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

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

IMPORT CB, ProxiedObj, VBT;

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

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

  Rec = RECORD
    whatChanged : VBT.KeySym;
    wentDown    : BOOLEAN;
    modifiers   : VBT.Modifiers;
  END;

KeyCB.Rec is a record type containing information about a key event. whatChanged is the key that went up or down, wentDown indicates whether it went down or up. modifiers is the set of modifiers (Shift, Control, Mouse Buttons, etc.) that was active when the transition 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.KeyRec.}

END KeyCB.