Dart: Strategy pattern in Dart

dart_strategy_01.dart

// Strategy pattern in Dart with abstract base class.
abstract class Renderer {
  static const TEXT = const TextRenderer('text');
  static const HTML = const HtmlRenderer('html');
  static const JSON = const JsonRenderer('json');

  final String name;
  const Renderer._internal(this.name);
  void render();
}

class TextRenderer extends Renderer {
  const TextRenderer(String name) : super._internal(name);
  void render() => print('Rendering to $name...');
}

class HtmlRenderer extends Renderer {
  const HtmlRenderer(String name) : super._internal(name);
  void render() => print('Rendering to $name...');
}

class JsonRenderer extends Renderer {
  const JsonRenderer(String name) : super._internal(name);
  void render() => print('Rendering to $name...');
}

class Context {
  Renderer _renderer;
  Context(this._renderer);
  export() => _renderer.render();
}

void main() {
  var context = new Context(Renderer.JSON);
  context.export();
}

dart_strategy_02.dart

// Strategy pattern in Dart with abstract base class and factory constructor.
abstract class Renderer {
  final String name;
  static Map<String, Renderer> _cache;

  factory Renderer(String name) {
    if (_cache == null) {
      _cache = {};
    }

    if (_cache.containsKey(name)) {
      return _cache[name];
    } else {
      var renderer;
      switch (name) {
        case 'text':
          renderer = new TextRenderer(name);
          break;
        case 'html':
          renderer = new HtmlRenderer(name);
          break;
        case 'json':
          renderer = new JsonRenderer(name);
          break;
        default:
          throw new ArgumentError('Unknown renderer: $name');
      }
      _cache[name] = renderer;
      return renderer;
    }
  }

  const Renderer._internal(this.name);
  void render();
}

class TextRenderer extends Renderer {
  TextRenderer(String name) : super._internal(name);
  void render() => print('Rendering $name...');
}

class HtmlRenderer extends Renderer {
  HtmlRenderer(String name) : super._internal(name);
  void render() => print('Rendering $name...');
}

class JsonRenderer extends Renderer {
  JsonRenderer(String name) : super._internal(name);
  void render()  => print('Rendering $name...');
}

class Context {
  Renderer _renderer;
  Context(this._renderer);
  export() => _renderer.render();
}

void main() {
  var context = new Context(new Renderer('json'));
  context.export();
}