rejigged the base systems classes and interfaces

Source/MonoGame.Extended.Entities/Systems/EntitySystem.cs

@@ -0,0 +1,47 @@
+using MonoGame.Extended.Collections;
+
+namespace MonoGame.Extended.Entities.Systems
+{
+    public abstract class EntitySystem : ISystem
+    {
+        protected EntitySystem(AspectBuilder aspectBuilder)
+        {
+            _aspectBuilder = aspectBuilder;
+        }
+
+        public void Dispose()
+        {
+            if (World != null)
+            {
+                World.EntityManager.EntityAdded -= OnEntityAdded;
+                World.EntityManager.EntityRemoved -= OnEntityRemoved;
+            }
+        }
+
+        public virtual void Initialize(EntityWorld world)
+        {
+            World = world;
+
+            var aspect = _aspectBuilder.Build(World.ComponentManager);
+            _subscription = new EntitySubscription(World.EntityManager, aspect);
+            World.EntityManager.EntityAdded += OnEntityAdded;
+            World.EntityManager.EntityRemoved += OnEntityRemoved;
+            World.EntityManager.EntityChanged += OnEntityChanged;
+
+            Initialize(world.ComponentManager);
+        }
+
+        public abstract void Initialize(IComponentMapperService mapperService);
+
+        private readonly AspectBuilder _aspectBuilder;
+        private EntitySubscription _subscription;
+
+        public EntityWorld World { get; private set; }
+
+        protected virtual void OnEntityChanged(int entityId) { }
+        protected virtual void OnEntityAdded(int entityId) { }
+        protected virtual void OnEntityRemoved(int entityId) { }
+
+        public Bag<int> ActiveEntities => _subscription.ActiveEntities;
+    }
+}