A quick example of the visitor pattern using the Dart programming language.

about.md

Visitor Pattern in Dart

A couple nice references on the visitor pattern:

1. Source Making - Visitor
2. Crafting Interpreters

The code in this Gist implements a simple visitor using nonsensical, but reasonably clear, types.

We have a set of primitive-alikes upon which we would like to perform various operations. We use a visitor to implement the "biggify" operation.

biggify_visitor.dart

import 'crazy_primitive.dart';
import 'crazy_visitor.dart';

class BiggifyVisitor extends CrazyVisitor {
  @override
  void visitInt(CrazyInt element) {
    element.value += 100;
  }

  @override
  void visitString(CrazyString element) {
    element.value.toUpperCase();
  }
}

crazy_primitive.dart

import 'crazy_visitor.dart';

abstract class CrazyPrimitive<T> {
  T value;

  void accept(CrazyVisitor visitor);
}

class CrazyDouble extends CrazyPrimitive<double> {

  /// This type does nothing upon accepting a visitor.
  /// One drawback of the visitor pattern is that it is
  /// more difficult to add additional types since every
  /// new type requires a new method to be added on the
  /// base visitor class, which then potentially
  /// requires all concrete visitors to be updated.
  @override
  void accept(CrazyVisitor visitor) {}
}

class CrazyString extends CrazyPrimitive<String> {
  @override
  String value;

  @override
  void accept(CrazyVisitor visitor) {
    visitor.visitString(this);
  }
}

class CrazyInt extends CrazyPrimitive<int> {
  @override
  int value;

  @override
  void accept(CrazyVisitor visitor) {
    visitor.visitInt(this);
  }
}

crazy_visitor.dart

import 'crazy_primitives.dart';

abstract class CrazyVisitor {
  void visitString(CrazyString element);

  void visitInt(CrazyInt element);
}