Binding a command defined in a datatemplate - wpf

Question

I know there are few answers on this topic. But none of them was working in my case.
I have a ListView with a style and an ItemContainerStyle. In the ItemContainer Style, I define some triggers in order to use a different DataTemplate depending if the item in the list is selected or not. Then, finally in the Datatemplate I have a context menu with a command. The problem is how to bind the command to the viewmodel.
This is the ListView:
<ListView
x:Name="lstPersons"
Grid.Row="1"
Style="{StaticResource ListViewStyle}"
ItemContainerStyle="{StaticResource ItemContainerStyle}"
DataContext="{Binding}"
ItemsSource="{Binding Path=Persons}"
Tag="{Binding}"
SelectedItem="{Binding Path=SelectedPerson, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
</ListView>
and these are the styles, datatemplates and contextmenu (defined in a resource dictionary).
The commands in the context menu do not work....:
<ContextMenu x:Key="SelectedItemContextMenu">
<MenuItem
Header="Do Something"
Command="{Binding Path=DataContext.DoSomethingCmd, ElementName=LayoutRoot}">
</MenuItem>
<MenuItem
Header="Do Something"
Command="{Binding PlacementTarget.Tag.DoSomethingCmd, RelativeSource={RelativeSource AncestorType=ContextMenu}}">
</MenuItem>
</ContextMenu>
<DataTemplate
x:Key="ItemTemplate">
<Canvas
Margin="4"
Width="60"
Height="60"
Background="LightGray">
<TextBlock
Foreground="Black"
Margin="2 0 0 0"
Opacity="0.5"
FontFamily="Segoe UI"
Text="{Binding Path=FirstName}" />
</Canvas>
</DataTemplate>
<DataTemplate
x:Key="ItemSelectedTemplate">
<Grid>
<Border
BorderBrush="Black"
BorderThickness="1"
Margin="3"
ContextMenu="{DynamicResource SelectedItemContextMenu}">
<Canvas
Width="60"
Height="60"
Background="LightBlue">
<TextBlock
Foreground="Black"
Margin="2 0 0 0"
Opacity="0.5"
FontFamily="Segoe UI"
Text="{Binding Path=FirstName}" />
</Canvas>
</Border>
</Grid>
</DataTemplate>
<!--style of the listviewitem-->
<Style
TargetType="{x:Type ListViewItem}"
x:Key="ItemContainerStyle">
<Setter
Property="ContentTemplate"
Value="{StaticResource ItemTemplate}" />
<Style.Triggers>
<Trigger
Property="IsSelected"
Value="True">
<Setter
Property="ContentTemplate"
Value="{StaticResource ItemSelectedTemplate}" />
</Trigger>
</Style.Triggers>
</Style>
<!--style of the listview-->
<Style
TargetType="{x:Type ListBox}"
x:Key="ListViewStyle">
<Setter
Property="Template">
<Setter.Value>
<ControlTemplate
TargetType="{x:Type ListBox}">
<Grid>
<Border>
<ScrollViewer
Focusable="false">
<WrapPanel
IsItemsHost="True"
Orientation="Horizontal"
Width="{Binding (FrameworkElement.ActualWidth), RelativeSource={RelativeSource AncestorType=ScrollContentPresenter}}"/>
</ScrollViewer>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Answer 1

Your ContextMenu is used inside a data template. I will be put in a different name scope of "LayoutRoot" and ElementName binding won't work. Also, the PlacementTarget of your context menu is the Border, and you've not setup any Tag on it. So the second command won't work either.
It looks like you are implement the commands on the ListBox level (or LayoutRoot?). It might be easier to put your context menu on the ListBox, and use ListBox.SelectedItem to find the current selection and apply your logic on it.

Answer 2

You can use RelativeSource:
<ContextMenu x:Key="SelectedItemContextMenu">
<MenuItem
Header="Do Something"
Command="{Binding Path=DataContext.DoSomethingCmd, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}">
</MenuItem>
</ContextMenu>

Answer 3

You should probably be using RoutedCommands instead of VM commands in this case. You would bind the RoutedCommand to the ContextMenu, and since you only need static object references for that, finding them shouldn't be a problem. Then you'd set up appropriate CommandBindings on the controls that should handle the commands (either ListView or ListViewItem, depending on whether you want the List-ViewModel or the Item-ViewModel to handle the command). These controls will know their ViewModels, so binding to them will not be a problem there. Through the process of Command Routing, which is built-in in WPF, the context menu will find the proper target for its command automatically.
For guidance on how to set up CommandBindings in a MVVM-friendly way, you might want to refer to http://wpfglue.wordpress.com/2012/05/07/commanding-binding-controls-to-methods/