The Visitor design pattern represents an operation to be performed on the elements of an object structure. This pattern lets you define a new operation without changing the classes of the elements on which it operates.
A visualization of the classes and objects participating in this pattern.
The classes and objects participating in this pattern include:
Visitor)
IncomeVisitor, VacationVisitor)
Element)
Employee)
Employees)
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.
copyusing System;
using System.Collections.Generic;
namespace Visitor.Structural
{
/// <summary>
/// Visitor Design Pattern
/// </summary>
public class Program
{
public static void Main(string[] args)
{
// 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>
public abstract class Visitor
{
public abstract void VisitConcreteElementA(
ConcreteElementA concreteElementA);
public abstract void VisitConcreteElementB(
ConcreteElementB concreteElementB);
}
/// <summary>
/// A 'ConcreteVisitor' class
/// </summary>
public 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>
public 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>
public abstract class Element
{
public abstract void Accept(Visitor visitor);
}
/// <summary>
/// A 'ConcreteElement' class
/// </summary>
public class ConcreteElementA : Element
{
public override void Accept(Visitor visitor)
{
visitor.VisitConcreteElementA(this);
}
public void OperationA()
{
}
}
/// <summary>
/// A 'ConcreteElement' class
/// </summary>
public class ConcreteElementB : Element
{
public override void Accept(Visitor visitor)
{
visitor.VisitConcreteElementB(this);
}
public void OperationB()
{
}
}
/// <summary>
/// The 'ObjectStructure' class
/// </summary>
public class ObjectStructure
{
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);
}
}
}
}
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.
using System;
using System.Collections.Generic;
namespace Visitor.RealWorld
{
/// <summary>
/// Visitor Design Pattern
/// </summary>
public class Program
{
public static void Main(string[] args)
{
// Setup employee collection
Employees employee = new Employees();
employee.Attach(new Clerk());
employee.Attach(new Director());
employee.Attach(new President());
// Employees are 'visited'
employee.Accept(new IncomeVisitor());
employee.Accept(new VacationVisitor());
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Visitor' interface
/// </summary>
public interface IVisitor
{
void Visit(Element element);
}
/// <summary>
/// A 'ConcreteVisitor' class
/// </summary>
public class IncomeVisitor : IVisitor
{
public void Visit(Element element)
{
Employee employee = element as Employee;
// Provide 10% pay raise
employee.Income *= 1.10;
Console.WriteLine("{0} {1}'s new income: {2:C}",
employee.GetType().Name, employee.Name,
employee.Income);
}
}
/// <summary>
/// A 'ConcreteVisitor' class
/// </summary>
public class VacationVisitor : IVisitor
{
public void Visit(Element element)
{
Employee employee = element as Employee;
// Provide 3 extra vacation days
employee.VacationDays += 3;
Console.WriteLine("{0} {1}'s new vacation days: {2}",
employee.GetType().Name, employee.Name,
employee.VacationDays);
}
}
/// <summary>
/// The 'Element' abstract class
/// </summary>
public abstract class Element
{
public abstract void Accept(IVisitor visitor);
}
/// <summary>
/// The 'ConcreteElement' class
/// </summary>
public class Employee : Element
{
private string name;
private double income;
private int vacationDays;
// Constructor
public Employee(string name, double income,
int vacationDays)
{
this.name = name;
this.income = income;
this.vacationDays = vacationDays;
}
public string Name
{
get { return name; }
set { name = value; }
}
public double Income
{
get { return income; }
set { income = value; }
}
public int VacationDays
{
get { return vacationDays; }
set { vacationDays = value; }
}
public override void Accept(IVisitor visitor)
{
visitor.Visit(this);
}
}
/// <summary>
/// The 'ObjectStructure' class
/// </summary>
public class Employees
{
private List<Employee> employees = new List<Employee>();
public void Attach(Employee employee)
{
employees.Add(employee);
}
public void Detach(Employee employee)
{
employees.Remove(employee);
}
public void Accept(IVisitor visitor)
{
foreach (Employee employee in employees)
{
employee.Accept(visitor);
}
Console.WriteLine();
}
}
// Three employee types
public class Clerk : Employee
{
// Constructor
public Clerk()
: base("Kevin", 25000.0, 14)
{
}
}
public class Director : Employee
{
// Constructor
public Director()
: base("Elly", 35000.0, 16)
{
}
}
public class President : Employee
{
// Constructor
public President()
: base("Eric", 45000.0, 21)
{
}
}
}
The .NET optimized code demonstrates the
same real-world situation as above but uses modern, built-in .NET features,
such as, generics, reflection, LINQ, lambda functions, etc.
You can find an example on our Singleton pattern page.
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