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Document visitor

Export or inspect document nodes while keeping those operations outside the document-node classes.

Problem

A document system must support operations such as export and inspection across node types without coupling every new operation to the document-node hierarchy.

Related patterns

Visitor

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Implementations

Visitor

TypeScriptApplication

TypeScript shape operation visitors

Moves rendering and measurement operations into visitors so shape classes remain stable when new operations are introduced.

TypeScript
interface ShapeVisitor<Result> {
  visitCircle(circle: Circle): Result;
  visitRectangle(rectangle: Rectangle): Result;
}

interface Shape {
  accept<Result>(visitor: ShapeVisitor<Result>): Result;
}

class Circle implements Shape {
  constructor(readonly radius: number) {}

  accept<Result>(visitor: ShapeVisitor<Result>) {
    return visitor.visitCircle(this);
  }
}

class Rectangle implements Shape {
  constructor(
    readonly width: number,
    readonly height: number,
  ) {}

  accept<Result>(visitor: ShapeVisitor<Result>) {
    return visitor.visitRectangle(this);
  }
}

class AreaVisitor implements ShapeVisitor<number> {
  visitCircle(circle: Circle) {
    return Math.PI * circle.radius ** 2;
  }

  visitRectangle(rectangle: Rectangle) {
    return rectangle.width * rectangle.height;
  }
}

Associated technologies