CGI::Push

See the current Perl documentation for CGI::Push.

Here is our local, out-dated (pre-5.6) version:

CGI::Push - Simple Interface to Server Push

    use CGI::Push qw(:standard);

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

    sub next_page {
        my($q,$counter) = @_;
        retur
    

    

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.



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


These have the same meaning as the like-named parameters in CGI::header().




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 {
        header(-refresh=>'5; URL=finished',
               -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 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.



CAVEATS

This is a new module. It hasn't been extensively tested.



AUTHOR INFORMATION

be used and modified freely, but 
I do request that this copyright notice remain
attached to the file. You may modify this module as you wish, but if you
redistribute a modified version, please attach a note listing the
modifications you have made.


Address bug reports and comments to: lstein@genome.wi.mit.edu



BUGS

This section intentionally left blank.



SEE ALSO

Carp, CGI