MonoGame.Extended/Source/MonoGame.Extended.Entities/EntityProcessingSystem.cs

// Original code derived from: 
// https://github.com/thelinuxlich/artemis_CSharp/blob/master/Artemis_XNA_INDEPENDENT/System/EntityProcessingSystem.cs

// --------------------------------------------------------------------------------------------------------------------
// <copyright file="EntityProcessingSystem.cs" company="GAMADU.COM">
//     Copyright © 2013 GAMADU.COM. All rights reserved.
//
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//           conditions and the following disclaimer.
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// </copyright>
// <summary>
//   Class EntityProcessingSystem.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

using System.Collections.Generic;
using Microsoft.Xna.Framework;

namespace MonoGame.Extended.Entities
{
    public abstract class EntityProcessingSystem : ProcessingSystem
    {
        protected EntityProcessingSystem()
        {
        }

        private readonly Dictionary<Entity, int> _activeEntitiesLookup = new Dictionary<Entity, int>();

        internal int Index;
        internal Aspect Aspect;

        private readonly List<Entity> _activeEntities = new List<Entity>();
        public IReadOnlyCollection<Entity> ActiveEntities => _activeEntities;

        internal virtual void RefreshEntityComponents(Entity entity)
        {
            var contains = entity.SystemBits[Index];
            var isInterested = Aspect.Matches(entity.ComponentBits);

            if (contains)
                Remove(entity);
            else if (isInterested)
                Add(entity);
        }

        public virtual void OnEntityAdded(Entity entity) { }
        public virtual void OnEntityDisabled(Entity entity) { }
        public virtual void OnEntityEnabled(Entity entity) { }
        public virtual void OnEntityRemoved(Entity entity) { }

        protected override void Process(GameTime gameTime)
        {
            base.Process(gameTime);

            for (var i = _activeEntities.Count - 1; i >= 0; i--)
            {
                var entity = _activeEntities[i];
                Process(gameTime, entity);
            }
        }

        protected abstract void Process(GameTime gameTime, Entity entity);

        protected bool IsInterestedIn(Entity entity)
        {
            return Aspect.Matches(entity.ComponentBits);
        }

        internal void Add(Entity entity)
        {
            if (entity == null)
                return;

            entity.SystemBits[Index] = true;

            if (_activeEntitiesLookup.ContainsKey(entity))
                return;

            _activeEntitiesLookup.Add(entity, _activeEntities.Count);
            _activeEntities.Add(entity);

            OnEntityAdded(entity);
        }

        internal void Remove(Entity entity)
        {
            if (entity == null)
                return;

            entity.SystemBits[Index] = false;

            int activeEntityIndex;
            if (!_activeEntitiesLookup.TryGetValue(entity, out activeEntityIndex))
                return;
            _activeEntitiesLookup.Remove(entity);

            var swapEntity = _activeEntities[_activeEntities.Count - 1];

            if (entity != swapEntity)
                _activeEntitiesLookup[swapEntity] = activeEntityIndex;

            _activeEntities[activeEntityIndex] = swapEntity;
            _activeEntities.RemoveAt(_activeEntities.Count - 1);

            OnEntityRemoved(entity);
        }
    }  
}