Perspective - visible portion of an interactor

Description

       A  perspective  object  describes  which  part  of  an  interactor's total display area is visible in the
       interactor's canvas.  The main use of perspectives is for scrolling, panning,  and  zooming  interactors.
       The member variables (x0, y0) define the origin of the interactor's display area; width and height define
       the  total  height  of  the  area.  The member variables (curx, cury) define the lower-left corner of the
       interactor's canvas relative to the total area; curwidth and curheight define how much of the total  area
       is displayed in the canvas.

       The  member  variables  sx,  sy,  lx, and ly specify small and large increments in each dimension.  These
       increments are used by a scroller to implement line and page scrolling.

Name

       Perspective - visible portion of an interactor

Public Operations

Perspective()
              Define a new perspective with all member variables initialized to zero.

       voidAttach(Interactor*)
              Add to the list of  interactors  that  are  interested  in  updates  to  the  perspective.   These
              interactors reflect the perspective visually; examples include scrollers and sliders.

       voidDetach(Interactor*)
              Remove  an  interactor  from  the  list  of those interested in updates.  This operation should be
              called from the interactor's destructor.

       voidInit(Coordx0,Coordy0,Coordwidth,Coordheight)
              Initialize the perspective display area.

       voidUpdate()
              Notify all the interactors interested in the perspective that it has changed.   The  perspective's
              interactor should call Update after modifying its perspective.