1SOAP::WSDL::Generator::UVsiesritCoorn(t3r)ibuted Perl DoScOuAmPe:n:tWaStDiLo:n:Generator::Visitor(3)
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NAME

6       SOAP::WSDL::Generator::Visitor - SOAP::WSDL's Visitor-based Code
7       Generator
8

DESCRIPTION

10       SOAP::WSDL features a code generating facility. This code generation
11       facility (in fact there are several of them) is implemented as Visitor
12       to SOAP::WSDL::Base-derived objects.
13
14   The Visitor Pattern
15       The Visitor design pattern is one of the object oriented design pattern
16       described by [GHJV1995].
17
18       A Visitor is an object implementing some behaviour for a fixed set of
19       classes, whose implementation would otherwise need to be scattered
20       across those classes' implementations.
21
22       Visitors are usually combined with Iterators for traversing either a
23       list or tree of objects.
24
25       A Visitor's methods are called using the so-called double dispatch
26       technique.  To allow double dispatching, the Visitor implements one
27       method for every class to be handled, whereas every class implements
28       just one method (commonly named "access"), which does nothing more than
29       calling a method on the reference given, with the self object as
30       parameter.
31
32       If all this sounds strange, maybe an example helps. Imagine you had a
33       list of person objects and wanted to print out a list of their names
34       (or address stamps or everything else you like). This can easily be
35       implemented with a Visitor:
36
37           package PersonVisitor;
38           use Class::Std;    # handles all basic stuff like constructors etc.
39
40           sub visit_Person {
41               my ( $self, $object ) = @_;
42               print "Person name is ", $object->get_name(), "\n";
43           }
44
45           package Person;
46           use Class::Std;
47           my %name : ATTR(:name<name> :default<anonymous>);
48
49           sub accept { $_[1]->visit_Person( $_[0] ) }
50
51           package main;
52           my @person_from = ();
53           for (qw(Gamma Helm Johnson Vlissides)) {
54               push @person_from, Person->new( { name => $_ } );
55           }
56
57           my $visitor = PersonVisitor->new();
58           for (@person_from) {
59               $_->accept($visitor);
60           }
61
62           # will print
63           Person name is Gamma
64           Person name is Helm
65           Person name is Johnson
66           Person name is Vlissides
67
68       While using this pattern for just printing a list may look a bit over-
69       sized, it may become handy if you need multiple output formats and
70       different classes to operate on.
71
72       The main benefits using visitors are:
73
74       ·   Grouping related behaviour in one class
75
76           Related behaviour for several classes can be grouped together in
77           the Visitor class. The behaviour can easily be changed by changing
78           the code in one class, instead of having to change all the visited
79           classes.
80
81       ·   Cleaning up the data classes' implementations
82
83           If classes holding data also implement several different output
84           formats or other (otherwise unrelated) behaviour, they tend to get
85           bloated.
86
87       ·   Adding behaviour is easy
88
89           Swapping out the visitor class allows easy alterations of
90           behaviour. So on a list of Persons, one Visitor may print address
91           stamps, while another one prints out a phone number list.
92
93       Of course, there are also drawbacks in the visitor pattern:
94
95       ·   Changes in the visited classes are expensive
96
97           If one of the visited classes changes (or is added), all visitors
98           must be updated to reflect this change. This may be rather
99           expensive if classes change often.
100
101       ·   The visited classes must expose all data required
102
103           Visitors may need to use the internals of a class. This may result
104           in fidelling with a object's internals, or a bloated interface in
105           the visited class.
106
107       Visitors are usually accompanied by a Iterator. The Iterator may be
108       implemented in the visited classes, in the Visitor, or somewhere else
109       (in the example it was somewhere else).
110
111       The Iterator decides which object to visit next.
112
113   Why SOAP::WSDL uses the Visitor pattern for Code Generation
114       Code generation in SOAP::WSDL means generating various artefacts:
115
116       ·   Typemaps
117
118           For every WSDL definition, a Typemap is created. The Typemap is
119           used later as an aid in parsing the SOAP XML messages.
120
121       ·   Type Classes
122
123           For every type defined in the WSDL's schema, a Type Class is
124           generated.
125
126           These classes are instantiated later as a result of parsing SOAP
127           XML messages.
128
129       ·   Interface Classes
130
131           For every service, a interface class is generated. This class is
132           later used by programmers accessing the service
133
134       ·   Documentation
135
136           Both Type Classes and Interface Classes include documentation.
137           Additional documentation may be generated as a hint for
138           programmers, or later for mimicking .NET's .asmx example pages.
139
140       All these behaviours could well be (and have historically been)
141       implemented in the classes holding the WSDL data. This made these
142       classes rather bloated, and made it hard to change behaviour (like
143       supporting SOAP Headers, supporting atomic types, and other features
144       which were missing from early versions of SOAP::WSDL).
145
146       Implementing these behaviours in Visitor classes eases adding new
147       behaviours, and reducing the incompletenesses still inherent in
148       SOAP::WSDL's WSDL and XML schema implementation.
149
150   Implementation
151       accept
152
153       SOAP::WSDL::Base defines an accept method which expects a Visitor as
154       only parameter.
155
156       The method visit_Foo_Bar is called on the visitor, with the self object
157       as parameter.
158
159       The actual method name is constructed this way:
160
161       ·   SOAP::WSDL is stripped from the class name
162
163       ·   All remaining  :: s are replaced by _
164
165       Example:
166
167       When visiting a SOAP::WSDL::XSD::ComplexType object, the method
168       visit_XSD_ComplexType is called on the visitor.
169
170   Writing your own visitor
171       SOAP::WSDL eases writing your own visitor. This might be required if
172       you need some special output format from a WSDL file or want to feed
173       your own serializer/deserializer pair with custom configuration data.
174       Or maybe you want to generate C# code from it...
175
176       To write your own code generating visitor, you should subclass
177       SOAP::WSDL::Generator::Visitor. It implements (empty) default methods
178       for all SOAP::WSDL data classes:
179
180       ·   visit_Definitions
181
182       ·   visit_Binding
183
184       ·   visit_Message
185
186       ·   visit_Operation
187
188       ·   visit_OpMessage
189
190       ·   visit_Part
191
192       ·   visit_Port
193
194       ·   visit_PortType
195
196       ·   visit_Service
197
198       ·   visit_SoapOperation
199
200       ·   visit_Types
201
202       ·   visit_XSD_Schema
203
204       ·   visit_XSD_ComplexType
205
206       ·   visit_XSD_Element
207
208       ·   visit_XSD_SimpleType
209
210       In your Visitor, you must implement visit_Foo methods for all classes
211       you wish to visit.
212
213       The SOAP::WSDL::Generator::Visitor implementations include part of
214       their own Iterator (which means they know how to find the next objects
215       to visit). You may or may not choose to implement a separate Iterator.
216
217       Letting a visitor implementing its own Iterator visit a WSDL definition
218       is as easy as writing something like this:
219
220        my $visitor = MyVisitor->new();
221        my $parser = SOAP::WSDL::Expat::WSDLParser->new();
222        my $definitions = $parser->parse_file('my.wsdl'):
223
224        $definitions->_accept( $visitor );
225
226       If you need an iterator following the somewhat crude path of
227       dependencies in a WSDL1.1 definition, you might want to look at
228       SOAP::WSDL::Generator::Iterator::WSDL11.
229

REFERENCES

231       ·   [GHJV1995]
232
233           Erich Gamma, Richard Helm, Ralph E. Johnson, John Vlissides,
234           (1995): Design Patterns. Elements of Reusable Object-Oriented
235           Software.  Addison-Wesley Longman, Amsterdam.
236
238       Copyright 2004-2008 Martin Kutter.
239
240       This file is part of SOAP-WSDL. You may distribute/modify it under the
241       same terms as perl itself
242

AUTHOR

244       Martin Kutter <martin.kutter fen-net.de>
245

REPOSITORY INFORMATION

247        $Rev: 391 $
248        $LastChangedBy: kutterma $
249        $Id: Client.pm 391 2007-11-17 21:56:13Z kutterma $
250        $HeadURL: https://soap-wsdl.svn.sourceforge.net/svnroot/soap-wsdl/SOAP-WSDL/trunk/lib/SOAP/WSDL/Client.pm $
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254perl v5.32.0                      2020-07-28 SOAP::WSDL::Generator::Visitor(3)
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