How could items in groupItems communicate/interact

[helloearth012]

My groupItems has two items: rectangle and point

1. When insert, I would draw a rectangle, after this action is done, I hope a point could be drew automatically in the bottom left corner of this rectangle.
2. After insertion is done, it's visualization phase. No matter how I move/resize the rectangle, the point should always in the bottom left corner of this rectangle. But I should be able to slide the point on the bottom line of the rectangle.

Above operation involves communication between groupItems, since point needs to know the coordinates of rectangle to draw. However, in current code, it seems even for groupItems, each item is isolated. How could I make two items interact and communicate in current framework.