update position engine

content/pong/ball/bouncing/motion_engine.md

@@ -195,6 +195,30 @@ namespace PongFE.Messages
 
 This is pretty straightforward. We'll use this message to set an entity's position.
 
+In **PongFE/Engines/UpdatePositionEngine.cs**:
+
+```cs
+using Encompass;
+using PongFE.Components;
+using PongFE.Messages;
+
+namespace PongFE.Engines
+{
+    [Receives(typeof(UpdatePositionMessage))]
+    [Writes(typeof(PositionComponent))]
+    public class UpdatePositionEngine : Engine
+    {
+        public override void Update(double dt)
+        {
+            foreach (ref readonly var message in ReadMessages<UpdatePositionMessage>())
+            {
+                SetComponent(message.Entity, new PositionComponent(message.Position));
+            }
+        }
+    }
+}
+```
+
 Now we can send these messages in **MotionEngine**.
 
 ```cs