DLPS Classes

Subclassing in TextClass

DLXS middleware is written in Object-Oriented Perl. There are superclasses (or base classes) of objects that have basic member data and run generally applicable methods. Collection specific tailoring can be accomplished for collections that require different behavior . Many look and feel issues can be handled by collection HTML specific templates.

However, different searching or different display, will require some Perl code modification. This is accomplished through subclassing.

This section will cover how to make a collection specific subclass of TextClass.pm so that we can change some SGML to HTML filtering.

The general idea is the following:

  1. Make a subclass of TextClass.pm. By convention we name TextClass subclasses based on the collection id. For the purposes of this discussion, if your collection id is "digbooks", the subclass module would be called "DigbooksTC.pm". This file should be in the $DLXSROOT/cgi/t/text/TextClass subdirectory.
  2. The subclass module needs to have certain Perl code early on that makes this file a subclass of TextClass.pm. See below.
  3. In the collmgr, enter "TextClass/DigbooksTC" in the subclassmodule field. This will tell the middleware to use the code in this module when possible for this collection.

The easiest way to create a TextClass subclass module is to copy an existing subclass module. The simplest version is SampleTC.pm.


package SampleTC;
       # enable strict under development
       if ( $ENV{'DLPS_DEV'} )
           require "strict.pm";
   use TextClass ();
   use TextClassUtils ();
   use DlpsUtils qw( :DEFAULT );

   use Foo;     # Remove this line or change to "use" other subclass needed modules

   # Subclass of TextClass.pm module
   use vars qw( @ISA );   
   @ISA = qw( TextClass );
   ## **********************************************************************
   ## this is a subclass of TextClass
   ## for different search and filtering behavior.  
   ## The intent with "Sample" is to provide a template to guide subclass
   ## creation
   # ----------------------------------------------------------------------
   # NAME      : new (Defaults to BaseClass::new
   # PURPOSE   : create new SampleTC object
   # CALLED BY : main
   # CALLS     : SampleTC->_initialize
   # INPUT     : see TextClass::_initialize
   # RETURNS   : 
   # NOTES     :
   # ----------------------------------------------------------------------
   # ----------------------------------------------------------------------
   # NAME      : _initialize
   # PURPOSE   : create structure for TextClass object
   # CALLED BY : new
   # CALLS     :
   # INPUT     : see new
   # RETURNS   :
   # NOTES     :
   # ----------------------------------------------------------------------
   sub _initialize
       my $self = shift;
       $self->SUPER::_initialize( @_ );
   ## ----------------------------------------------------------------------

After copying the file, you will need to change the package name to match the collection. In the example case,

package DigbooksTC;

You can replace "use Foo;" with any "use" statements you might need, that the base class (TextClass.pm) doesn't already "use."

The code that refers to the @ISA array is needed to tell Perl what base class this module is a subclass of. This does not need changing.

Now, any method that you create here that has the same name as a method in the base class (TextClass.pm) will be run instead of the method in the base class.

Any method you create here that does not exist in the base class can also be run by the middleware when dealing with this collection.

Of course, when writing methods, the first parameter passed in, implicitly, is the object itself. So, "$self = shift;" should always be the first line in any class's method.

Finally, at any time, you can call "$self->SUPER::NameOfMethodInBaseClass( )". This will run the method that is defined in the base class. Often it is convenient to create a method by the same name as the base class's method, run the SUPER's version of it and then add some more code. Or vice versa; that is, run some code and then call SUPER's method.

Here is an example of method overriding from NietzscheTC.pm. You can see that the TextFilter method is short. First it runs another method called FilterMilestones (one that exists only in this subclass, not in the base class code) and then runs SUPER's version of TextFilter. In this case, it was important to modify the SGML to be filtered (in $sRef) before the usual filtering was done. In other cases, one might want to run the superclass's filtering first and then change something in that result, in which case, SUPER::TextFilter would be called before the other subclass-specific filtering.

   # ----------------------------------------------------------------------
   # NAME         : TextFilter
   # PURPOSE      :
   #              :
   # CALLED BY    : ScopedResultsFilter
   # CALLS        : SUPER, and other methods if needed, be they
   #              : in superclass or in this subclass
   # INPUT        : $sRef (reference to search result string
   # RETURNS      : $results (filtered)
   # GLOBALS      : NONE
   # NOTES        : 
   # ----------------------------------------------------------------------
   sub TextFilter
       my $self = shift;
       my ( $sRef, $collid, $idno, $cgi, $dso, $iel ) = @_;

       # &FilterMilestones( $sRef );   # example of call to any other methods needed

       my $results = $self->SUPER::TextFilter( @_ );
       return $results;
   # ----------------------------------------------------------------------
   # NAME         : FilterMilestones
   # PURPOSE      :
   # CALLED BY    : SubClass::TextFilter
   # CALLS        :
   # INPUT        :
   # RETURNS      :
   # GLOBALS      :
   # NOTES        : To filter this collection's very specific MILESTONE tag
   #              :        syntax, e.g.:
   #              : <MILESTONE UNIT="Aphorism" ED="kgw" N="III-3.9"> <MILESTONE 
   #              : UNIT="Aphorism" ED="ksa" N="7.13">
   # ----------------------------------------------------------------------
   sub FilterMilestones
       my $sRef = shift;
       # ignore/remove all "page" type MILESTONEs
       $$sRef =~ s|<MILESTONE UNIT=\"page\".*?>||gs;
       # handle other types
       $$sRef =~ s|<MILESTONE UNIT=\".*?\" ED=\"(.*?)\" N=\"(.*?)\">|<br>\n\[$1 ed\., $2\]|gs;

Remember that, for this collection to use this subclassed code, you need to update the subclassmodule field for this collection's entry in the Collection Manager.