- 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"
- use interfaces/abstract classes -- change the implementation if it is needed / multiple implementation could be created
- initiate and manage the element (objectStructure)
- Call Visitor methods on objectStructure
- Classes that contain methods that iterate through the object structure and apply operations to its elements
implements an Accept operation that takes a visitor as an argument
class ConcreteElementA : Element
public override void Accept(Visitor i_visitor)
- 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.