Visitor Pattern

About

• defines and performs new operations on all the elements of an existing structure, without altering its classes
• the visitor design pattern is a way of separating an algorithm from an object structure it operates on
• using the visitor pattern allows to conform with the open/closed principle
  
• "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification"
  
• inheritance
• use interfaces/abstract classes -- change the implementation if it is needed / multiple implementation could be created

Client:

• initiate and manage the element (objectStructure)
• Call Visitor methods on objectStructure

Visitor:

• Classes that contain methods that iterate through the object structure and apply operations to its elements

Element1, Element2
implements an Accept operation that takes a visitor as an argument

class ConcreteElementA : Element

{

  public override void Accept(Visitor i_visitor)

  {

    visitor.VisitElement1(this);

  }
...
}

Samples:

• Animals that do different things: barks, run, ...
• Different operation (getWage(), getPTO() ) on different type of employees (manager, programmer, tester)

Visitor Pattern in C#:

• System.Reflection API
  
• Viewing metadata
• Performing type discovery
• Late binding to methods and properties
• Creating types at runtime with Reflection.Emit

            Type[]: Will hold a list of type names

            GetType( ): Will return a class name

            MethodInfo: Can return all parameters and the return type of a method

            Invoke: Will call the method described in a variable of type MethodInfo

• Use Visitor Pattern to do different operation on tree (binary tree)

Shortcomings of the Visitor pattern

• The return type of visiting methods have to be known in advance. 
• A lot of code has to be written to prepare the use of visitors: the visitor class with one abstract method per class, and a accept method per class. This code is tedious and boring to write. 
• If a visitor pattern has not been written in the first time, the hierarchy has to be modified to implement it.