How do I handle lists/arrays in Unity DOTS/ECS?

Question

I'm new to ECS and am having issues finding the right data structure to use.

For context, I have a galaxy with solar systems that produce resources at a given rate (ResourceProducer component). Resources are tallied in a stockpile (ResourceStockpile) and the individual types are defined in a Resources enum.

public struct ResourceProducer : IComponentData {
    public Resources resourceType;
    public float quantity;
    public float interval;
}

(ResourceStockpile is the same but no interval)

I had originally intended to have multiple producers and stockpiles per location, one per resource type, but Unity objects to having multiple of the same component on a single entity (and I've seen some Stack Overflow posts supporting this philosophy).

So I then created a

public struct ResourceQuantity {
    public Resources resourceType;
    public float quantity;
}

and changed my stockpile to use

public ResourceQuantity[] resources;

(Planning to worry about production rate later as I can't represent it here. Also, I'd prefer a list, but an array seemed more appropriate given the error below)

But now I get an issue in my baker....

public class ResourceStockpileBaker : Baker<ResourceStockpileAuthoring> {
    public override void Bake(ResourceStockpileAuthoring authoring) {
        Entity entity = this.GetEntity(new TransformUsageFlags());
        AddComponent(entity, new ResourceStockpile {
            resources = authoring.resources
        }) ;
    }
}

AddComponent fails to compile with

The type 'ResourceStockpile' must be a non-nullable value type, along with all fields at any level of nesting, in order to use it as parameter 'T' in the generic type or method 'IBaker.AddComponent(Entity, in T)'

It feels like I may be dealing with an XY problem... I'm going down a rabbit hole without knowing it's the right one.

What's the correct way to represent an entity that produces multiple resources at different rates, to be accumulated in a stockpile for later usage?

Answer 1

If you want a component that doesn't have a 1:1 relationship with entities but a n:1 relationship, as in "an entity can have multiple instances of that component", then you need a Dynamic Buffer Component.

A dynamic buffer component is basically a component that is not a single structure but an array of structures.

I actually created a ECS prototype of a game that seems very similar to yours. That was about a year ago, so considering the speed with which DOTS evolves it probably no longer follows the idiomatic style of the week. But this is how I solved this back then.

This is my ResourceStorage structure. Each entity with an inventory has one per resource.

public enum ResourceType : short {
    METAL = 0,
    FOOD = 1
    // Remember to update the InternalBufferCapacity of the ResourceStorage buffer component below when adding more resources
}

[InternalBufferCapacity(2)] 
public struct ResourceStorage : IBufferElementData {
    public ResourceType type;
    public float max;
    public float current;
}

This is the code that adds an inventory to an entity:

public void SetResourceStorageData(IEnumerable<ResourceStorage> values, Entity entity) {
    EntityManager manager = World.DefaultGameObjectInjectionWorld.EntityManager;
    DynamicBuffer<ResourceStorage> buffer;
    buffer = manager.AddBuffer<ResourceStorage>(entity);

    foreach (ResourceStorage entry in values) {
        buffer.Add(entry);
    }
}

This is the resource production system that increases it:

public partial class ResourceProductionSystem : SystemBase
{
    protected override void OnUpdate()
    {

        float deltaTime = SystemAPI.Time.DeltaTime;
        
        Entities.ForEach((DynamicBuffer<model.ResourceStorage> storages) => {
            for (var i = 0; i < storages.Length; i++) {
                var storage = storages[i];
                storage.current += deltaTime;
                storage.current = math.clamp(storage.current, 0.0f, storage.max);
                storages[i] = storage;                
            }
        }).ScheduleParallel();
    }
}