REF XPT_CLIENTCALLBACK                          Jonathan Meyer, Aug 1991

       COPYRIGHT University of Sussex 1993. All Rights Reserved.

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<<<<<<<<<<<<<<<<<<<<<                             >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<   MANAGEMENT OF CLIENT      >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<         CALLBACKS           >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<                             >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

This  REF  file     describes     the     procedures     supplied     by
LIB * XPT_CLIENTCALLBACK  which  provides  a   way  of  adding   "client
callbacks" to a widget.

         CONTENTS - (Use <ENTER> g to access required sections)

  1   Introduction

  2   Client Callback Procedures



---------------
1  Introduction
---------------

Client  callbacks  bear  no  relation   to  X  Toolkit  callbacks   (see
REF * XT_CALLBACK), except  insofar  as they  share  the same  style  of
application programming interface.

Client callbacks are  useful in  situations where  applications want  to
adopt a  callback-style interface  to widgets,  but wish  to extend  the
number of  callback  list  names  known to  a  widget,  rather  than  be
restricted to the list of  names (and their associated semantics)  known
internally by  a  widget. Client  callbacks  provide a  modular  way  of
extending the concept of a callback.

Client callback  procedures have  the  same calling  form as  X  Toolkit
callbacks:

            P(widget, client_data, call_data)




-----------------------------
2  Client Callback Procedures
-----------------------------

XptAddClientCallback(widget, name, widentproc, client_data)  [procedure]
        Add a client callback procedure widentproc to the callback  list
        named  name   of  the   widget  widget,   passing  client   data
        client_data. widentproc is  a Pop11 word,  ident, or  procedure.
        name and client_data are arbritrary Pop11 objects.

        Note that client callback lists are identified by testing for an
        exact equality (ie. ==) for the name argument. Thus, if you  use
        an  lconstant  or   sectionised  strings  for   name,  you   can
        effectively create `private' callback lists which.

        If widget did not previously have a client callback list  called
        name, a new callback list for name is created.


XptRemoveClientCallback(widget, name, widentproc,            [procedure]
                                       client_data)
        Remove the callback  instance specified by  widentproc and  DATA
        from the callback list  named name of  widget widget. Note  that
        the first client  callback with  the same  (ie. class_=)  client
        data and identical (ie. ==) widentproc is removed.


XptRemoveAllClientCallbacks(widget, name)                    [procedure]
        Removes all callbacks in the  callback list named name from  the
        widget widget.


XptCallClientCallbacks(widget, name, call_data)              [procedure]
XptCallClientCallbacks(widget, name, call_data, defer)
        Call all the callback procedures in the callback list named name
        of the  widget widget,  passing  call_data (an  arbitrary  Pop11
        object) as the call argument to the callback procedures. If  the
        optional argument defer is true, * external_defer_apply is  used
        to call  the  callback list  outside  of the  context  of  any X
        toolkit callbacks.


XptCallAllClientCallbacks(name, call_data)                   [procedure]
        A special convenience procedure that calls all client  callbacks
        on client callback lists  named name on  all widgets which  have
        registered client  callbacks. Then  calls  XFlush to  flush  the
        output buffers after calling the callback routines.


XptHasClientCallbacks(widget, name) -> bool                  [procedure]
        Returns true  if the  widget has  some client  callbacks in  the
        callback list named name, and false otherwise.




--- C.x/x/pop/ref/xpt_clientcallback
--- Copyright University of Sussex 1993. All rights reserved.