Visitor Design Pattern
definition
Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
Frequency of use: low
sample code in C#
This structural code demonstrates the Visitor pattern in which an object traverses an object structure and performs the same operation on each node in this structure. Different visitor objects define different operations.
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// Visitor pattern — Structural example
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using System; using System.Collections.Generic;
namespace DoFactory.GangOfFour.Visitor.Structural { /// <summary> /// MainApp startup class for Structural /// Visitor Design Pattern. /// </summary> class MainApp { static void Main() { // Setup structure ObjectStructure o = new ObjectStructure(); o.Attach(new ConcreteElementA()); o.Attach(new ConcreteElementB());
// Create visitor objects ConcreteVisitor1 v1 = new ConcreteVisitor1(); ConcreteVisitor2 v2 = new ConcreteVisitor2();
// Structure accepting visitors o.Accept(v1); o.Accept(v2);
// Wait for user Console.ReadKey(); } }
/// <summary> /// The ‘Visitor’ abstract class /// </summary> abstract class Visitor { public abstract void VisitConcreteElementA( ConcreteElementA concreteElementA); public abstract void VisitConcreteElementB( ConcreteElementB concreteElementB); }
/// <summary> /// A ‘ConcreteVisitor’ class /// </summary> class ConcreteVisitor1 : Visitor { public override void VisitConcreteElementA( ConcreteElementA concreteElementA) { Console.WriteLine(“{0} visited by {1}”, concreteElementA.GetType().Name, this.GetType().Name); }
public override void VisitConcreteElementB( ConcreteElementB concreteElementB) { Console.WriteLine(“{0} visited by {1}”, concreteElementB.GetType().Name, this.GetType().Name); } }
/// <summary> /// A ‘ConcreteVisitor’ class /// </summary> class ConcreteVisitor2 : Visitor { public override void VisitConcreteElementA( ConcreteElementA concreteElementA) { Console.WriteLine(“{0} visited by {1}”, concreteElementA.GetType().Name, this.GetType().Name); }
public override void VisitConcreteElementB( ConcreteElementB concreteElementB) { Console.WriteLine(“{0} visited by {1}”, concreteElementB.GetType().Name, this.GetType().Name); } }
/// <summary> /// The ‘Element’ abstract class /// </summary> abstract class Element { public abstract void Accept(Visitor visitor); }
/// <summary> /// A ‘ConcreteElement’ class /// </summary> class ConcreteElementA : Element { public override void Accept(Visitor visitor) { visitor.VisitConcreteElementA(this); }
public void OperationA() { } }
/// <summary> /// A ‘ConcreteElement’ class /// </summary> class ConcreteElementB : Element { public override void Accept(Visitor visitor) { visitor.VisitConcreteElementB(this); }
public void OperationB() { } }
/// <summary> /// The ‘ObjectStructure’ class /// </summary> class ObjectStructure { private List<Element> _elements = new List<Element>();
public void Attach(Element element) { _elements.Add(element); }
public void Detach(Element element) { _elements.Remove(element); }
public void Accept(Visitor visitor) { foreach (Element element in _elements) { element.Accept(visitor); } } } } |
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Output
ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1 ConcreteElementA visited by ConcreteVisitor2 ConcreteElementB visited by ConcreteVisitor2 |
This real-world code demonstrates the Visitor pattern in which two objects traverse a list of Employees and performs the same operation on each Employee. The two visitor objects define different operations — one adjusts vacation days and the other income.
