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

A {\em mouse event callback object} is an object which has one method, invoke. Each geometric object has a stack of mouse callback objects. When a mouse event mr is relayed to a particular geometric o, the the message cb.invoke(mr) is sent to the top callback object on o's mouse callback stack (if o's mouse 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 MouseCB;

IMPORT CB, Point, ProxiedObj, VBT;

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

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

  Rec = RECORD
    pos2D       : Point.T;
    whatChanged : VBT.Button;
    modifiers   : VBT.Modifiers;
    clickType   : VBT.ClickType;
  END;

MouseCB.Rec is a record type containing information about a mouse event. pos2D is the position of the mouse when the button transition occurred. whatChanged is the button that went up or down. modifiers is the set of modifiers (Shift, Control, Mouse Buttons, etc.) that was active when the transition took place. clickType indicates whether the button went down or up, and whether any other button was already or still down.

{\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.MouseRec.}

END MouseCB.