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//# DisplayEH.h: WorldCanvas refresh event handler
//# Copyright (C) 1993,1994,1995,1996,1998,1999,2000,2002
//# Associated Universities, Inc. Washington DC, USA.
//#
//# This library is free software; you can redistribute it and/or modify it
//# under the terms of the GNU Library General Public License as published by
//# the Free Software Foundation; either version 2 of the License, or (at your
//# option) any later version.
//#
//# This library is distributed in the hope that it will be useful, but WITHOUT
//# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
//# FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
//# License for more details.
//#
//# You should have received a copy of the GNU Library General Public License
//# along with this library; if not, write to the Free Software Foundation,
//# Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
//#
//# Correspondence concerning AIPS++ should be addressed as follows:
//# Internet email: aips2-request@nrao.edu.
//# Postal address: AIPS++ Project Office
//# National Radio Astronomy Observatory
//# 520 Edgemont Road
//# Charlottesville, VA 22903-2475 USA
//#
//# $Id$
//# (Note: below is one of just 3 DL classes not in trialdisplay/...)
namespace casa { //# NAMESPACE CASA - BEGIN
// <summary>
// class for handling any type of DisplayEvent.
// </summary>
//
// <prerequisite>
// <li> <linkto class="DisplayEvent">DisplayEvent</linkto>
// <li> Understanding of Display library event-handling methodology
// </prerequisite>
//
// <etymology>
// DisplayEH : generic display event handler--A handler for any type of
// display library event.
// </etymology>
//
// <synopsis>
// DisplayEH has an overridable callback that allows it to handle any type
// of base class DisplayEvent. It is intended to allow different types
// of handlers (for different types of event) to reside on the same callback
// list.
// </synopsis>
//
// <motivation>
// Different types of events (and event-handling routines) are needed in the
// display library, and more are likely to be needed in future. However,
// creating still more specialized handler lists in places like WC for
// each new type of event seems impractical. WorldCanvas and PixelCanvas
// already maintain 3 such lists each. This generic event handler