In accordance with the terminology used by Trestle, we refer to mouse movements as to {\em position events}. A position event is represented by a record PositionCB.Rec.

A {\em position event callback object} is an object which has one method, invoke. Each geometric object has a stack of position callback objects. When a position event pr is relayed to a particular geometric o, the the message cb.invoke(pr) is sent to the top callback object on o's position callback stack (if o's position callback stack is empty, the event is simply dropped). It is cb's reponsibility to perform whatever action is appropriate in the current context.

INTERFACE PositionCB;

IMPORT CB, Point, ProxiedObj, VBT;

TYPE
  T <: Public;
  Public = ProxiedObj.T OBJECT
  METHODS
    init () : T;
    invoke (pr : Rec) RAISES {CB.BadMethod};
  END;

PositionCB.T is the abstract class of position callback objects. If o is a geometric object, c is the top object in its position callback stack, and a position event pr gets relayed to o, then cb.invoke(pr) will be called. It is up to the user to create subclasses of PositionCB.T that handle position events in a given context appropriately.

  Rec = RECORD
    pos2D     : Point.T;
    modifiers : VBT.Modifiers;
  END;

PositionCB.Rec is a record type containing information about a position event. pos2D is the position of the mouse when the movement occurred. modifiers is the set of modifiers (Shift, Control, Mouse Buttons, etc.) that was active when the movement took place.

{\em NOTE: There is a fair chance that I will add other fields to Rec, once we have gained more experience with event handling. So far, I pretty much mimick (part of) what is there in VBT.PositionRec.}

END PositionCB.