Widget equivalent for HUDs "Get Actors in Selection Rectangle"

Hey,

i am currently trying to create an RTS-style game, with the camera in birds-eye view and the player can select units on the ground.

Now i am trying to implement the multi-selection of units by drawing a box around them.
I implemented the first part of this system, the actual drawing of this MultiSelection-box, with widgets (actually i am using a CanvasPanel with a BorderWidget) … and this actually works reasonably well, BUT i dont know how to select all overlapping Units of this Widget.

I found the mentioned method “Get Actors in Selection Rectangle” on the HUD, is there anything similar for Widgets?

Have you ever found anything for this?