Visitor Design Pattern - Behavioral

1.1     Intent

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.

Separates the structure of an object from the operations that act on the structure.

1.2     Motivation

·         Consider a compiler that represents programs as abstract syntax trees. It will need to perform operations on abstract syntax trees for "static semantic" analyses like checking that all variables are defined. It will also need to generate code. It will be confusing to have type-checking code mixed with pretty-printing code or flow analysis code. It would be better if each new operation could be added separately, and the ode classes were independent of the operations that apply to them.

·         We can have both by packaging related operations from each class in a separate object, called a visitor, and passing it to elements of the abstract syntax tree as it's traversed. When an element accepts the visitor, it sends a request to the visitor that encodes the element's class. It also includes the element as an argument. The visitor will then execute the operation for that element—the operation that used to be in the class of the element.

·         Modular systems are easier to maintain.

·         Separation of functionality makes cleaner code.

1.3     Structure

1.4     Participants

 The classes and/or objects participating in this pattern are:

·         Visitor  (Visitor)

o   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)

o   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)

o   defines an Accept operation that takes a visitor as an argument.

·         ConcreteElement  (Employee)

o   implements an Accept operation that takes a visitor as an argument

·         ObjectStructure  (Employees)

o   can enumerate its elements

o   may provide a high-level interface to allow the visitor to visit its elements

o   may either be a Composite (pattern) or a collection such as a list or a set

 

1.5     Applicability

·         An object structure contains many classes of objects with differing interfaces…

·         Many distinct and unrelated operations need to be performed on objects in an object structure…

·         The classes defining the object structure rarely change, but you often want to define new operations over the structure…

1.6     Consequences

·         +flexibility: visitor & object structure are independent

·         +localized functionality

·         –circular dependency between Visitor & Element interfaces

·         –Visitor brittle to new ConcreteElementclasses

1.7     Sample Code

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.

namespace Xpanxion.Visitor {     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);             }         }     } }

 

1.8     Sample Code2

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.

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();         }     } }

 

1.9     C# Iterators and Visitor Pattern

The iterators are coded directly into the class to be iterated. For example:

public IEnumerable<int> Visit(SomeVisitor visitor)

{

int position = 0;

for(int i = 0; i < Count; i++)

{

yield return position;

data[i].Accept(visitor);

position++;

}

}

This can then be used in a foreach construct:

foreach(int position in someObject.Visit(this))

{

// Do something with position here.

}

Where 'someObject' is the object to traverse, the class of which contains the iterator code above, and 'this' is an instance of a class derived from 'SomeVisitor.'

So as the object is traversed, the iterator first returns the position information. Here it is just a simple integer, but it could be anything, something much more complex, perhaps. Then the Accept method is called.

The idea is that you can traverse a collection where the Visitor design pattern is used without having to explicitely call the Accept method; the iterator takes care of that for you. Plus, you get additional information about the traversal that can be used as well. Care would have to be taken so that you know which comes first, the value returned from the iterator or the Accept method being called.

Hope this helps.

Thanks & Regards,

Arun Manglick