Introduce poor man's ECS

[jjyr]

Introduce a little bit ECS to improve the composition.

The previous implementation used the EntType trait to define entity callbacks, but it encountered several limitations:

• Accessing fields of EntType requires downcasting, which is cumbersome.
• During callbacks, it's impossible to access the self EntType from the World, as the callback holds a reference to it.

It turns out, in Rust, our only choice is ECS. The traditional OOP approch always holds the reference, which prevent us from accessing entities in a unified method.

Of cause, this ECS is different with that ECS, we have so many ECS. The bevy ECS is the most popular and powerful implementation in the rust world, and it is very inspiring. But I found it's too complicated for me, the query and system schedule is powerful but it feel like overkilled to me. I have tried bevy ECS, and my experience tell me I want something simpler, no complex queries and just callback style code.

I came across a blog post, Archetypal ECS Considered Harmful, which inspired me. It reassured me that there are others who also seek a simpler ECS.

What I truly need is exactly what the post discusses - the compositional capabilities of ECS. Components are the key. By introducing components, we gain the ability to dynamically access data and provide a unified way to work with entities, solving the problems I faced.

Turning into ECS, I refactor the old EntType into EntHooks, a component, Any entity providing this component will be called by the engine. I also remove the Entity struct, moving its fields into seperate components: Transform, Phisics, Sprite.

Some system, such as collision detection, need state, so I borrowed the idea of Resource from bevy. I added a CollisionSet resource, entities can be added into CollisionSet to enable the collision detection.

Here’s an example of accessing an entity's component:

// Get mutable ent ref
let mut ent = w.get_mut(ent).unwrap();
// get mutable transform component ref
let transform = ent.get_mut::<Transform>().unwrap();

See Demo for a full example.