1 files changed, 61 insertions, 0 deletions
diff --git a/Plugins/MonoGame.Extended/source/MonoGame.Extended.Entities/EntitySubscription.cs b/Plugins/MonoGame.Extended/source/MonoGame.Extended.Entities/EntitySubscription.cs
new file mode 100644
index 0000000..6766b0f
--- /dev/null
+++ b/Plugins/MonoGame.Extended/source/MonoGame.Extended.Entities/EntitySubscription.cs
@@ -0,0 +1,61 @@
+using System;
+using MonoGame.Extended.Collections;
+
+namespace MonoGame.Extended.Entities
+{
+    internal class EntitySubscription : IDisposable
+    {
+        private readonly Bag<int> _activeEntities;
+        private readonly EntityManager _entityManager;
+        private readonly Aspect _aspect;
+        private bool _rebuildActives;
+
+        internal EntitySubscription(EntityManager entityManager, Aspect aspect)
+        {
+            _entityManager = entityManager;
+            _aspect = aspect;
+            _activeEntities = new Bag<int>(entityManager.Capacity);
+            _rebuildActives = true;
+
+            _entityManager.EntityAdded += OnEntityAdded;
+            _entityManager.EntityRemoved += OnEntityRemoved;
+            _entityManager.EntityChanged += OnEntityChanged;
+        }
+
+        private void OnEntityAdded(int entityId)
+        {
+            if (_aspect.IsInterested(_entityManager.GetComponentBits(entityId)))
+                _activeEntities.Add(entityId);
+        }
+
+        private void OnEntityRemoved(int entityId) => _rebuildActives = true;
+        private void OnEntityChanged(int entityId) => _rebuildActives = true;
+
+        public void Dispose()
+        {
+            _entityManager.EntityAdded -= OnEntityAdded;
+            _entityManager.EntityRemoved -= OnEntityRemoved;
+        }
+
+        public Bag<int> ActiveEntities
+        {
+            get
+            {
+                if (_rebuildActives)
+                    RebuildActives();
+
+                return _activeEntities;
+            }
+        }
+
+        private void RebuildActives()
+        {
+            _activeEntities.Clear();
+
+            foreach (var entity in _entityManager.Entities)
+                OnEntityAdded(entity);
+
+            _rebuildActives = false;
+        }
+    }
+}
+\ No newline at end of file