CODESAMPLE

Visitor - TypeScript

Visitor TypeScript

The Visitor pattern lets you define a new operation without changing the classes of the objects on which it operates. It’s useful when you have a complex object structure and want to add functionality that depends on the specific types of objects within that structure. Here, we define Element and concrete elements like ConcreteElementA and ConcreteElementB. The Visitor interface declares visit methods for each concrete element. ConcreteVisitor1 and ConcreteVisitor2 implement these visits to perform different operations. The ObjectStructure (a simple array in this case) accepts a visitor and calls the appropriate visit method on each element. This TypeScript implementation leverages interfaces and type safety, common practices in the language, to ensure correct visitor dispatching.

// Element interface
interface Element {
  accept(visitor: Visitor): void;
}

// Concrete Elements
class ConcreteElementA implements Element {
  operationA(): string {
    return "ConcreteElementA operation";
  }

  accept(visitor: Visitor): void {
    visitor.visitConcreteElementA(this);
  }
}

class ConcreteElementB implements Element {
  operationB(): string {
    return "ConcreteElementB operation";
  }

  accept(visitor: Visitor): void {
    visitor.visitConcreteElementB(this);
  }
}

// Visitor interface
interface Visitor {
  visitConcreteElementA(element: ConcreteElementA): void;
  visitConcreteElementB(element: ConcreteElementB): void;
}

// Concrete Visitors
class ConcreteVisitor1 implements Visitor {
  visitConcreteElementA(element: ConcreteElementA): void {
    console.log("ConcreteVisitor1 visited ConcreteElementA: " + element.operationA());
  }

  visitConcreteElementB(element: ConcreteElementB): void {
    console.log("ConcreteVisitor1 visited ConcreteElementB: " + element.operationB());
  }
}

class ConcreteVisitor2 implements Visitor {
  visitConcreteElementA(element: ConcreteElementA): void {
    console.log("ConcreteVisitor2 visited ConcreteElementA: " + element.operationA().toUpperCase());
  }

  visitConcreteElementB(element: ConcreteElementB): void {
    console.log("ConcreteVisitor2 visited ConcreteElementB: " + element.operationB().repeat(2));
  }
}

// Object Structure
class ObjectStructure {
  private elements: Element[];

  constructor(elements: Element[]) {
    this.elements = elements;
  }

  traverse(visitor: Visitor): void {
    this.elements.forEach(element => {
      element.accept(visitor);
    });
  }
}

// Example Usage
const elements = [new ConcreteElementA(), new ConcreteElementB(), new ConcreteElementA()];
const objectStructure = new ObjectStructure(elements);

const visitor1 = new ConcreteVisitor1();
const visitor2 = new ConcreteVisitor2();

objectStructure.traverse(visitor1);
objectStructure.traverse(visitor2);