RelativeSource and Popup - wpf

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.

Answer 2

You can make small hack: setup DataContext via resources.
<Grid.Resources>
<Style TargetType="TextBlock">
<Setter Property="DataContext" Value="{Binding ElementName=myGrid, Path=DataContext}" />
</Style>
</Grid.Resources>

Answer 3

As others have mentioned, it's because the Popup is not part of the visual tree. Instead, you can use the Popup's PlacementTarget property to get back to the visual tree:
<Grid x:Name="LayoutRoot" Background="White" Height="100" Width="200">
<Popup IsOpen="True">
<TextBlock Text="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Popup}},
Path=PlacementTarget.DataContext}" />
</Popup>
</Grid>

Answer 4

Popups are not part of the visual tree.
Relative Source "Gets or sets the binding source by specifying its location relative to the position of the binding target (MSDN)". Since Popups are not part of the visual tree of the control that is showing it, it will not be able to resolve anything outside of the popup.