Treeview ContextMenu binding Cut and Paste to logical tree

Question

This seems like a pretty common scenario but I can't figure out how to bind the menu items to disable when there is nothing in the clipboard.

I've decided against using the Windows clipboard and instead store the actual object in a reference variable on the UserControl called NodeClipboard. Since it is strongly typed and implements INotifyProperty it is a lot more useful to me than the Windows clipboard.

Binding to the individual item works fine though it is extremely verbose because you can't set EventHandlers within resources without using the Style Event Setters.

It sort of looks like this...

<UserControl x:Name="PART_Root">
    <TreeView x:Name="PART_Tree" ItemsSource="{Binding ElementName=PART_Root, Path=RootItemContainer}">
        <TreeView.Resources>
            <ContextMenu x:Key="ContextMenu">
                <ContextMenu.Style>
                    <Style TargetType="ContextMenu">
                        <!-- I use this event to select the tree view item otherwise it is actually pretty difficult to know what item you right clicked on -->
                        <EventSetter Event="Opened" Handler="ContextMenu_Opened"/>
                    </Style>
                </ContextMenu.Style>
                <MenuItem Header="Cut">
                    <MenuItem.Style>
                        <Style TargetType="MenuItem">
                            <EventSetter Event="Click" Handler="CutNode_Click"/>
                            <Style.Triggers>
                                <!-- This binding is fine because it binds to the item that was right clicked on -->
                                <DataTrigger Binding="{Binding Path=IsRoot}" Value="True">
                                    <Setter Property="IsEnabled" Value="False"/>
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </MenuItem.Style>
                </MenuItem>
                <MenuItem Header="Paste">
                    <MenuItem.Style>
                        <Style TargetType="MenuItem">
                            <EventSetter Event="Click" Handler="PasteNode_Click"/>
                            <!-- This binding always fails because ContextMenu lives outside of the logical tree -->
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding ElementName=PART_Root, Path=NodeClipboard" Value="{x:Null}">
                                    <Setter Property="IsEnabled" Value="False"/>
                                </DataTrigger>
                            </Style.Triggers>
                    </Style>
                    </MenuItem.Style>
                </MenuItem>
            </ContextMenu>
            <Style TargetType="TreeViewItem">
                <Setter Property="ContextMenu" Value="{StaticResource ContextMenu}"/>
            </Style>
            <HierarchicalDataTemplate DataType="{x:Type local:Node}" ItemsSource="{Binding Path=Children}">
                <TextBlock Text="{Binding Path=Id}"/>
            </HierarchicalDataTemplate>
        </TreeView.Resources>
    </TreeView>
</UserControl>

The key part that doesn't work is this here:

                        <!-- This binding always fails because ContextMenu lives outside of the logical tree -->
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding ElementName=PART_Root, Path=NodeClipboard" Value="{x:Null}">
                                <Setter Property="IsEnabled" Value="False"/>
                            </DataTrigger>
                        </Style.Triggers>

I've tried using relative source which results in the same problem. The only solution I've thought of so far is making two context menus, one with Paste enabled and one without, and switching the context menu on the style on the TreeViewItem style's ContextMenu setter like so...

<TreeView.Resources>
    <Style TargetType="TreeViewItem">
        <Setter Property="ContextMenu" Value="{StaticResource ContextMenu_PasteEnabled}"/>
        <Style.Triggers>
            <DataTrigger Binding="{Binding ElementName=PART_Root, Path=NodeClipboard" Value="{x:Null}">
                <Setter Property="ContextMenu" Value="{StaticResource ContextMenu_PasteDisabled}"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</TreeView.Resources>

Answer 1

Use the Clipboard class. Then you can use the ContainsText method to determine if there is any data on the clipboard.

http://msdn.microsoft.com/en-us/library/system.windows.forms.clipboard.aspx