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Having arrived at this structure, we can now process it in one of two broad ways: Categorize and summarize the different types of elements making up the course. Apply the structure to the marks recorded for the students to get their results. Both applications involve the Visitor pattern: we set up the structure and then visit it to perform the operations. We ll focus on the first task here and consider the second in the discussion of the Interpreter pattern.
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Design
The Visitor pattern has two distinct parts: there are the classes that make up an object structure, and then there are the methods that will be applied to the object structure. These are the visitors. The UML for the pattern is given in Figure 10-3.
<<interface>> IVisitor
Visitor1 +VisitorMethod1( ) Client objectStructure : Element visitor1 : Visitor1 visitor2 : Visitor2 Element
Visitor2 +VisitorMethod2( )
ElementA
ElementB
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10: Behavioral Patterns: Visitor, Interpreter, and Memento
The Client defines and creates an object structure and populates it with data. This process is not part of the pattern, however. The object structure is specified by a hierarchy of Element classes, three of which are shown here: Element, ElementA, and ElementB. The Visitor methods, VisitorMethod1 and VisitorMethod2, are defined in separate classes. Objects of these classes conform to an IVisitor interface. Although the object structure and Visitors are separate, some implementations of the Visitor pattern might require a small change to the objects to make them visitor-ready. The players in the Visitor pattern are:
Client
A class that creates and maintains the object structure; instantiates and calls the Visitors
ObjectStructure
A composite structure of several related element types
Element
The root element type for the object structure
ElementA and ElementB Subclasses of Element IVisitor
An interface for all the Visitor classes
Visitor1 and Visitor2
Classes that contain methods that iterate through the object structure and apply operations to its elements Note that the object structure is not necessarily a class hierarchy. In general, one node in the object structure can refer to another object of any type whatsoever, and the visitor can traverse to that other object. The interesting issue with subclass relationships among the object structure nodes is whether the visitor actions can easily be inherited. The Visitor pattern is intended for object structures, so it is often used in conjunction with the Composite pattern. The Composite pattern is responsible for setting up the multilevel structure on which the Visitor pattern then operates.
Implementation
Because the Visitor pattern is one of those with a distinction between the data and some additional and extraneous behavior, we shall develop it in three parts: The object structure The visitor(s) The client
Visitor Pattern |
QUIZ
Match the Visitor Pattern Players with the Course Illustration
To test whether you understand the Visitor pattern, cover the lefthand column of the table below and see if you can identify its players among the items from the illustrative example (Figure 10-1), as shown in the righthand column. Then check your answers against the lefthand column.
Client ObjectStructure Element ElementA, ElementB VisitorA VisitorB
The instructor The rules of the course (taken from the notation) Course Lab, Theory, Midterm, Exam Print the course rules Summarize the Lab and Theory totals
We shall illustrate this development with a small theoretical example and show two ways of implementing visitors, before going on to the larger example. The small example sets up a structure in levels, and then the client asks the visitor to count the number of top-level nodes. One might imagine that it would be easy to add this functionality to the nodes themselves and simply traverse the structure. However, with the Visitor pattern, we can add new behavior after the object structure has effectively been sealed off. On the other side, the object structure might evolve, and we would like the visiting process to be unaffected. Although this small example does not show the power of the Visitor pattern, it does reveal the independence of the different players and how this independence reduces coupling. The program is shown in Example 10-1. It starts with setting up the object structure, in a namespace called ObjectStructure (lines 7 37). This is all fairly standard. The initializing of the structure is done in client on lines 70 82, and we can see that it has nine ordinary nodes of the type Element and two of the type ElementWithLink, which go down a next level. The only other business in the client is to instantiate the visitor, call it, and report on the number of elements (lines 84 87).
1 2 3 4 5 6 7 8 9 using System; using ObjectStructure; // Visitor Pattern - Example Judith Bishop October 2007 // Sets up an object structure and visits it namespace ObjectStructure { abstract class IElement {
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