Binding a Popup to another control's relative screen position

Question

I'm writing an XBAP with a complex Popup (Canvas Z-index of 99 with a grid on it...) that I would like to "attach" to the button that opens it and follow that button around wherever it goes on the screen. For example, if the button is in a ListBox or an XamDataGrid I would like the popup to follow the button as it scrolls through. If it is beneath an Expander I want it to stay attached to the button when the expander forces it to move, etc.

Any Ideas?

Answer 1

When using a Popup, neither PlacementTarget nor CustomPopupPlacementCallback is used after the popup has originally appeared. So any use of these properties will not allow the popup to track the button as it moves.

Several ways occur to me of achieving what you desire:

1. Attach a custom Adorner to the button and put the popup inside it. Disadvantage: Popup is not connected to Button or surrounding elements, so it won't inherit properties & DataContext from them.

2. Attach a custom Adorner to the button. This adorner will get measure and arrange calls when the button moves relative to the AdornerLayer, allowing you to manually update the Popup position. As long as your AdornerDecorator doesn't move relative to your Window (eg if it is the direct child of the Window), you can easily detect the AdornerLayer being moved by monitoring changes to Window size. Disadvantage: Complex to code & get right.

3. Don't use a Popup at all. Instead wrap the button in a <Grid> alongside a <Canvas> with zero width and height and the desired position. Inside the <Canvas> add the UserControl for the popup with an appropriate ZIndex. It will extend past the edge f the Canvas, which is just fine in WPF. Instead of using a Popup control just control the visibility of the UserControl. Disadvantage: Will not really be totally on top of all other objects, can't extend off edge of window (may not be an issue for XBAP, though).