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CGI::Push(3)
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 reference to a subroutine that is
responsible for drawing each new page, an interval delay, and an optional
subroutine 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 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 $counter times";
}
You are of course free to refer to create and use global variables
within your draw routine in order to achieve special effects.
-last_page
This optional parameter points to a reference to the subroutine
responsible for drawing the last page of the series. It is called
after the -next_page routine returns a false value. The subroutine
itself should have exactly the same calling conventions as the
-next_page routine.
-type
This optional parameter indicates the content type of each page. It
defaults to "text/html". Normally the module assumes that each page is
of a homogenous MIME type. However if you provide either of the magic
values "heterogeneous" or "dynamic" (the latter provided for the
convenience of those who hate long parameter names), you can specify
the MIME type -- and other header fields -- on a per-page basis. See
"heterogeneous pages" for more details.
-delay
This indicates the delay, in seconds, between frames. Smaller delays
refresh the page faster. Fractional values are allowed.
If not specified, -delay will default to 1 second
-cookie, -target, -expires, -nph
These have the same meaning as the like-named parameters in
CGI::header().
If not specified, -nph will default to 1 (as needed for many servers,
see below).
Heterogeneous Pages
Ordinarily all pages displayed by CGI::Push share a common MIME type.
However by providing a value of "heterogeneous" or "dynamic" in the
do_push() -type parameter, you can specify the MIME type of each page on a
case-by-case basis.
If you use this option, you will be responsible for producing the HTTP
header for each page. Simply modify your draw routine to look like this:
sub my_draw_routine {
my($q,$counter) = @_;
return header('text/html'), # note we're producing the header here
start_html('testing'),
h1('testing'),
"This page called $counter times";
}
You can add any header fields that you like, but some (cookies and status
fields included) may not be interpreted by the browser. One interesting
effect is to display a series of pages, then, after the last page, to
redirect the browser to a new URL. Because redirect() does b<not> work,
the easiest way is with a -refresh header field, as shown below:
sub my_draw_routine {
my($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";
}
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 argument 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 prefixing the script's name with "nph-". Recognition
of NPH scripts happens automatically with WebSTAR and Microsoft IIS. Users
of other servers should see their documentation 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 disable 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: lstein@cshl.org
BUGS
This section intentionally left blank.
SEE ALSO
CGI::Carp, CGI
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