主要用途是对聚集采取某种操作，统一执行。
访问者模式适用于数据结构相对未定的系统，它把数据结构和作用于结构上的操作之间的耦合解脱开，使得操作集合可以相对自由地演化。
比如雇员对象，我们先只有名称、年龄、工资这些属性，可以先定义访问者类来分离结构和操作
participants
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
using System;
using System.Collections.Generic;
namespace DoFactory.GangOfFour.Visitor.RealWorld
{
///
/// MainApp startup class for Real-World
/// Visitor Design Pattern.
///
class MainApp
{
///
/// Entry point into console application.
///
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.ReadKey();
}
}
///
/// The 'Visitor' interface
///
interface IVisitor
{
void Visit(Element element);
}
///
/// A 'ConcreteVisitor' class
///
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);
}
}
///
/// A 'ConcreteVisitor' class
///
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);
}
}
///
/// The 'Element' abstract class
///
abstract class Element
{
public abstract void Accept(IVisitor visitor);
}
///
/// The 'ConcreteElement' class
///
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;
}
// Gets or sets the name
public string Name
{
get { return _name; }
set { _name = value; }
}
// Gets or sets income
public double Income
{
get { return _income; }
set { _income = value; }
}
// Gets or sets number of vacation days
public int VacationDays
{
get { return _vacationDays; }
set { _vacationDays = value; }
}
public override void Accept(IVisitor visitor)
{
visitor.Visit(this);
}
}
///
/// The 'ObjectStructure' class
///
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 e in _employees)
{
e.Accept(visitor);
}
Console.WriteLine();
}
}
// Three employee types
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)
{
}
}
}