Visitor

Example

interface Visitable {
  accept(visitor: IVisitor): void;
}

interface IVisitor {
  visitMainItem?(mainItem: MainItem): void;
  visitSideItem?(sideItem: SideItem): void;
}

class MainItem implements Visitable {
  accept(visitor: IVisitor) {
    if (visitor.visitMainItem) {
      visitor.visitMainItem(this);
    }
  }
}

class SideItem implements Visitable {
  accept(visitor: IVisitor) {
    if (visitor.visitSideItem) {
      visitor.visitSideItem(this);
    }
  }
}

class LogVisitor implements IVisitor {
  visitMainItem(mainItem: MainItem): void {
    console.log('Log mainItem, and add new logics');
  }

  visitSideItem(sideItem: SideItem): void {
    console.log('Log sideItem, and add new logics');
  }
}

class DecorateVisitor implements IVisitor {
  visitMainItem(mainItem: MainItem): void {
    console.log('Decorate mainItem, and add new logics');
  }

  visitSideItem(sideItem: SideItem): void {
    console.log('Decorate sideItem, and add new logics');
  }
}

class ItemsGroup implements Visitable {
  private _items: Object[];

  constructor() {
    this._items = [];
  }

  public addItem(item: Object) {
    this._items.push(item);
  }

  public popItem() {
    return this._items.pop();
  }

  accept(visitor: IVisitor): void {
    this._items.map((item: Visitable) => {
      item.accept(visitor);
    });
  }
}

(function main() {
  const group = new ItemsGroup();
  group.addItem(new MainItem());
  group.addItem(new SideItem());

  const mainVisitor = new LogVisitor();
  const sideVisitor = new DecorateVisitor();

  group.accept(mainVisitor);
  group.accept(sideVisitor);

  // remove
  const removedItem = group.popItem();
  console.log('removedItem:', removedItem); // SideItem

  // attempt to accept again
  group.accept(mainVisitor);
  group.accept(sideVisitor); // not accepted this time
})();
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