如何使用 MultiBinding 在上下文菜单中隐藏分隔符？

Question

我正在 wpf 树视图上使用上下文菜单，并且我几乎可以满足我的需求。在解释问题之前，让我先解释一下上下文菜单的 XAML 定义的作用。

对于上下文菜单中的每个菜单项，我们都有一个命令，可以根据命令 CanExecute 方法禁用或启用菜单项。每个命令都会根据 CanExecute 的结果设置相应菜单项的 IsEnabled 属性。

每个菜单项的 IsEnabled 都绑定到 BooleanToVisibilityConverter，它将 IsEnabled bool 值转换为 Collapse 或 Visible 值，以绑定菜单项的 Visibility 属性。这再次工作得很好，我的菜单项显示和隐藏得很好。

现在来说说问题。在下面的 XAML 中，分隔符上方有两个菜单项（addCategoryMenuItem 和 removeCategoryMenuItem）。我正在尝试通过 IMultiValueConverter (MultiBooleanToVisibilityConverter) 的自定义实现对这两个菜单项的 IsEnabled 属性进行多重绑定，以便在禁用这两个菜单项时，我可以将分隔符的可见性属性设置为折叠，从而在菜单项被禁用。

对于 Converter(MultiBooleanToVisibilityConverter) 中的 Convert 方法，参数值（对象 [] 值）我在数组中得到两个包含值“{DependencyProperty.UnsetValue}”的项目。这些不能转换为布尔值，因此无法计算出我的可见性值。

也许与 MultiBinding 中使用的 ElementName 有关。找不到元素吗？我尝试过使用RelativeSource，即查找祖先等。但我只是感到困惑。我花了几个小时在这上面，所以现在我把它留给社区。

亲切的问候

穆罕默德

<ContextMenu x:Key="CategoryMenu">
    <ContextMenu.ItemContainerStyle>
        <Style TargetType="{x:Type Control}">
            <Setter Property="Visibility" Value="{Binding Path=IsEnabled, RelativeSource={RelativeSource Self}, Mode=OneWay, Converter={StaticResource booleanToVisibilityConverter}}" />
        </Style>
    </ContextMenu.ItemContainerStyle>
    <ContextMenu.Items>
        <MenuItem x:Name="addCategoryMenuItem" Header="add category" Command="{Binding AddCategory}">
            <MenuItem.Icon>
                <Image Source="/Images/add.png" Width="16" Height="16" />
            </MenuItem.Icon>
        </MenuItem>
        <MenuItem x:Name="removeCategoryMenuItem" Header="remove category" Command="{Binding RemoveCategory}">
            <MenuItem.Icon>
                <Image Source="/Images/remove.png" Width="16" Height="16" />
            </MenuItem.Icon>
        </MenuItem>
        <Separator>
            <Separator.Visibility>
                <MultiBinding Converter="{StaticResource multiBooleanToVisibilityConverter}">
                    <Binding Mode="OneWay" ElementName="addCategoryMenuItem" Path="IsEnabled" />
                    <Binding Mode="OneWay" ElementName="removeCategoryMenuItem" Path="IsEnabled" />
                </MultiBinding>
            </Separator.Visibility>
        </Separator>
        <MenuItem x:Name="refreshCategoryMenuItem" Header="refresh" Command="{Binding RefreshCategory}">
            <MenuItem.Icon>
                <Image Source="/Images/refresh.png" Width="16" Height="16" />
            </MenuItem.Icon>
        </MenuItem>
    </ContextMenu.Items>
</ContextMenu>

Answer 1

我扩展了普通分隔符来创建一个分隔符，该分隔符可以根据父 ItemsControl 中的其他项目自动确定是否应显示它。

public class AutoVisibilitySeparator : Separator
{
    public AutoVisibilitySeparator()
    {
        if (DesignerProperties.GetIsInDesignMode(this))
            return;

        Visibility = Visibility.Collapsed; // Starting collapsed so we don't see them disappearing

        Loaded += OnLoaded;
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        // We have to wait for all siblings to update their visibility before we update ours.
        // This is the best way I've found yet. I tried waiting for the context menu opening or visibility changed, on render and lots of other events
        Dispatcher.BeginInvoke(new Action(UpdateVisibility), DispatcherPriority.Render);
    }

    private void UpdateVisibility()
    {
        var showSeparator = false;

        // Go through each sibling of the parent context menu looking for a visible item before and after this separator
        var foundThis = false;
        var foundItemBeforeThis = false;
        foreach (var visibleItem in ((ItemsControl)Parent).Items.OfType<UIElement>().Where(i => i.Visibility == Visibility.Visible || i == this))
        {
            if (visibleItem == this)
            {
                // If there were no visible items prior to this separator then we hide it
                if (!foundItemBeforeThis)
                    break;

                foundThis = true;
            }
            else if (visibleItem is AutoVisibilitySeparator || visibleItem is Separator)
            {
                // If we already found this separator and this next item is not a visible item we hide this separator
                if (foundThis)
                    break;

                foundItemBeforeThis = false; // The current item is a separator so we reset the search for an item
            }
            else
            {
                if (foundThis)
                {
                    // We found a visible item after finding this separator so we're done and should show this
                    showSeparator = true;
                    break;
                }

                foundItemBeforeThis = true;
            }
        }

        Visibility = showSeparator ? Visibility.Visible : Visibility.Collapsed;
    }
}

Answer 2

好的，经过一番休息，我已经解决了这个问题。我必须使用RelativeSource 和FindAncestor 来获取上下文菜单对象，然后访问项目集合，然后使用索引器值来获取菜单项。我认为如果我可以使用菜单项名称会更好，因为我不喜欢代码或 xaml 中的幻数。

<Separator>
    <Separator.Visibility>
        <MultiBinding Converter="{StaticResource multiBooleanToVisibilityConverter}">
            <Binding Mode="OneWay" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type ContextMenu}}" Path="Items[0].IsEnabled" />
            <Binding Mode="OneWay" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type ContextMenu}}" Path="Items[1].IsEnabled" />
        </MultiBinding>
    </Separator.Visibility>
</Separator>

Answer 3

Michael Olsen 的想法非常好 - 我修改了它，以便分隔符检查分隔符之前和之后是否有有效的项目。

public class AutoSeparator : Separator
{
    public AutoSeparator()
    {
        if (DesignerProperties.GetIsInDesignMode(this))
            return;

        Visibility = Visibility.Collapsed; // Starting collapsed so we don't see them disappearing

        Loaded += OnLoaded;
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        Dispatcher.BeginInvoke(new Action(UpdateVisibility), DispatcherPriority.Render);
    }

    private void UpdateVisibility()
    {
        ItemCollection items = ((ItemsControl)Parent).Items;
        int index = items.IndexOf(this);

        if (index == -1)
            return;

        int i;
        for (i = index - 1; i >= 0; i--)
        {
            if (items[i] is UIElement uiElement)
            {
                // Invisible item cannot be valid predecessor
                if (uiElement.Visibility != Visibility.Visible)
                    continue;
                // Separator is invalid predecessor
                else if (uiElement is Separator or AutoSeparator)
                    return;
                // Anything else is valid predecessor
                else
                    break;
            }
        }

        // No valid item found before separator
        if (i < 0)
            return;

        for (i = index + 1; i < items.Count; i++)
        {
            if (items[i] is UIElement uiElement)
            {
                // Invisible item cannot be valid successor
                if (uiElement.Visibility != Visibility.Visible)
                    continue;
                // Separator is invalid successor
                else if (uiElement is Separator or AutoSeparator)
                    return;
                // Anything else is valid successor
                else
                    break;
            }
        }

        if (i >= items.Count)
            return;

        Visibility = Visibility.Visible;
    }
}