NAME

       SOAP::WSDL::Generator::Visitor - SOAP::WSDL's Visitor-based Code Generator

DESCRIPTION

       SOAP::WSDL features a code generating facility. This code generation facility (in fact there are several
       of them) is implemented as Visitor to SOAP::WSDL::Base-derived objects.

   The Visitor Pattern
       The Visitor design pattern is one of the object oriented design pattern described by [GHJV1995].

       A Visitor is an object implementing some behaviour for a fixed set of classes, whose implementation would
       otherwise need to be scattered across those classes' implementations.

       Visitors are usually combined with Iterators for traversing either a list or tree of objects.

       A Visitor's methods are called using the so-called double dispatch technique.  To allow double
       dispatching, the Visitor implements one method for every class to be handled, whereas every class
       implements just one method (commonly named "access"), which does nothing more than calling a method on
       the reference given, with the self object as parameter.

       If all this sounds strange, maybe an example helps. Imagine you had a list of person objects and wanted
       to print out a list of their names (or address stamps or everything else you like). This can easily be
       implemented with a Visitor:

           package PersonVisitor;
           use Class::Std;    # handles all basic stuff like constructors etc.

           sub visit_Person {
               my ( $self, $object ) = @_;
               print "Person name is ", $object->get_name(), "\n";
           }

           package Person;
           use Class::Std;
           my %name : ATTR(:name<name> :default<anonymous>);

           sub accept { $_[1]->visit_Person( $_[0] ) }

           package main;
           my @person_from = ();
           for (qw(Gamma Helm Johnson Vlissides)) {
               push @person_from, Person->new( { name => $_ } );
           }

           my $visitor = PersonVisitor->new();
           for (@person_from) {
               $_->accept($visitor);
           }

           # will print
           Person name is Gamma
           Person name is Helm
           Person name is Johnson
           Person name is Vlissides

       While using this pattern for just printing a list may look a bit over-sized, it may become handy if you
       need multiple output formats and different classes to operate on.

       The main benefits using visitors are:

       •   Grouping related behaviour in one class

           Related behaviour for several classes can be grouped together in the Visitor class. The behaviour can
           easily be changed by changing the code in one class, instead of having to change all the visited
           classes.

       •   Cleaning up the data classes' implementations

           If classes holding data also implement several different output formats or other (otherwise
           unrelated) behaviour, they tend to get bloated.

       •   Adding behaviour is easy

           Swapping out the visitor class allows easy alterations of behaviour. So on a list of Persons, one
           Visitor may print address stamps, while another one prints out a phone number list.

       Of course, there are also drawbacks in the visitor pattern:

       •   Changes in the visited classes are expensive

           If one of the visited classes changes (or is added), all visitors must be updated to reflect this
           change. This may be rather expensive if classes change often.

       •   The visited classes must expose all data required

           Visitors may need to use the internals of a class. This may result in fidelling with a object's
           internals, or a bloated interface in the visited class.

       Visitors are usually accompanied by a Iterator. The Iterator may be implemented in the visited classes,
       in the Visitor, or somewhere else (in the example it was somewhere else).

       The Iterator decides which object to visit next.

   Why SOAP::WSDL uses the Visitor pattern for Code Generation
       Code generation in SOAP::WSDL means generating various artefacts:

       •   Typemaps

           For every WSDL definition, a Typemap is created. The Typemap is used later as an aid in parsing the
           SOAP XML messages.

       •   Type Classes

           For every type defined in the WSDL's schema, a Type Class is generated.

           These classes are instantiated later as a result of parsing SOAP XML messages.

       •   Interface Classes

           For every service, a interface class is generated. This class is later used by programmers accessing
           the service

       •   Documentation

           Both Type Classes and Interface Classes include documentation. Additional documentation may be
           generated as a hint for programmers, or later for mimicking .NET's .asmx example pages.

       All these behaviours could well be (and have historically been) implemented in the classes holding the
       WSDL data. This made these classes rather bloated, and made it hard to change behaviour (like supporting
       SOAP Headers, supporting atomic types, and other features which were missing from early versions of
       SOAP::WSDL).

       Implementing these behaviours in Visitor classes eases adding new behaviours, and reducing the
       incompletenesses still inherent in SOAP::WSDL's WSDL and XML schema implementation.

   Implementation
       accept

       SOAP::WSDL::Base defines an accept method which expects a Visitor as only parameter.

       The method visit_Foo_Bar is called on the visitor, with the self object as parameter.

       The actual method name is constructed this way:

       •   SOAP::WSDL is stripped from the class name

       •   All remaining  :: s are replaced by _

       Example:

       When visiting a SOAP::WSDL::XSD::ComplexType object, the method visit_XSD_ComplexType is called on the
       visitor.

   Writing your own visitor
       SOAP::WSDL eases writing your own visitor. This might be required if you need some special output format
       from a WSDL file or want to feed your own serializer/deserializer pair with custom configuration data. Or
       maybe you want to generate C# code from it...

       To write your own code generating visitor, you should subclass SOAP::WSDL::Generator::Visitor. It
       implements (empty) default methods for all SOAP::WSDL data classes:

       •   visit_Definitions

       •   visit_Binding

       •   visit_Message

       •   visit_Operation

       •   visit_OpMessage

       •   visit_Part

       •   visit_Port

       •   visit_PortType

       •   visit_Service

       •   visit_SoapOperation

       •   visit_Types

       •   visit_XSD_Schema

       •   visit_XSD_ComplexType

       •   visit_XSD_Element

       •   visit_XSD_SimpleType

       In your Visitor, you must implement visit_Foo methods for all classes you wish to visit.

       The SOAP::WSDL::Generator::Visitor implementations include part of their own Iterator (which means they
       know how to find the next objects to visit). You may or may not choose to implement a separate Iterator.

       Letting a visitor implementing its own Iterator visit a WSDL definition is as easy as writing something
       like this:

        my $visitor = MyVisitor->new();
        my $parser = SOAP::WSDL::Expat::WSDLParser->new();
        my $definitions = $parser->parse_file('my.wsdl'):

        $definitions->_accept( $visitor );

       If you need an iterator following the somewhat crude path of dependencies in a WSDL1.1 definition, you
       might want to look at SOAP::WSDL::Generator::Iterator::WSDL11.

REFERENCES

       •   [GHJV1995]

           Erich Gamma, Richard Helm, Ralph E. Johnson, John Vlissides, (1995): Design Patterns. Elements of
           Reusable Object-Oriented Software.  Addison-Wesley Longman, Amsterdam.

LICENSE AND COPYRIGHT

       Copyright 2004-2008 Martin Kutter.

       This file is part of SOAP-WSDL. You may distribute/modify it under the same terms as perl itself

AUTHOR

       Martin Kutter <martin.kutter fen-net.de>

REPOSITORY INFORMATION

        $Rev: 391 $
        $LastChangedBy: kutterma $
        $Id: Client.pm 391 2007-11-17 21:56:13Z kutterma $
        $HeadURL: https://soap-wsdl.svn.sourceforge.net/svnroot/soap-wsdl/SOAP-WSDL/trunk/lib/SOAP/WSDL/Client.pm $