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 $counter times";
}
You are of course free to refer to create and use global variables within
your draw routine in order to achieve special effects.
- -last_page
- This optional parameter points to a reference to the subroutine
responsible for drawing the last page of the series. It is called after
the -next_page routine returns a false value. The subroutine itself should
have exactly the same calling conventions as the -next_page routine.
- -type
- This optional parameter indicates the content type of each page. It
defaults to "text/html". Normally the module assumes that each
page is of a homogeneous MIME type. However if you provide either of the
magic values "heterogeneous" or "dynamic" (the latter
provided for the convenience of those who hate long parameter names), you
can specify the MIME type -- and other header fields -- on a per-page
basis. See "heterogeneous pages" for more details.
- -delay
- This indicates the delay, in seconds, between frames. Smaller delays
refresh the page faster. Fractional values are allowed.
If not specified, -delay will default to 1 second
- -cookie, -target, -expires, -nph
- These have the same meaning as the like-named parameters in
CGI::header().
If not specified, -nph will default to 1 (as needed for many servers, see
below).
Heterogeneous Pages¶
Ordinarily all pages displayed by CGI::Push share a common MIME type. However by
providing a value of "heterogeneous" or "dynamic" in the
do_push() -type parameter, you can specify the MIME type of each page
on a case-by-case basis.
If you use this option, you will be responsible for producing the HTTP header
for each page. Simply modify your draw routine to look like this:
sub my_draw_routine {
my($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.
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