Chapter 11

The next call, to start_mode(), sets  list  as the default run mode. When the 

CGI is called without an rm parameter set, the run mode will be  list.  This is gen 

erally what happens when the user first hits the application which is why it's 

called start_mode() and not default_mode().

Finally, run_modes() sets up the run mode table. The keys are names of run 

modes and the values are method names. Notice that the application defines a run 

mode named  new  but uses the method name new_message(). Using new() would 

have caused problems since CGI::Application defines a new() method already and 

the BBS class is derived from CGI::Application.

After that, the class implements each of its run modes as a separate method. 

For example, the list() method looks like this:

# show list of messages

sub list {

   my $self = shift;

   my $query = $self >query;

   my $output;

   # ...

   return $output;

}

Since run modes are methods, they receive their object as the first parameter 

($self). The inherited query() method is used to create and return a CGI.pm 

object. As a result, the internals of the function may be very similar to the pure 

CGI.pm implementation shown earlier. However, there is one major difference 

CGI::Application run mode methods must never print directly to STDOUT. Instead, 

they return their output.

CAUTION Never print to STDOUT from a CGI::Application run mode. All 

output must be returned as a string.

Since all run modes are methods, transferring control from one to another is 

easy. For example, the save() method is intended to show the list of messages 

when it finishes saving the message. To do this it just calls list() at the end of the 

method and returns the result:

262