Changing node color when other node is dragged above it

I would like to implement a group creation when a node is dragged onto another node (like in android phone).
As soon as the node is dragged over another node, i would like that the underneath node to change its color or opacity.
How can i do it ?

Set GraphObject | GoJS API and GraphObject | GoJS API on the Node.

For example: Org Chart Editor

Works perfect,