add Spawner engine type

encompass-cs/Engine.cs

@@ -8,9 +8,9 @@ namespace Encompass
 {
     public abstract class Engine
     {
-        private readonly List<Type> mutateComponentTypes = new List<Type>();
+        internal readonly List<Type> mutateComponentTypes = new List<Type>();
-        private readonly List<Type> emitMessageTypes = new List<Type>();
+        internal readonly List<Type> emitMessageTypes = new List<Type>();
-        private readonly List<Type> readMessageTypes = new List<Type>();
+        internal readonly List<Type> readMessageTypes = new List<Type>();
 
         private EntityManager entityManager;
         private ComponentManager componentManager;

encompass-cs/Engines/Detector.cs

@@ -12,7 +12,7 @@ namespace Encompass.Engines
 
         public IEnumerable<Type> ComponentTypes { get { return componentTypes; } }
 
-        protected Detector()
+        protected Detector() : base()
         {
             var detectsAttribute = GetType().GetCustomAttribute<Detects>(false);
             if (detectsAttribute != null)

encompass-cs/Engines/Spawner.cs

@@ -0,0 +1,28 @@
+using System.Reflection;
+
+namespace Encompass.Engines
+{
+    public abstract class Spawner<TMessage> : Engine where TMessage : struct, IMessage
+    {
+        protected Spawner() : base()
+        {
+            var readsAttribute = GetType().GetCustomAttribute<Reads>(false);
+            if (readsAttribute != null)
+            {
+                readsAttribute.readMessageTypes.Add(typeof(TMessage));
+            }
+
+            readMessageTypes.Add(typeof(TMessage));
+        }
+
+        public override void Update(double dt)
+        {
+            foreach (var message in ReadMessages<TMessage>())
+            {
+                Spawn(message);
+            }
+        }
+
+        protected abstract void Spawn(TMessage message);
+    }
+}

encompass-cs/WorldBuilder.cs

@@ -36,6 +36,11 @@ namespace Encompass
             return entityManager.CreateEntity();
         }
 
+        public void EmitMessage<TMessage>(TMessage message) where TMessage : struct, IMessage
+        {
+            messageManager.AddMessage(message);
+        }
+
         public Engine AddEngine<TEngine>(TEngine engine) where TEngine : Engine
         {
             engine.AssignEntityManager(entityManager);

test/SpawnerTest.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Encompass;
+using Encompass.Engines;
+using NUnit.Framework;
+
+namespace Tests
+{
+    public class SpawnerTest
+    {
+        struct TestComponent : IComponent { }
+        struct SpawnMessageA : IMessage { }
+
+        static Entity resultEntity;
+
+        class TestSpawner : Spawner<SpawnMessageA>
+        {
+            protected override void Spawn(SpawnMessageA message)
+            {
+                resultEntity = CreateEntity();
+                resultEntity.AddComponent(new TestComponent());
+            }
+        }
+
+        [Test]
+        public void RunsSpawnMethodOnMessageRead()
+        {
+            var worldBuilder = new WorldBuilder();
+            worldBuilder.AddEngine(new TestSpawner());
+
+            worldBuilder.EmitMessage(new SpawnMessageA());
+
+            var world = worldBuilder.Build();
+
+            world.Update(0.01);
+
+            Assert.That(resultEntity.HasComponent<TestComponent>(), Is.True);
+
+            var id = resultEntity.id;
+
+            world.Update(0.01);
+
+            Assert.That(resultEntity.id, Is.EqualTo(id));
+        }
+    }
+}