← Back to scenarios
ScenarioCatalog DB

Shape visitor

Apply rendering or measurement operations to shapes without changing the shape classes.

Problem

A graphics system needs to render and measure multiple shape types while keeping those operations separate from the stable shape hierarchy.

Related patterns

Visitor

View pattern →

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