Applying ECS to a simple game

I’ve been going back to work on my Entity-Component-System framework. Aside from being a side project, I will also plan to use it for my voxel platform game. I’ve already created a Minimum Viable Product using it, which is the Bomberman clone game I mentioned a few posts back. Animations are still very buggy, and there is no AI implemented, but a barebones 2-4 player version is working.

Previously I initialized all the Components, Systems, and Entity templates in the framework code. While I did this for testing out the game, it’s not good for portability, so I had to remove all that initialization code out and update the framework so that it can accept new Components, Systems and templates from outside.

Finally, I isolated the code into its own assembly, so it would be possible to just link it as a DLL. This also meant I had to remove any XNA/MonoGame specific classes and all the rendering will have to do be done from outside. In short, it’s really is meant for game logic only, and your game will have to handle the rendering separately.

The framework itself is lightweight (and I hope it stays that way), and only consists of 5 files/classes: Component, EntityManager, EntitySystem, EntityTemplate, and SystemManager. The SystemManager handles all the high level control and flow of the EntitySystems, which you make custom systems from. EntityTemplate is a simple class used as a blueprint to add Components that define an Entity, and is deep-cloneable. EntityManager handles the creation of Entities from these templates, and also the organization of its components. Despite its name, there is no Entity class. I think I wil rename this manager to “ComponentManager” in another revision.

The Bomberman game has the following components:

  • Bomb
  • Collision
  • InputContext
  • PlayerInfo
  • PowerUp
  • ScreenPosition
  • Spread
  • Sprite
  • TilePosition
  • TimedEffect

They are used by the following systems:

  • BombSystem
  • CollisionSystem
  • ExplosionSystem
  • InputSystem
  • MovementSystem
  • PlayerSystem
  • PowerUpSystem
  • TileSystem

Some of the systems are more generic than others. There are a couple of systems like the Bomb system or Power-up system that have very specific logic pertaining to the game, while others like the Input system are pretty abstract and can be adapted to other game types with little or no change. Some are Drawable systems so they have an extra Draw() function that is called in a separate part of the game loop.

The funny thing is that I was going to talk about using Messages in this update, but in the time between, I did away with them completely. Messages were a static list of Message objects that was in the base System class. They were mostly used for one-time player triggered events (like setting a bomb) and every system had access to them, but I decided to just pass along the InputContext component into the systems that will be dependent on player input.

Setup and Gameplay

The game is started by initializing all the components and systems and then creating the entire set of Entities using a Level class. This class has only one job- to lay out the level. Specifically, it adds the components needed to make the tiles, sprites and players. My implementation of the game pre-allocates 9 Bomb entities (the maximum a player can have) for each player.

Each player can be custom controlled but right now that’s facing issues now that I moved from invoking methods to instantiate new Entities, to deep-cloning them. This works as well as long as none of the component have reference types.

The only Component that has reference types is the InputContext component as it needs to keep a Dictionary of the available control mappings. This breaks with deep-cloning and thus with multiple players, they all share the same control scheme. Other than that, it makes the component too bloated, especially with helper functions to initialize the mappings. So I am figuring out how to use value types only to represent an arbitrary group of control mappings.

The game starts immediately after setup, and every InputContext that is tied in with a PlayerInfo controls a player. Movement around the level is handled with the Movement System, while placing and remote-detonating bombs is handled with the Bomb System.

The Input System detects key and button presses from an InputContext’s available control mappings, and changes two integers, “current action” and “current state”, based on it. It is up to other systems to determine what it should do with these values, if needed.

The Tile System is responsible for keeping sprites aligned to a tile grid, or giving them their closest “tile coordinates” which is important in knowing where a bomb should be placed, for example.

Collision System is self-explanatory. It handles different collision types varying by enum, to differentiate solid objects, destructible objects or damaging objects, as well as the response (it pushes players off walls, for example). If a player walks into an explosion, the Collision System knows.

An Explosion System is used to propagate the explosions in pre-set directions. By default it’s in all 4 cardinal directions with a bomb’s Power attribute copied to a Spread component, subtracting one with each tile. It keeps creating more explosions until this attribute reaches 0 or it hits a wall.

The Powerup System handles tracking tile locations with the players’ own tile locations so if two identical locations are found, we know a player is over a power-up and it can be applied.

There used to be a system for drawing sprites, but I decided to remove it and have the rendering be done outside the ECS scope. This makes the code more portable and you can use your own renderer.

Now that the game is now done (with minimal specs), I am now ready to extend its use to produce one of the games I have wanted to make for a while, a top-down arena style shooter. This game will have similarities with components and systems for player movement, tile collision, and power-ups (which will be changed simply to items). I plan to make it in 2D at first but eventually switch the renderer to 3D and also offer customizable maps.

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