Game Entity System learning

Entity systems are simple in concept:

• Every ‘thing’ in system has an id
• The entity system (hopefully a library if you are reading this) stores data components against each id
• Data components can be colour, size, mass or position

Then you write a listener system which ‘acts’ on each of the entities in the system, which decides what to do.

Done, explaination over. Although obviously its more complex to actually use than that, but not by much.

My attempt at learning this was a re-hash of a game I had already made: Asteroids (assuming you have a game engine already, and can use it effectively, using an entity system is rather easy)

I’ll skip over all the details (because im lazy and because its written in like 60 places online already), the general idea is that game objects are really just a guid/id/int. This is really me just typing for fun I swear.

And you write these listener systems which just act on entities with the data components that it works with. Examples:

• position, shape, colour and size could alow a DisplaySystem can use these to show them on the screen
• position and velocity could allow a MovementSystem to update the position to show motion
• health and hit used by a DamageSystem can reduce the health and record game interactions
• postiion and size used by a CollisionSystem could allow objects to bounce/explode/reduce health

…well i hope you get the point.

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The next concept which is important is to not set the same entity type on the same entity. This is because the last one to set it will win, causing a race condition.

From the great help I recieved on the jMonkeyEngine forums (@ here) preventing these situations is very important and can be done with something called a tagged entity. Basic logic is that you create a new entity which has an ‘affects’ data component. So when the systems use it the resulting info is applied to the tagged entity not the one the component is actually against. This allows many systems to update an entity but not overstep other systems.

For example, say you have a display only particle, but you want it to be affected by gravity and be affected by external collisions. What would normally happen is there would be like a GravitySystem which sets the acceleration on the object. And another system like CollisionSystem which sets the acceleration based on impulses it receives from the environment. These are now setting the acceleration on the entity at the same time, causing only one of the acclerations to get registered.

In this case using a tagged entity is useful, instead of the systems editing the acceleration data component directly, they create a tagged entity which refers to the base entity. And both accelerations can be applied.