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CGI::Screen - Perl extension for easy creation of multi screen CGI scripts


  use CGI::Screen;
  use vars qw(@ISA);
  @ISA = qw(CGI::Screen);

  my $query = __PACKAGE__->new;



This is alpha software. User visible changes can happen any time.


CGI::Screen is a subclass of CGI which allows the esay(TM) creation of simple multi screen CGI scripts. By 'multi screen' I mean scripts which present different screens to the user when called with different parameters. This is the common case for scripts linking to themselves.

To use CGI::Screen, you have to subclass it. For each screen you want to present to the user, you must create a method screen_namescreen_. This method has to produce the HTML code for the screen. CGI::Screen does generate HTTP headers and an HTML framework for you. The HTML framework already contains the FORM tags. You can customize the HTTP headers and the HTML framework by providing callback methods.

CGI::Screen keeps track of the CGI parameters used in your screen and passes old parameters which are not used in the current screen.

It highjacks the parameters screen_* to dispatch the different screens the script implements. The screen_user and screen_passwd fields are used if you enable the builtin simple authentication. In general you should advice your HTTP server to do authentication. But sometimes it is convenient to check the authentication at the script level. Especially if you do not have access to your server's configuration.

The constructor new

If the first parameter of new is the string -screen the second argument must be a hash reference specifying the options for the subclass. Other parameters are passed to the constructor of CGI.


Normaly the history of pages will not be extended if the current page is the same as the last page. So looping on a page will not change the result of the last_screen method. If the option -dont_cut_loops is provided and true, the page will recorded twice. A third visit will be interpreted as jump back to the first visit.

That sounds weird. Will have to figure out a way to recognize back jumps independent of the history.

Adding Screens

All applications should provide a main screen by defining a method main_screen. This method is called if no (existing) screen is specified in the parameters. The method is called with three arguments: The query object, the screen name and the screen title (More precisely the third parameter, if present, is the text on the button or anchor which caused the jump to this page).

So the minimal application looks like this:

  use CGI::Screen;
  use vars qw(@ISA);
  @ISA = qw(CGI::Screen);
  my $query = __PACKAGE__->new;
  sub main_screen {
    my $query = shift;
    print $query->p('This is the Main Screen');

That is not too exciting. Let us add a second screen and allow navigation between the screens:

  sub main_screen {
    my $query = shift;
      $query->p('This is the Main Screen'),
      $query->goto_screen('second', 'Another Screen');
  sub second_screen {
    my $query = shift;
      $query->p('This is the Other Screen'),
      $query->goto_screen('main', 'Back to Main Screen');


Moving between screens

Use the method goto_screen to produce a button for switching to another screen. You can also produce an anchor instead of a button by calling link_to_screen instead of goto_screen. You may pass additional parameters to encode:

    for my $docid (keys %score) {
      print $query->link_to_screen('display', $title{$docid},
                                   'docid' => $docid,
                                   'score' => $score{$docid});

For convenience, CGI::Screen keeps track of the last screen for you so that you can link to the previous page. Note that only the last seven screens are saved:

  my $screen = $query->last_screen;
    $query->p("You came from screen $screen. Press "),
    $query->p(" to go back");

last_screen returns screen name and title in list context and screen name in scalar context. Do not use the CGI parameters screen_last_* since they are changed before you can get hold of them ;-P

The callbacks

All callbacks are called with three arguments: The query object, the screen name and the screen title (= button/anchor text). Callbacks should return a string.


The application method returns a string which is used in the default title and headline callbacks. The Default method returns the string "CGI::Screen Test" and should definitely be overwritten by your application.


The result of the method is used in the HTTP header and in the default headline. It defaults to the application.


The headline method should return a chunk of HTML code to start the Screen. It defaults to the title enclosed in H1 tags.


To enable password authentication, define a method check_auth_user. The dispatcher will call the method with the user and password entered by the user. The method should return true if the authentication succeeded and false otherwise. The dispatcher will present the login_screen if the authentication failed.

  sub check_auth_user {
    my ($query, $user, $passwd) = @_;

    $user eq 'pfeifer';

For IP address based authentication define the method check_auth_ip.

  sub check_auth_ip {
    my ($query, $ipaddr) = @_;

   $ipaddr =~ /^(193\.96\.65\.|139\.4\.36\.)/;

If you do not like the default login screen, overwrite with your own login_screen. Use the CGI parameters screen_user and screen_passwd.

Customizing the Title

You may provide a custom title method to generate a title for your screens.

  sub title {
    my ($query, $screen)  = shift;
    $query->application . ': ' . $screen;

Customizing the Headline

You may provide a custom headline method to generate a HTML chunk to start your screens.

  sub headline { $_[0]->h1(title(@_)) }

You should overwrite the application method if you use the default title and headline.

  sub application { 'CGI::Screen Test' }

Customizing the Trailer

For a custom Trailer, define the trailer method.

  sub trailer {
    my ($query, $screen)  = shift;
    "End of Screen $screen";

Multiple Forms

If you want to have multiple forms on one screen, call the method new_form.

  sub multi_screen {
     my $query = shift;

       $query->p('This is the Main Screen'),

Non HTML screens

You can create non HTML screens by defining a name_data method instead of a <name>_screen method. For data screens you have to generate HTTP headers yourself.

  sub gif_data {
    my $query = shift;
    print $query->header(
                         -type    => 'image/gif',
                         -status  => '200 OK',
                         -expires => '+120s',
    my $font  = $query->param('font');
    my $w     = GD::Font->$font()->width;
    my $h     = GD::Font->$font()->height;
    my $im    = GD::Image->new((length($query->param('foo'))+2)*$w,$h);
    my $white = $im->colorAllocate(255,255,255);
    my $red   = $im->colorAllocate(255,0,0);
    my $black = $im->colorAllocate(0,0,0);
    print $im->gif;

Keeping parameter values

CGI::Screen keeps track of the CGI parameters used in the current form. It simply looks at the first parameter in any call to a CGI method. If the first parameter is -name, the second parameter is marked as used parameter. CGI::Screen passed all current parameter values not used in hidden fields or in the query string of an anchor. So do not use old style CGI calls to bypass this mechanism or you will end up with multiple values for the parameters.

If you want to get rid of a parameter, you must explicitly call the delete method of CGI.


Support for importing from is incomplete.


Ulrich Pfeifer <>


The CGI(3) manual and the demo CGI script eg/screen included in the distribution.


I wish to thank Andreas Koenig for the fruitful discussion about the design of this module.


The CGI::Screen module is Copyright (c) 1997,1998 Ulrich Pfeifer. Germany. All rights reserved.

You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.

2 POD Errors

The following errors were encountered while parsing the POD:

Around line 499:

'=item' outside of any '=over'

Around line 516:

You forgot a '=back' before '=head2'