Pattern: sub-system

If you want to re-use logic in your namespace, it can be useful to create sub-systems: private system that bundles often re-used logic together.

As an example, let’s say you are building an NFT project. You have an OwnerTable and BalanceTable in your namespace representing token ownership and the total balance of each address. In order to comply with the ERC-721 spec, you need to modify both tables atomically and emit an event a Transfer event on a contract.

You have many different places in your code that that transfers NFTs from one address to another (a game, a raffle, an auction, etc) and you would like to make sure this code exists only in one place to avoid errors.

You could use a library, but it would increase the code size of each system. You could use a public library, but deployment tools today don’t support re-using public libraries really well (they tend to re-deploy them over and over again), and it’s not possible to upgrade them unless you do some proxy magic. With World, you could deploy this piece of logic in a system — let’s call it the TransferSystem — and set its openAccess to false. Now only systems in the same namespace can execute a transfer through this system, and there are no risk of security coming from having external EOAs or systems in different namespaces transfer NFTs directly by being able to call that system.

import { mudConfig } from "@latticexyz/world/register";
 
export default mudConfig({
  namespace: "nftgame",
  systems: {
    TransferSystem: {
      name: "transfer",
      openAccess: false, // it's a subsystem now!
    },
    GameSystem: {...},
    RaffleSystem: {...},
    MintSystem: {...},
  },
  tables: {
    BalanceTable: {
      keySchema: { owner: "address"},
      valueSchema: { amount: "uint256" },
    },
    OwnerTable: {
      keySchema: { token: "uint256" },
      valueSchema: { owner: "address" },
    },
  },
});

And the Transfer system could look like this:

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;
import { System } from "@latticexyz/world/src/System.sol";
import { BalanceTable } from "../codegen/tables/BalanceTable.sol";
import { OwnerTable } from "../codegen/tables/OwnerTable.sol";
 
contract TranferSystem is System {
  function transfer(uint256 token, address to) public returns () {
    address from = OwnerTable.get(token);
    OwnerTable.set(token, to);
    Balance.set(from, Balance.get(from) - 1);
    Balance.set(to, Balance.get(to) + 1);
    emit Transfer(from, to, token);
  }
}