and the TextBox's SelectedText property .但是，我无法让解决方案发挥作用。这是我的非工作代码，我试图在第二个文本框中显示第一个文本框的选定文本。

查看:

SelectedText 和 Text 只是我的 ViewModel 中的字符串属性。

<TextBox Text="{Binding Path=Text, UpdateSourceTrigger=PropertyChanged}"  Height="155" HorizontalAlignment="Left" Margin="68,31,0,0" Name="textBox1" VerticalAlignment="Top" Width="264" AcceptsReturn="True" AcceptsTab="True" local:TextBoxHelper.SelectedText="{Binding SelectedText, UpdateSourceTrigger=PropertyChanged, Mode=OneWayToSource}" />
        <TextBox Text="{Binding SelectedText, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" Height="154" HorizontalAlignment="Left" Margin="82,287,0,0" Name="textBox2" VerticalAlignment="Top" Width="239" />

文本框助手

 public static class TextBoxHelper
{
    #region "Selected Text"
    public static string GetSelectedText(DependencyObject obj)
    {
        return (string)obj.GetValue(SelectedTextProperty);
    }

    public static void SetSelectedText(DependencyObject obj, string value)
    {
        obj.SetValue(SelectedTextProperty, value);
    }

    // Using a DependencyProperty as the backing store for SelectedText.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedTextProperty =
        DependencyProperty.RegisterAttached(
            "SelectedText",
            typeof(string),
            typeof(TextBoxHelper),
            new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, SelectedTextChanged));

    private static void SelectedTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        TextBox tb = obj as TextBox;
        if (tb != null)
        {
            if (e.OldValue == null && e.NewValue != null)
            {
                tb.SelectionChanged += tb_SelectionChanged;
            }
            else if (e.OldValue != null && e.NewValue == null)
            {
                tb.SelectionChanged -= tb_SelectionChanged;
            }

            string newValue = e.NewValue as string;

            if (newValue != null && newValue != tb.SelectedText)
            {
                tb.SelectedText = newValue as string;
            }
        }
    }

    static void tb_SelectionChanged(object sender, RoutedEventArgs e)
    {
        TextBox tb = sender as TextBox;
        if (tb != null)
        {
            SetSelectedText(tb, tb.SelectedText);
        }
    }
    #endregion

}

我究竟做错了什么？

Answer 1

这不起作用的原因是没有引发属性更改回调(因为来自 VM 的绑定(bind)值与属性元数据中指定的默认值相同)。但更根本的是，当所选文本设置为 null 时，您的行为将分离。在这种情况下，我倾向于有另一个附加属性，它只是用于启用对所选文本的监视，然后可以绑定(bind) SelectedText 属性。所以，像这样:

#region IsSelectionMonitored
public static readonly DependencyProperty IsSelectionMonitoredProperty = DependencyProperty.RegisterAttached(
    "IsSelectionMonitored",
    typeof(bool),
    typeof(PinnedInstrumentsViewModel),
    new FrameworkPropertyMetadata(OnIsSelectionMonitoredChanged));

[AttachedPropertyBrowsableForType(typeof(TextBox))]
public static bool GetIsSelectionMonitored(TextBox d)
{
    return (bool)d.GetValue(IsSelectionMonitoredProperty);
}

public static void SetIsSelectionMonitored(TextBox d, bool value)
{
    d.SetValue(IsSelectionMonitoredProperty, value);
}

private static void OnIsSelectionMonitoredChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    TextBox tb = obj as TextBox;
    if (tb != null)
    {
        if ((bool)e.NewValue)
        {
            tb.SelectionChanged += tb_SelectionChanged;
        }
        else
        {
            tb.SelectionChanged -= tb_SelectionChanged;
        }

        SetSelectedText(tb, tb.SelectedText);
    }
}
#endregion

#region "Selected Text"
public static string GetSelectedText(DependencyObject obj)
{
    return (string)obj.GetValue(SelectedTextProperty);
}

public static void SetSelectedText(DependencyObject obj, string value)
{
    obj.SetValue(SelectedTextProperty, value);
}

// Using a DependencyProperty as the backing store for SelectedText.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty SelectedTextProperty =
    DependencyProperty.RegisterAttached(
        "SelectedText",
        typeof(string),
        typeof(TextBoxHelper),
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, SelectedTextChanged));

private static void SelectedTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    TextBox tb = obj as TextBox;
    if (tb != null)
    {
        tb.SelectedText = e.NewValue as string;            
    }
}

static void tb_SelectionChanged(object sender, RoutedEventArgs e)
{
    TextBox tb = sender as TextBox;
    if (tb != null)
    {
        SetSelectedText(tb, tb.SelectedText);
    }
}
#endregion

然后在您的 XAML 中，您必须将该属性添加到您的第一个 TextBox:

<TextBox ... local:TextBoxHelper.IsSelectionMonitored="True" local:TextBoxHelper.SelectedText="{Binding SelectedText, Mode=OneWayToSource}" />