CODESAMPLE

Visitor - Dart

Visitor Dart

The Visitor pattern allows you to define a new operation without modifying the classes of the objects on which it operates. It’s achieved by moving the operation to a separate “visitor” class that has a visit method for each type of object it can handle. This promotes the Open/Closed Principle.

This Dart implementation defines a basic shape hierarchy (Circle, Square) and a Shape interface. The Visitor interface declares a visit method for each Shape type. ConcreteVisitor implements the Visitor to perform specific operations (e.g., calculating area, printing details). The accept method in each Shape takes a Visitor and calls the appropriate visit method on it, enabling the visitor to operate on the shape. This is idiomatic Dart due to its use of interfaces and method overriding, aligning with its object-oriented nature.

// Define the element interface
abstract class Shape {
  void accept(Visitor visitor);
}

// Concrete elements
class Circle implements Shape {
  final double radius;

  Circle(this.radius);

  @override
  void accept(Visitor visitor) {
    visitor.visitCircle(this);
  }
}

class Square implements Shape {
  final double side;

  Square(this.side);

  @override
  void accept(Visitor visitor) {
    visitor.visitSquare(this);
  }
}

// Define the visitor interface
abstract class Visitor {
  void visitCircle(Circle circle);
  void visitSquare(Square square);
}

// Concrete visitor: Area calculation
class AreaVisitor implements Visitor {
  double totalArea = 0;

  @override
  void visitCircle(Circle circle) {
    totalArea += 3.14159 * circle.radius * circle.radius;
  }

  @override
  void visitSquare(Square square) {
    totalArea += square.side * square.side;
  }
}

// Concrete visitor: Printing details
class PrintVisitor implements Visitor {
  @override
  void visitCircle(Circle circle) {
    print('Circle with radius: ${circle.radius}');
  }

  @override
  void visitSquare(Square square) {
    print('Square with side: ${square.side}');
  }
}

void main() {
  final circle = Circle(5.0);
  final square = Square(4.0);

  // Calculate area
  final areaVisitor = AreaVisitor();
  circle.accept(areaVisitor);
  square.accept(areaVisitor);
  print('Total area: ${areaVisitor.totalArea}');

  // Print details
  final printVisitor = PrintVisitor();
  circle.accept(printVisitor);
  square.accept(printVisitor);
}