💔 Breaking the rules

This is the not-really-ECS part, but may make your game development way faster.

🚪 State machines

A state machine is pretty much a component with logic. The actual implementation in the game looks more or less like this:

internal class BuyFromShop : StateMachine
{
    public BuyFromShop()
    {
        State(Idle);
    }

    private IEnumerator<Wait> Idle()
    {
        yield return Wait.ForMessage<InteractMessage>();
        yield return GoTo(ShowShop);
    }

    private IEnumerator<Wait> ShowShop()
    {
        while (true)
        {
            yield return Wait.NextFrame;
        }
    }
}

This will be wrapped in a StateMachineComponent<BuyFromShop> component, and you can fetch this component with entity.GetStateMachine(), so you can only have one state machine per entity.

🔔 Interactions

This is when an entity wants to send an interaction to another entity. This is a real example of an interaction that adds a child from a prefab to another entity:

public readonly struct AddChildOnInteraction : Interaction
{
    [JsonProperty]
    private readonly AssetRef<PrefabAsset> _child = new();

    [JsonProperty]
    private readonly string? _name = null;

    public AddChildOnInteraction() { }

    public void Interact(World world, Entity interactor, Entity? interacted)
    {
        if (interacted is null)
        {
            return;
        }

        Entity child = _child.Asset.CreateAndFetch(world);
        interacted.AddChild(child.EntityId, _name);
    }
}

🧑‍👧‍👦 Families

This is where ECS libraries are rather obscure about it. We have a family hierarchy, in which entities hold references to other entities (with their id) that are parents or children. A component that will be affected by this hierarchy will inherit from IParentRelativeComponent, so Bang can send notifications whenever the parent component was modified.

If a parent gets destroyed, its children also get destroyed. If a child gets destroyed, the parent will no longer reference them.