johnnystefan/pattern-visitor

Skill: Implement Visitor Pattern

Purpose

Separates an algorithm from the object structure on which it operates, allowing you to add new operations to existing object structures without modifying them.

Procedural Instructions

1. Declare Visitor Interface: Define a "visit" method for each class in the object structure.
2. Declare Element Interface: Add an accept(visitor) method to the base of the object structure.
3. Implement Accept in Elements: Each concrete class calls the visitor method corresponding to its own class (Double Dispatch).
4. Implement Concrete Visitors: Create a class for each new operation that needs to be performed across the object structure.

TypeScript Example

interface Shape {
  accept(v: Visitor): void;
}

class Circle implements Shape {
  accept(v: Visitor) {
    v.visitCircle(this);
  }
}

interface Visitor {
  visitCircle(c: Circle): void;
  visitRectangle(r: Rectangle): void;
}

class XMLExportVisitor implements Visitor {
  visitCircle(c: Circle) {
    console.log('Exporting Circle to XML...');
  }
  visitRectangle(r: Rectangle) {
    /* ... */
  }
}

Payoff

• OCP: Add new behaviors that work with objects of different classes without changing those classes.
• SRP: Moves multiple versions of the same behavior into a single Visitor class.