CGI Application Modules for CPAN

CGI::Application is an abstract base class in object oriented terminology. This 

means it provides all its functionality by serving as a parent class. You can't use 

CGI::Application directly, you have to create a subclass. The start of Listing 11 1, 

placed in a file called BBS.pm, does just that:

package BBS;

use CGI::Application;

@ISA = qw(CGI::Application);

This creates a new module called BBS that inherits from CGI::Application.

Next, the class implements the one required method, setup(). The setup()

method is called from CGI::Application >new(). It is responsible for setting up the 

run modes for the class. Run modes provide a direct implementation of the events 

shown in the state machine for this application. They replace the if elsif structure 

of the earlier example.

The setup() method works by calling methods inherited from CGI::Application, 

specifically mode_param(), start_mode(), and run_modes():

sub setup {

  my $self = shift;

  $self >mode_param("rm");

  $self >start_mode("list");

  $self >run_modes(list  => "list",

                   save   => "save",

                   new    => "new_message",

                   read   => "read",

                   reply  => "reply",

                   search => "search");

}

The mode_param() method tells CGI::Application that the CGI parameter with the 

name rm will control the run mode of the program. This means that each HTML 

form sent to this CGI will have a parameter called rm set to a run mode. For example, 

this might take the form of a hidden input field:

When the form containing this tag is submitted, the application will enter the 

 save  run mode. By tracking the run mode in a single parameter, CGI::Application 

always knows which event is being called. In contrast to the heuristic if elsif struc 

ture seen earlier, this system is durable and simple to understand. CGI::Application 

simply looks up the value of the rm parameter in the table passed to run_modes()

and finds a method to call in the application class.

26

261

1