Actor built in components should be extensible - "Extending" built-in components on Actors

[eonarheim]

Actor build in components should be extensible

See discord discussion https://discord.com/channels/1195771303215513671/1198087869282140170/1198087869282140170

Discussed in #2895

^{Originally posted by mattjennings January 19, 2024}
I've been attempting to implement my own physics system. Thanks to ECS it's been straight forward to swap out the Motion & Collision systems with my own. This lets me still make use of Actor's built-in components such as body and collider, which is great! That is, until I inevitably need to extend either of those components with some custom behaviour to supplement the system, and this has put me in a bit of a pickle.

Consider this scenario: I want to add a set of raycasts pointing in each direction to aid in collision solving for my physics system. I could make a new separate component, but I'd like to leverage the existing body component on the actor as I think this is where it makes the most sense to be.

I could compose my own actor, but taking a peak at the default one shows there's a lot to replicate there. I thought about copying & pasting this into my own, but it uses an internal watch function.

I thought maybe I could replace the body component with my own:

class MyBodyComponent extends ex.BodyComponent {}

class MyActor extends ex.Actor {
    constructor() {
       super({})
       this.removeComponent('ex.body')
       this.addComponent(new MyBodyComponent())
    }

    public get body() { // <- called by super() before I've added it in my constructor
         return this.get(MyBodyComponent)
    }
}

super() accesses this.body which will call my custom getter, and throw an error because it won't exist yet.

So at this point I've had to settle with mutating the body, which is fine, but difficult to write cleanly

class MyActor extends ex.Actor {
    constructor() {
       super({})
       extendBody(this.body)
    }
}

extendBody(body) {
     body.myCustomProperty = 123
}

Any suggestions on how else this could be approached? I feel like composing my own Actor is the way to go here. If so, I wonder if there's a middle ground here between a bare entity and a custom Actor that Excalibur could offer.