如何在数据上下文设置为祖先时设置绑定

Question

我想知道如何在我的视图模型工作选项卡视图模型中将绑定设置为公共变量。我在下面举了两个例子，

Foreground="{Binding MenuItemForeground}"和

Foreground="{Binding MenuItemForeground, RelativeSource={RelativeSource AncestorType=UserControl}}"

但他们都不认识MenuItemForeground。

<TextBlock VerticalAlignment="Center" FontWeight="Bold" Margin="1 0 0 0" DataContext="{Binding Path=DataContext, RelativeSource={RelativeSource AncestorType=UserControl}}" >
    <TextBlock.ContextMenu>
        <ContextMenu>
            <MenuItem Header="Close tab" cal:Message.Attach="[Click] = [CloseTab()]" />
            <MenuItem Header="Close other tabs" cal:Message.Attach="[Click] = [CloseOtherTabs()]" IsEnabled="False" Foreground="{Binding MenuItemForeground, RelativeSource={RelativeSource AncestorType=UserControl}}"/>
            <MenuItem Header="Close all tabs" cal:Message.Attach="[Click] = [CloseAllTabs()]" IsEnabled="False" Foreground="{Binding MenuItemForeground}"/>
         </ContextMenu>
    </TextBlock.ContextMenu>
</TextBlock>

工作选项卡视图模型

public Brush MenuItemForeground { get; set; }
public void CloseTab(){...}
public void CloseOtherTab(){...}
public void CloseAllTabs(){...}

Answer 1

上下文菜单不是可视化树的一部分，因此它不会继承父控件 DataContext。因此，您也不能使用祖先语法。

一种解决方案是使用绑定代理。

public class BindingProxy : Freezable
{
    #region Overrides of Freezable
    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }
    #endregion
    public object Data
    {
        get { return (object)GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }
    // Using a DependencyProperty as the backing store for Data.
    // This enables animation, styling, binding, etc...
    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register("Data",
                                    typeof(object),
                                    typeof(BindingProxy),
                                    new UIPropertyMetadata(null));
}

它在 XAML 中的用法。

<TextBlock VerticalAlignment="Center" 
           FontWeight="Bold" 
           Margin="1 0 0 0"
           DataContext="{Binding Path=DataContext, RelativeSource={RelativeSource AncestorType=UserControl}}">
    <TextBlock.Resources>
        <helper:BindingProxy x:Key="proxy"
                             Data="{Binding }" />
    </TextBlock.Resources>
    <TextBlock.ContextMenu>
        <MenuItem Header="Close tab" 
                  cal:Message.Attach="[Click] = [CloseTab()]" />
        <MenuItem Header="Close other tabs" 
                  cal:Message.Attach="[Click] = [CloseOtherTabs()]"
                  IsEnabled="False"
                  Foreground="{Binding Source={StaticResource proxy}, Data.MenuItemForeground}"/>
        <MenuItem Header="Close all tabs" 
                  cal:Message.Attach="[Click] = [CloseAllTabs()]"
                  IsEnabled="False"
                  Foreground="{Binding Source={StaticResource proxy}, Data.MenuItemForeground}"/>
     </ContextMenu>
</TextBlock.ContextMenu>

不要忘记在窗口/用户控件的顶部声明帮助程序命名空间导入。