主要用途是對聚集採取某種操作，統一執行。
訪問者模式適用於資料結構相對未定的系統，它把資料結構和作用於結構上的操作之間的耦合解脫開，使得操作集合可以相對自由地演化。
比如雇員物件，我們先只有名稱、年齡、工資這些屬性，可以先定義訪問者類來分離結構和操作
the classes and/or objects participating in this pattern are:
concretevisitor(incomevisitor, vacationvisitor)
element(element)
concreteelement(employee)
objectstructure(employees)
using system;
using system.collections.generic;
namespace dofactory.gangoffour.visitor.realworld
///
//////
visitor design pattern.
///class
//////
///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' inte***ce
///inte***ce
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(" 's new income: ",
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(" 's new vacation days: ",
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
set
// gets or sets income
public
double income
get
set
// gets or sets number of vacation days
public
int vacationdays
get
set
public
override
void accept(ivisitor visitor)
visitor.visit(this);
///
///the 'objectstructure' class
///class
employees
private
list
_employees = new
list
();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)
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