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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
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The classes and/or objects participating in this pattern are:
- Visitor (Visitor)
- declares a Visit operation for each class of ConcreteElement in the object
structure. The operation's name and signature identifies the class that sends
the Visit request to the visitor. That lets the visitor determine the concrete
class of the element being visited. Then the visitor can access the elements
directly through its particular interface
- ConcreteVisitor (IncomeVisitor, VacationVisitor)
- implements each operation declared by Visitor. Each operation implements
a fragment of the algorithm defined for the corresponding class or object in the
structure. ConcreteVisitor provides the context for the algorithm and stores its
local state. This state often accumulates results during the traversal of the
structure.
- Element (Element)
- defines an Accept operation that takes a visitor as an argument.
- ConcreteElement (Employee)
- implements an Accept operation that takes a visitor as an argument
- ObjectStructure (Employees)
- can enumerate its elements
- may provide a high-level interface to
allow the visitor to visit its elements
- may either be a Composite (pattern) or a collection such as a list or a set
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.
Show code
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// Visitor pattern -- Structural example
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using System;
using System.Collections;
namespace DoFactory.GangOfFour.Visitor.Structural
{
// MainApp test application
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.Read();
}
}
// "Visitor"
abstract class Visitor
{
public abstract void VisitConcreteElementA(
ConcreteElementA concreteElementA);
public abstract void VisitConcreteElementB(
ConcreteElementB concreteElementB);
}
// "ConcreteVisitor1"
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);
}
}
// "ConcreteVisitor2"
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);
}
}
// "Element"
abstract class Element
{
public abstract void Accept(Visitor visitor);
}
// "ConcreteElementA"
class ConcreteElementA : Element
{
public override void Accept(Visitor visitor)
{
visitor.VisitConcreteElementA(this);
}
public void OperationA()
{
}
}
// "ConcreteElementB"
class ConcreteElementB : Element
{
public override void Accept(Visitor visitor)
{
visitor.VisitConcreteElementB(this);
}
public void OperationB()
{
}
}
// "ObjectStructure"
class ObjectStructure
{
private ArrayList elements = new ArrayList();
public void Attach(Element element)
{
elements.Add(element);
}
public void Detach(Element element)
{
elements.Remove(element);
}
public void Accept(Visitor visitor)
{
foreach (Element e in elements)
{
e.Accept(visitor);
}
}
}
}
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Output
ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1
ConcreteElementA visited by ConcreteVisitor2
ConcreteElementB visited by ConcreteVisitor2
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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.
Show code
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// Visitor pattern -- Real World example
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using System;
using System.Collections;
namespace DoFactory.GangOfFour.Visitor.RealWorld
{
// MainApp startup application
class MainApp
{
static void Main()
{
// Setup employee collection
Employees e = new Employees();
e.Attach(new Clerk());
e.Attach(new Director());
e.Attach(new President());
// Employees are 'visited'
e.Accept(new IncomeVisitor());
e.Accept(new VacationVisitor());
// Wait for user
Console.Read();
}
}
// "Visitor"
interface IVisitor
{
void Visit(Element element);
}
// "ConcreteVisitor1"
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);
}
}
// "ConcreteVisitor2"
class VacationVisitor : IVisitor
{
public void Visit(Element element)
{
Employee employee = element as Employee;
// Provide 3 extra vacation days
Console.WriteLine("{0} {1}'s new vacation days: {2}",
employee.GetType().Name, employee.Name,
employee.VacationDays);
}
}
class Clerk : Employee
{
// Constructor
public Clerk() : base("Hank", 25000.0, 14)
{
}
}
class Director : Employee
{
// Constructor
public Director() : base("Elly", 35000.0, 16)
{
}
}
class President : Employee
{
// Constructor
public President() : base("Dick", 45000.0, 21)
{
}
}
// "Element"
abstract class Element
{
public abstract void Accept(IVisitor visitor);
}
// "ConcreteElement"
class Employee : Element
{
string name;
double income;
int vacationDays;
// Constructor
public Employee(string name, double income,
int vacationDays)
{
this.name = name;
this.income = income;
this.vacationDays = vacationDays;
}
// Properties
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);
}
}
// "ObjectStructure"
class Employees
{
private ArrayList employees = new ArrayList();
public void Attach(Employee employee)
{
employees.Add(employee);
}
public void Detach(Employee employee)
{
employees.Remove(employee);
}
public void Accept(IVisitor visitor)
{
foreach (Employee e in employees)
{
e.Accept(visitor);
}
Console.WriteLine();
}
}
}
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Output
Clerk Hank's new income: $27,500.00
Director Elly's new income: $38,500.00
President Dick's new income: $49,500.00
Clerk Hank's new vacation days: 14
Director Elly's new vacation days: 16
President Dick's new vacation days: 21
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This .NET optimized code demonstrates the
same real-world situation as above but uses modern, built-in .NET features.
Show code
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// Visitor pattern -- .NET optimized
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See our Singleton page for a .NET optimized code sample.
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