# 1. Create your first component and system

Most things (entities) in our game are going to be placed on a 2D map, things like players, terrain, and interactive tiles. We’ll represent this by creating a Position component that we can add to each entity. And we’ll create a Move system that we can call to move entities to a different position.

Create a new file in packages/contracts/src/components called PositionComponent.sol. We can inherit from MUD’s CoordComponent.

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;
import "std-contracts/components/CoordComponent.sol";

uint256 constant ID = uint256(keccak256("component.Position"));

contract PositionComponent is CoordComponent {
  constructor(address world) CoordComponent(world, ID) {}
}

Let’s also create a MoveSystem.sol in packages/contracts/src/systems to move our entities by updating their position component value.

In ECS, every “thing” in the world is an entity, and an entity is just an ID. Our game will have players, each represented as an entity with their own ID. Since each game client talks to the contracts via a wallet address (msg.sender), we can just use that as the entity ID. And MUD provides an addressToEntity helper for converting between an address and entity ID. This address-as-an-ID pattern will make our lives easier when updating the player’s component values and is also what MUD uses internally when registering components and systems.

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;
import { System, IWorld } from "solecs/System.sol";
import { getAddressById, addressToEntity } from "solecs/utils.sol";
import { PositionComponent, ID as PositionComponentID, Coord } from "components/PositionComponent.sol";

uint256 constant ID = uint256(keccak256("system.Move"));

contract MoveSystem is System {
  constructor(IWorld _world, address _components) System(_world, _components) {}

  function execute(bytes memory args) public returns (bytes memory) {
    return executeTyped(abi.decode(args, (Coord)));
  }

  function executeTyped(Coord memory coord) public returns (bytes memory) {
    uint256 entityId = addressToEntity(msg.sender);

    PositionComponent position = PositionComponent(getAddressById(components, PositionComponentID));
    position.set(entityId, coord);
  }
}

Any time we add a component or system, we need to update the deploy.json, which MUD uses to deploy and connect our components and systems. Because we’re setting the PositionComponent value in our MoveSystem, we must also declare that it needs writeAccess.

It's worth noting that hot reloading doesn't work for components yet, because we can't automatically migrate your data if it changes shape. We're working on bringing this to MUD, but in the meantime, any component changes will require you restart the yarn dev:contracts service to redeploy all contracts.

(We no longer need the project template’s counter component or increment system, so we can remove those too.)

{
  "components": ["PositionComponent"],
  "systems": [
    {
      "name": "MoveSystem",
      "writeAccess": ["PositionComponent"]
    }
  ]
}

There’s not much to see yet, so let’s get this wired up in the client next.