Linky Pinky Ponky

AppTypes.sol

library AppTypes {
  struct ListItem { 
      string content;
  }
  struct List {
      uint256 id;
      uint256 listItemsCount;
      ListItem[] listItems;
  }
  struct State {
      mapping(LIST => uint256) id;
      mapping(uint256 => List) lists;
  }
  enum Model { 
      LIST
  }
}

Main.sol

import "./SVGList.sol";
import "./AppTypes.sol";

library Main {

    using SVGList for AppTypes.List;
        
    function newListWithContent(AppTypes.State storage currentState, string memory content) public returns(uint256 newListId) {
        uint256 newListId = ++currentState.id[AppTypes.Model.LIST];
        AppTypes.List storage newList = currentState.lists[newListId];
        currentState.lists[currentState.id[AppTypes.Model.LIST]] = newList;
        newList.id = newListId;
        newList.addContent(content);
        return newListId;
     }

     function addContentToList(AppTypes.State storage currentState, uint256 listId, string memory content) public {
        AppTypes.List storage currentList = currentState.lists[listId];
        currentList.addContent(content);
     }
    
}

Primary.sol

import "./Main.sol";
import "./AppTypes.sol";

contract Primary {
  using Main for AppTypes.State;

  AppTypes.State storage state;

  function newListWithContent(string memory content) public returns(uint256 newListId) {
    return state.newListWithContent(content);
  }

  function addContentToList(uint256 listId, string memory content) public {
    state.addContentToList(listId, content);
  }

  function getContent(uint256 listId, uint256 index) public view returns (string memory) {
    return state.getContent(listId, index);
  }

  function getId(AppTypes.Model model) public view returns (uint256) {
    return state.id[model];
  }

}

SVGList.sol

import "./AppTypes.sol";

library SVGList {

 function addContent(AppTypes.List storage currentList, string memory content) public {
    AppTypes.ListItem storage newListItem = currentList.listItems[++currentList.listItemsCount];
    newListItem.content = content;
    currentList.listItems[currentList.listItemsCount] = newListItem;
 }

 function getContent(AppTypes.List storage currentList, uint256 index) public view returns (string memory) {
    return currentList.listItems[index].content;
 }

}