ECS and MUD

ECS and MUD

If you are coming from MUDv1 or traditional video game engine, you might be familiar with the Entity Component System data modelling technique.

MUDv2 exposes a different base data-model: tables. However, it is quite easy to model your state in the ECS way, and use client-side data-store like recs to query your state in an ECS-native way.

Modelling your state with ECS

To model your data in an ECS-native way, represent every component as a table with a bytes32 key. Do not use mutliple keys for components.

Defining a table schema in the MUD config defaults to a single bytes32 key if no keySchema is provided.

Note: we recommend adding the UniqueEntityModule to your World. It allows system to request a fresh entity ID that hasn't been used before. It is useful when creating new entities dynamically.

import { mudConfig } from "@latticexyz/world/register";
 
export default mudConfig({
  tables: {
    PlayerComponent: "bool",
    PositionComponent: {
      schema: { x: "int32", y: "int32" },
    },
    NameComponent: "string",
    DamageComponent: uint256,
    HealthComponent: uint256,
  },
  modules: [
    {
      name: "UniqueEntityModule",
      root: true,
      args: [],
    },
  ],
});

Here, we defined five components:

  1. The PlayerComponent, which attaches a bool value to an entity. We can use it to represent entities as players.
  2. The PositionComponent, which attaches a { x : int32, y : int32 } vector to an entity. We can use it to place entities in our game at a specific position.
  3. The NameComponent, which attaches a string value to an entity. We can use it to name entities.
  4. The DamageComponent which attaches a uint256 value to an entity. We can use it to configure the amount of damage dealt by an entity.
  5. The LifeComponent which attaches a uint256 value to an entity. We can use it to represent the amount of life an entity has left.

Creating entities

To create entities, we simply get a fresh entity ID that hasn't been used. Another ECS modelling technique is using an address as entity ID. It is useful for representing things like wallets in ECS and attaching components to them.

Creating a new entity

import { getUniqueEntity } from "@latticexyz/world/src/modules/uniqueentity/getUniqueEntity.sol";
import { DamageComponent } from "../codegen/tables/DamageComponent.sol";
import { HealthComponent } from "../codegen/tables/HealthComponent.sol";
import { PositionComponent } from "../codegen/tables/PositionComponent.sol";
// let's create an entity at the center of the world
bytes32 newEntity = getUniqueEntity();
PositionComponent.set(newEntity, {x: 0, y: 0});
// with 10 damage
DamageComponent.set(newEntity, 10);
// and 100 health
HealthComponent.set(newEntity, 100);

Creating a new entity from an address

import { DamageComponent } from "../codegen/tables/DamageComponent.sol";
import { PlayerComponent } from "../codegen/tables/PlayerComponent.sol";
import { HealthComponent } from "../codegen/tables/HealthComponent.sol";
import { PositionComponent } from "../codegen/tables/PositionComponent.sol";
// let's cast the msg.sender
// note: we use _msgSender() with systems to a bytes32 to use it as an entity ID
bytes32 playerEntity = bytes32(uint256(uint160(_msgSender())));
// we spawn the player who sent this transaction at {10, 42}
PositionComponent.set(playerEntity, {x: 10, y: 42});
PlayerComponent.set(playerEntity, true);
// with 50 damage
DamageComponent.set(playerEntity, 50);
// and 600 health
HealthComponent.set(playerEntity, 600);

Writing ECS-native systems

Building up on the previous step, we can create an ECS-native system to spawn as a player and move, as well as creating monsters and moving them around.

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;
import { System } from "@latticexyz/world/src/System.sol";
import { getUniqueEntity } from "@latticexyz/world/src/modules/uniqueentity/getUniqueEntity.sol";
import { DamageComponent } from "../codegen/tables/DamageComponent.sol";
import { PlayerComponent } from "../codegen/tables/PlayerComponent.sol";
import { HealthComponent } from "../codegen/tables/HealthComponent.sol";
import { PositionComponent } from "../codegen/tables/PositionComponent.sol";
import { NameComponent } from "../codegen/tables/NameSystem.sol;
contract GameSystem is System {
  function spawn(string memory name) public returns () {
    // let's cast the msg.sender
    // note: we use _msgSender() with systems to a bytes32 to use it as an entity ID
    bytes32 playerEntity = bytes32(uint256(uint160(_msgSender())));
    PositionComponent.set(playerEntity, {x: 10, y: 42});
    PlayerComponent.set(playerEntity, true);
    DamageComponent.set(playerEntity, 50);
    HealthComponent.set(playerEntity, 600);
    // we let players set their name
    NameComponent.set(playerEntity, name)
  }
  function createMonster(int32 x, int32 y) returns () {
    bytes32 newEntity = getUniqueEntity();
    // we create the monster at the position specified in the arguments
    PositionComponent.set(newEntity, {x: x, y: y});
    DamageComponent.set(newEntity, 10);
    HealthComponent.set(newEntity, 100);
    NameComponent.set(newEntity, "Monster");
  }
  function move(int32 x, int32 y) returns () {
    // check if sender has already spawned
    bytes32 playerEntity = bytes32(uint256(uint160(_msgSender())));
    require(PlayerComponent.get(playerEntity), "player hasn't spawned");
    // move the entity associated with the sender address. you can't move other players!
    PositionComponent.set(playerEntity, {x: x, y: y})
  }
  function moveMonster(bytes32 entity, int32 x, int32 y) returns () {
    // check if the entity is not a player
    require(PlayerComponent.get(entity) == false, "can't move a player");
    PositionComponent.set(entity, {x: x, y: y})
  }
}

Querying client side with recs

Because we modelled our application state in an ECS-native way, we can easily query our application state in a reactive way with recs. Using React as an example (note that recs can be used with any framework), let's define three reactive queries: One that reacts to any player moving, another one that reacts to any monster moving, and finally one that reacts when your player moves.

import { useEntityQuery, useComponentValue } from "@latticexyz/react";
import { getComponentValueStrict, Has, Not } from "@latticexyz/recs";
const {
  components: { PositionComponent, NameComponent, PlayerComponent },
  playerEntity,
} = useMUD();
// A. Subscribe to all player position
//   1. subscribe to all entities that are players and have a position
const players = useEntityQuery([
  Has(Player) /** All entities with a Player component */,
  Has(Position) /** With a position */,
]);
//   2. map all entities to their position (they must have one, so we can use getComponentValueStrict)
const playerPositions = players.map((player) => getComponentValueStrict(PositionComponent, player));
// -----------------------------------
// B. Subscribe to all monster position
//   1. subscribe to all entities that are *not* players and have a position
const monsters = useEntityQuery([
  Not(Player) /** All entities without a player component */,
  Has(Position) /** With a position */,
]);
//   2. map all entities to their position (they must have one, so we can use getComponentValueStrict)
const monsterPositions = monsters.map((monster) => getComponentValueStrict(PositionComponent, monster));
// -----------------------------------
// C. Subscribe to our player's position and health
//   1. subscribe to the position of our player
const ourPosition = useComponentValue(PositionComponent, playerEntity);
//   2. subscribe to the health of our player
const ourHealth = useComponentValue(HealthComponent, playerEntity);
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