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CGI::Push(3p)    Perl Programmers Reference Guide   CGI::Push(3p)


NAME
       CGI::Push - Simple Interface to Server Push

SYNOPSIS
           use CGI::Push qw(:standard);

           do_push(-next_page=>\&next_page,
                   -last_page=>\&last_page,
                   -delay=>0.5);

           sub next_page {
               my($q,$counter) = @_;
               return undef if $counter >= 10;
               return start_html('Test'),
                      h1('Visible'),"\n",
                      "This page has been called ", strong($counter)," times",
                      end_html();
           }

           sub last_page {
               my($q,$counter) = @_;
               return start_html('Done'),
                      h1('Finished'),
                      strong($counter - 1),' iterations.',
                      end_html;
           }

DESCRIPTION
       CGI::Push is a subclass of the CGI object created by
       CGI.pm.  It is specialized for server push operations,
       which allow you to create animated pages whose content
       changes at regular intervals.

       You provide CGI::Push with a pointer to a subroutine that
       will draw one page.  Every time your subroutine is called,
       it generates a new page.  The contents of the page will be
       transmitted to the browser in such a way that it will
       replace what was there beforehand.  The technique will
       work with HTML pages as well as with graphics files,
       allowing you to create animated GIFs.

       Only Netscape Navigator supports server push.  Internet
       Explorer browsers do not.

USING CGI::Push
       CGI::Push adds one new method to the standard CGI suite,
       do_push().  When you call this method, you pass it a ref-
       erence to a subroutine that is responsible for drawing
       each new page, an interval delay, and an optional subrou-
       tine for drawing the last page.  Other optional parameters
       include most of those recognized by the CGI header()
       method.

       You may call do_push() in the object oriented manner or



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CGI::Push(3p)    Perl Programmers Reference Guide   CGI::Push(3p)


       not, as you prefer:

           use CGI::Push;
           $q = new CGI::Push;
           $q->do_push(-next_page=>\&draw_a_page);

               -or-

           use CGI::Push qw(:standard);
           do_push(-next_page=>\&draw_a_page);

       Parameters are as follows:

       -next_page
               do_push(-next_page=>\&my_draw_routine);

           This required parameter points to a reference to a
           subroutine responsible for drawing each new page.  The
           subroutine should expect two parameters consisting of
           the CGI object and a counter indicating the number of
           times the subroutine has been called.  It should
           return the contents of the page as an array of one or
           more items to print.  It can return a false value (or
           an empty array) in order to abort the redrawing loop
           and print out the final page (if any)

               sub my_draw_routine {
                   my($q,$counter) = @_;
                   return undef if $counter > 100;
                   return start_html('testing'),
                          h1('testing'),
                          "This page called $counter times";
               }

           You are of course free to refer to create and use
           global variables within your draw routine in order to
           achieve special effects.

       -last_page
           This optional parameter points to a reference to the
           subroutine responsible for drawing the last page of
           the series.  It is called after the -next_page routine
           returns a false value.  The subroutine itself should
           have exactly the same calling conventions as the
           -next_page routine.

       -type
           This optional parameter indicates the content type of
           each page.  It defaults to "text/html".  Normally the
           module assumes that each page is of a homogenous MIME
           type.  However if you provide either of the magic val-
           ues "heterogeneous" or "dynamic" (the latter provided
           for the convenience of those who hate long parameter
           names), you can specify the MIME type -- and other



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CGI::Push(3p)    Perl Programmers Reference Guide   CGI::Push(3p)


           header fields -- on a per-page basis.  See "heteroge-
           neous pages" for more details.

       -delay
           This indicates the delay, in seconds, between frames.
           Smaller delays refresh the page faster.  Fractional
           values are allowed.

           If not specified, -delay will default to 1 second

       -cookie, -target, -expires, -nph
           These have the same meaning as the like-named parame-
           ters in CGI::header().

           If not specified, -nph will default to 1 (as needed
           for many servers, see below).

       Heterogeneous Pages

       Ordinarily all pages displayed by CGI::Push share a common
       MIME type.  However by providing a value of "heteroge-
       neous" or "dynamic" in the do_push() -type parameter, you
       can specify the MIME type of each page on a case-by-case
       basis.

       If you use this option, you will be responsible for pro-
       ducing the HTTP header for each page.  Simply modify your
       draw routine to look like this:

           sub my_draw_routine {
               my($q,$counter) = @_;
               return header('text/html'),   # note we're producing the header here
                      start_html('testing'),
                      h1('testing'),
                      "This page called $counter times";
           }

       You can add any header fields that you like, but some
       (cookies and status fields included) may not be inter-
       preted by the browser.  One interesting effect is to dis-
       play a series of pages, then, after the last page, to
       redirect the browser to a new URL.  Because redirect()
       does b<not> work, the easiest way is with a -refresh
       header field, as shown below:

           sub my_draw_routine {
               my($q,$counter) = @_;
               return undef if $counter > 10;
               return header('text/html'),   # note we're producing the header here
                      start_html('testing'),
                      h1('testing'),
                      "This page called $counter times";
           }




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CGI::Push(3p)    Perl Programmers Reference Guide   CGI::Push(3p)


           sub my_last_page {
               return header(-refresh=>'5; URL=http://somewhere.else/finished.html',
                             -type=>'text/html'),
                      start_html('Moved'),
                      h1('This is the last page'),
                      'Goodbye!'
                      hr,
                      end_html;
           }

       Changing the Page Delay on the Fly

       If you would like to control the delay between pages on a
       page-by-page basis, call push_delay() from within your
       draw routine.  push_delay() takes a single numeric argu-
       ment representing the number of seconds you wish to delay
       after the current page is displayed and before displaying
       the next one.  The delay may be fractional.  Without
       parameters, push_delay() just returns the current delay.

INSTALLING CGI::Push SCRIPTS
       Server push scripts must be installed as no-parsed-header
       (NPH) scripts in order to work correctly on many servers.
       On Unix systems, this is most often accomplished by pre-
       fixing the script's name with "nph-".  Recognition of NPH
       scripts happens automatically with WebSTAR and Microsoft
       IIS.  Users of other servers should see their documenta-
       tion for help.

       Apache web server from version 1.3b2 on does not need
       server push scripts installed as NPH scripts: the -nph
       parameter to do_push() may be set to a false value to dis-
       able the extra headers needed by an NPH script.

AUTHOR INFORMATION
       Copyright 1995-1998, Lincoln D. Stein.  All rights
       reserved.

       This library is free software; you can redistribute it
       and/or modify it under the same terms as Perl itself.

       Address bug reports and comments to: lsteinATcshl.org

BUGS
       This section intentionally left blank.

SEE ALSO
       CGI::Carp, CGI









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