Windows Phone - 带有Tiles和ContextMenu的ListBox，IsEnabled绑定不起作用

Question

我有一个ListBox（绑定到一个可观察的集合），每个项目都显示为tile。 ListBoxItems的样式在我的App.xaml中定义，并与属性ItemContainerStyle链接。我想为每个tile显示一个ContextMenu，其中一个MenuItem为“Pin to Start”。这确实有效，但是我仍然启用了MenuItem，尽管我将IsEnabled属性绑定到返回false的ViewModel属性。如果我在我的XAML代码中将IsEnabled属性设置为false（没有绑定），我就会遇到与启用MenuItem相同的问题。

App.xaml

<Style x:Key="TileListBoxItemStyle" TargetType="ListBoxItem">
    <Setter Property="Padding" Value="0"/>
    <Setter Property="Margin" Value="12,12,0,0"/>
    <Setter Property="Background" Value="{StaticResource PhoneAccentBrush}"/>
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="Width" Value="210"/>
    <Setter Property="Height" Value="210"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ListBoxItem">
                <Border x:Name="LayoutRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="{TemplateBinding VerticalAlignment}">
                    <ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                    <toolkit:ContextMenuService.ContextMenu>
                        <toolkit:ContextMenu>
                            <toolkit:MenuItem 
                                Header="Pin to Start"
                                IsEnabled="{Binding IsNotPinned}"
                                Command="{Binding PinToStartCommand}" 
                                CommandParameter="{Binding}"/>
                        </toolkit:ContextMenu>
                    </toolkit:ContextMenuService.ContextMenu>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

VieModel属性：

public bool IsNotPinned
{
    get
    {
        //return !TileManager.IsTileAvailable(this);
        return false;
    }
}

public ICommand PinToStartCommand { get; private set; }

private void PinToStart(Item item)
{
    //...
}