Nerfed/Nerfed.Runtime/Systems/ParentSystem.cs

using MoonTools.ECS;
using Nerfed.Runtime.Components;

namespace Nerfed.Runtime.Systems
{
    public class ParentSystem : MoonTools.ECS.System
    {
        private readonly Filter parentsAddedFilter;
        private readonly Filter parentsRemovedFilter;
        private readonly Filter parentsFilter;

        public ParentSystem(World world) : base(world)
        {
            parentsAddedFilter = FilterBuilder.Include<Parent>().Exclude<PreviousParent>().Build();
            parentsRemovedFilter = FilterBuilder.Include<PreviousParent>().Exclude<Parent>().Build();
            parentsFilter = FilterBuilder.Include<Parent>().Include<PreviousParent>().Build();
        }

        public override void Update(TimeSpan delta)
        {
            // Update removed parents.
            foreach (Entity entity in parentsRemovedFilter.Entities)
            {
                // Do stuff here to update/remove child relations etc.
                PreviousParent previousParent = Get<PreviousParent>(entity);
                World.Unrelate<ChildRelation>(previousParent.parentEntity, entity);
                Remove<PreviousParent>(entity);
            }

            // Update added parents.
            foreach (Entity entity in parentsAddedFilter.Entities)
            {
                Parent parent = Get<Parent>(entity);

                if (Has<Parent>(parent.parentEntity) && Get<Parent>(parent.parentEntity).parentEntity == entity)
                {
                    Log.Warning($"Entity {entity} cannot be a parent of entity {parent.parentEntity}, because {parent.parentEntity} is the parent of {entity}");
                    Remove<Parent>(entity);
                    continue;
                }

                PreviousParent previousParent = new(parent.parentEntity);
                Set(entity, previousParent);
                World.Relate(parent.parentEntity, entity, new ChildRelation());
            }

            // Update relations if the parent has changed.
            foreach (Entity entity in parentsFilter.Entities)
            {
                Parent parent = Get<Parent>(entity);
                PreviousParent previousParent = Get<PreviousParent>(entity);

                if(parent.parentEntity != previousParent.parentEntity)
                {
                    World.Unrelate<ChildRelation>(previousParent.parentEntity, entity);
                    Set(entity, new PreviousParent(parent.parentEntity));
                    World.Relate(parent.parentEntity, entity, new ChildRelation());
                }
            }
        }
    }
}