wpf - RelativeSource and Popup

Question

The problem is that RelativeSource does not work in the following case. I use silverlight 5.

//From MainPage.xaml
<Grid x:Name="LayoutRoot" Background="White" Height="100" Width="200">
    <Popup IsOpen="True">
        <TextBlock Text="{Binding Path=DataContext, RelativeSource={RelativeSource AncestorType=Grid}}" />
    </Popup>
</Grid>

//From MainPage.xaml.cs
public MainPage()
{
    InitializeComponent();
    DataContext = "ololo";
}

If I set a breakpoint on the binding, I'll get Error:

System.Exception: BindingExpression_CannotFindAncestor.

If I use ElementName=LayoutRoot instead of RelativeSource, everything will be OK.

Why does the relative source binding not work?

Answer 1

Popup is like ContextMenu , ToolTip controls , They are not added to the VisualTree. For this you will have to do like

<Grid x:Name="LayoutRoot" Height="100" Width="200" Background="Black">
    <Popup Grid.Row="0"  x:Name="popup" DataContext="{Binding PlacementTarget.DataContext, RelativeSource={RelativeSource Mode=Self}}">
        <TextBlock Text="{Binding DataContext, ElementName=popup}" Background="Red" Width="30" Height="30" />
    </Popup>
</Grid>

public MainWindow()
    {
        InitializeComponent();
        DataContext = "abcd";
        popup.PlacementTarget = LayoutRoot; 
    }

I hope this will help.Not like in case of ContextMenu or Tooltip , here you will also have to specify the PlacementTarget.