The Visitor pattern is a behavioral design pattern that lets you separate algorithms from the objects on which they operate. This pattern allows you to define new operations on objects without changing the objects themselves. It is particularly useful when you need to perform operations on a set of objects with different types, and you want to add new operations without modifying the classes of the objects.

Key Characteristics:

  • Separation of Concerns: Separates the operations from the objects on which they operate.
  • Extensibility: Allows adding new operations without modifying the existing object structure.
  • Double Dispatch: The operation to be executed is determined by both the type of the visitor and the type of the element.

How It Works:

  1. Visitor Interface: Defines a visit method for each type of element. Each method represents an operation that can be performed on the element.
  2. Concrete Visitors: Implement the Visitor interface and provide specific implementations of the operations.
  3. Element Interface: Defines an accept() method that takes a visitor as an argument.
  4. Concrete Elements: Implement the Element interface and implement the accept() method to call the appropriate visit method on the visitor.

Example in PHP: 

  // Visitor Interface
interface Visitor {
    public function visitConcreteElementA(ConcreteElementA $element);
    public function visitConcreteElementB(ConcreteElementB $element);
}

// Concrete Visitor
class ConcreteVisitor implements Visitor {
    public function visitConcreteElementA(ConcreteElementA $element) {
        echo "Visiting ConcreteElementA\n";
    }

    public function visitConcreteElementB(ConcreteElementB $element) {
        echo "Visiting ConcreteElementB\n";
    }
}

// Element Interface
interface Element {
    public function accept(Visitor $visitor);
}

// Concrete Element A
class ConcreteElementA implements Element {
    public function accept(Visitor $visitor) {
        $visitor->visitConcreteElementA($this);
    }
}

// Concrete Element B
class ConcreteElementB implements Element {
    public function accept(Visitor $visitor) {
        $visitor->visitConcreteElementB($this);
    }
}

// Client Code
$elements = [new ConcreteElementA(), new ConcreteElementB()];
$visitor = new ConcreteVisitor();

foreach ($elements as $element) {
    $element->accept($visitor);
}
// Output:
// Visiting ConcreteElementA
// Visiting ConcreteElementB

Explanation:

  1. Visitor Interface: Visitor interface defines visitConcreteElementA() and visitConcreteElementB() methods for different element types.
  2. Concrete Visitor: ConcreteVisitor implements the Visitor interface and provides specific behavior for each type of element.
  3. Element Interface: Element interface defines the accept() method, which allows the visitor to visit the element.
  4. Concrete Elements: ConcreteElementA and ConcreteElementB implement the Element interface and call the corresponding visit method on the visitor in their accept() method.

Benefits:

  • Separation of Algorithms and Object Structures: Decouples the operations from the objects, making it easier to add new operations.
  • Flexibility: Allows adding new operations without changing the existing object classes.
  • Extensibility: Facilitates adding new types of elements and operations in the future.

Use Cases:

  • Object Structure Traversal: When you need to traverse and perform operations on a complex object structure.
  • Operations on Different Object Types: When you need to perform different operations on a set of objects with varying types.
  • Adding Operations without Modifying Classes: When you want to add new operations to existing classes without modifying their code.

The Visitor pattern is useful for separating operations from objects, enabling you to add new operations easily and allowing complex object structures to be traversed and manipulated in a flexible and extensible manner.