Basic Visitor Pattern Example

dartVisitorDesignPattern.dart

// Reference: http://en.wikipedia.org/wiki/Visitor_pattern
interface CarElementVisitor {
    void visitWheel(Wheel wheel);
    void visitEngine(Engine engine);
    void visitBody(Body body);
    void visitCar(Car car);
}

interface CarElement {
  void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}


class Wheel implements CarElement {
  String name;

  Wheel(String name) {
      this.name = name;
  }

  void accept(CarElementVisitor visitor) {
 /*
  * Wheel.accept(CarElementVisitor) overrides CarElement.accept(CarElementVisitor), so the call 
  * to accept is bound at run time. This can be considered the first dispatch.
  * The decision to call CarElementVisitor.visitWheel(Wheel) however, rather than the other 'visit'
  * methods in CarElementVisitor, can be made during compile time since 'this' is known at compile 
  * time to be a Wheel. Moreover, each subclass of CarElementVisitor overrides the visitWheel(Wheel),
  * which is another decision that is made at run time. This can be considered the second dispatch.
  */ 
  visitor.visitWheel(this);
  }
}

class Engine implements CarElement {
  void accept(CarElementVisitor visitor) {
      visitor.visitEngine(this);
  }
}

class Body implements CarElement {
  void accept(CarElementVisitor visitor) {
      visitor.visitBody(this);
  }
}

class Car implements CarElement {
  List<CarElement> elements;

  Car() {
      //create new Array of elements
      this.elements = new List<CarElement>();
      this.elements.add(new Wheel("front left")); 
      this.elements.add(new Wheel("front right"));
      this.elements.add(new Wheel("back left"));  
      this.elements.add(new Wheel("back right")); 
      this.elements.add(new Body()); 
      this.elements.add(new Engine());
  }

  void accept(CarElementVisitor visitor) {     
      for(CarElement elem in elements) {
          elem.accept(visitor);
      }
      visitor.visitCar(this); 
  }
  
}
  
  
class CarElementPrintVisitor implements CarElementVisitor {
    void visitWheel(Wheel wheel) {      
        print("Visiting ${wheel.name} wheel");
    }
 
    void visitEngine(Engine engine) {
        print("Visiting engine");
    }
 
    void visitBody(Body body) {
        print("Visiting body");
    }
 
    void visitCar(Car car) {      
        print("Visiting car");
    }
}

class CarElementDoVisitor implements CarElementVisitor {
  void visitWheel(Wheel wheel) {
      print("Kicking my ${wheel.name} wheel");
  }

  void visitEngine(Engine engine) {
      print("Starting my engine");
  }

  void visitBody(Body body) {
      print("Moving my body");
  }

  void visitCar(Car car) {
      print("Starting my car");
  }
}


void main() {
  Car car = new Car();
  car.accept(new CarElementPrintVisitor());
  car.accept(new CarElementDoVisitor());
}

I see the Dart engy's using this pattern a lot in their parsing code.

I know, its definitely a used pattern in dart2js. Might start to gather what common design patterns look like in dart, maybe something similar to dofactories (www.dofactory.com) code but for dart.

That's a great idea. :)