implemented aspect filtering builder

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

@@ -1,8 +1,26 @@
-namespace MonoGame.Extended.Entities.Systems
+using MonoGame.Extended.Collections;
+
+namespace MonoGame.Extended.Entities.Systems
 {
     public abstract class BaseSystem
     {
+        private readonly Aspect.Builder _aspectBuilder;
+
+        protected BaseSystem(Aspect.Builder aspect)
+        {
+            _aspectBuilder = aspect;
+        }
+
+        protected Aspect Aspect { get; private set; }
+
         public EntityWorld  World { get; internal set; }
+
         public abstract void Initialize(ComponentManager componentManager);
+
+        protected Bag<Entity> GetEntities()
+        {
+            var aspect = _aspectBuilder.Build(World.ComponentManager);
+            return World.EntityManager.Entities;
+        }
     }
 }

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

@@ -4,7 +4,8 @@ namespace MonoGame.Extended.Entities.Systems
 {
     public abstract class DrawSystem : BaseSystem
     {
-        protected DrawSystem()
+        protected DrawSystem(Aspect.Builder aspect)
+            : base(aspect)
         {
         }
 

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

@@ -4,7 +4,8 @@ namespace MonoGame.Extended.Entities.Systems
 {
     public abstract class UpdateSystem : BaseSystem
     {
-        protected UpdateSystem()
+        protected UpdateSystem(Aspect.Builder aspect)
+            : base(aspect)
         {
         }