the big ComponentMessage rework

TODO

@@ -1,11 +1,6 @@
-- guard rail tests for nonexistent lookups in manager classes
+- Change "Writes" To "Sends" and make it only take Messages
-- custom exception for nonexistent Entity ID
+- Add "Updates" for updating components
-- custom exception for nonexistent Component ID
 
-- special Engine kinds (detector, modifier, spawner)
+- so we have four attributes: Activates, Reads, Updates, and Sends
-
-- maybe AddEngine should take a constructed engine similarly to AddComponent?
-
-- component getters should return ValueTuple instead of KeyValuePair so we can do destructuring assignments
 
 - docs

encompass-cs/Attributes/Activates.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Encompass.Exceptions;
+
+namespace Encompass
+{
+    public class Activates : Attribute
+    {
+        public readonly HashSet<Type> activateTypes = new HashSet<Type>();
+
+        public Activates(params Type[] activateTypes)
+        {
+            foreach (var activateType in activateTypes)
+            {
+                var isComponent = activateType.GetInterfaces().Contains(typeof(IComponent));
+                if (!isComponent)
+                {
+                    throw new IllegalActivateTypeException("{0} must be a Component", activateType.Name);
+                }
+
+                this.activateTypes.Add(typeof(ComponentMessage<>).MakeGenericType(activateType));
+            }
+        }
+    }
+}

encompass-cs/Attributes/Reads.cs

@@ -5,22 +5,32 @@ using Encompass.Exceptions;
 
 namespace Encompass
 {
-    [System.AttributeUsage(System.AttributeTargets.Class)]
+    [AttributeUsage(AttributeTargets.Class)]
-    public class Reads : System.Attribute
+    public class Reads : Attribute
     {
-        public readonly HashSet<Type> readTypes;
+        public readonly HashSet<Type> readTypes = new HashSet<Type>();
 
         public Reads(params Type[] readTypes)
         {
             foreach (var readType in readTypes)
             {
-                if (!readType.GetInterfaces().Contains(typeof(IMessage)))
+                var isMessage = readType.GetInterfaces().Contains(typeof(IMessage));
+                var isComponent = readType.GetInterfaces().Contains(typeof(IComponent));
+
+                if (!isMessage && !isComponent)
                 {
-                    throw new IllegalReadTypeException("{0} must be a Message", readType.Name);
+                    throw new IllegalReadTypeException("{0} must be a Message or Component", readType.Name);
+                }
+
+                if (isComponent)
+                {
+                    this.readTypes.Add(typeof(ComponentMessage<>).MakeGenericType(readType));
+                }
+                else
+                {
+                    this.readTypes.Add(readType);
                 }
             }
-
-            this.readTypes = new HashSet<Type>(readTypes);
         }
     }
 }

encompass-cs/ComponentManager.cs

@@ -62,8 +62,7 @@ namespace Encompass
             entityIDToComponentIDs[entityID].Add(componentID);
             componentIDToEntityID[componentID] = entityID;
 
-            inactiveComponents.Add(componentID);
+            activeComponents.Add(componentID);
-            MarkForActivation(componentID);
 
             entitiesWithAddedComponents.Add(entityID);
 
@@ -89,11 +88,11 @@ namespace Encompass
             return GetComponentIDsByEntityID(entityID).Intersect(activeComponents).Select((id) => new ValueTuple<Guid, IComponent>(id, IDToComponent[id]));
         }
 
-        internal IEnumerable<ValueTuple<Guid, TComponent>> GetActiveComponentsByType<TComponent>() where TComponent : struct, IComponent
+        internal IEnumerable<ValueTuple<Guid, Guid, TComponent>> GetActiveComponentsByType<TComponent>() where TComponent : struct, IComponent
         {
             return typeToComponentIDs.ContainsKey(typeof(TComponent)) ?
-                typeToComponentIDs[typeof(TComponent)].Intersect(activeComponents).Select((id) => new ValueTuple<Guid, TComponent>(id, (TComponent)IDToComponent[id])) :
+                typeToComponentIDs[typeof(TComponent)].Intersect(activeComponents).Select((id) => new ValueTuple<Guid, Guid, TComponent>(GetEntityIDByComponentID(id), id, (TComponent)IDToComponent[id])) :
-                Enumerable.Empty<ValueTuple<Guid, TComponent>>();
+                Enumerable.Empty<ValueTuple<Guid, Guid, TComponent>>();
         }
 
         internal IEnumerable<ValueTuple<Guid, IComponent>> GetActiveComponentsByType(Type type)
@@ -103,16 +102,11 @@ namespace Encompass
                 Enumerable.Empty<ValueTuple<Guid, IComponent>>();
         }
 
-        internal ValueTuple<Guid, TComponent> GetActiveComponentByType<TComponent>() where TComponent : struct, IComponent
-        {
-            return GetActiveComponentsByType<TComponent>().Single();
-        }
-
         internal IEnumerable<ValueTuple<Guid, TComponent>> GetComponentsByEntityAndType<TComponent>(Guid entityID) where TComponent : struct, IComponent
         {
             var entityComponentsByType = GetComponentsByEntity(entityID).Where((pair) => componentIDToType[pair.Item1] == typeof(TComponent)).Select((pair) => new ValueTuple<Guid, TComponent>(pair.Item1, (TComponent)pair.Item2));
             var activeComponentsByType = GetActiveComponentsByType<TComponent>();
-            return activeComponentsByType.Intersect(entityComponentsByType);
+            return activeComponentsByType.Select((triple) => (triple.Item2, triple.Item3)).Intersect(entityComponentsByType);
         }
 
         internal IEnumerable<ValueTuple<Guid, IComponent>> GetComponentsByEntityAndType(Guid entityID, Type type)
@@ -180,22 +174,7 @@ namespace Encompass
             }
         }
 
-        internal void MarkForActivation(Guid componentID)
+        internal void Activate(Guid componentID)
-        {
-            componentsMarkedForActivation.Add(componentID);
-        }
-
-        internal void ActivateMarkedComponents()
-        {
-            foreach (var componentID in componentsMarkedForActivation)
-            {
-                Activate(componentID);
-            }
-
-            componentsMarkedForActivation.Clear();
-        }
-
-        private void Activate(Guid componentID)
         {
             if (inactiveComponents.Remove(componentID))
             {

encompass-cs/ComponentMessage.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace Encompass
+{
+    public struct ComponentMessage<TComponent> : IMessage where TComponent : struct, IComponent
+    {
+        public Entity entity;
+        public Guid componentID;
+        public TComponent component;
+    }
+}

encompass-cs/Engine.cs

@@ -10,6 +10,7 @@ namespace Encompass
     {
         internal readonly HashSet<Type> writeTypes = new HashSet<Type>();
         internal readonly HashSet<Type> readTypes = new HashSet<Type>();
+        internal readonly HashSet<Type> activateTypes = new HashSet<Type>();
 
         private EntityManager entityManager;
         private ComponentManager componentManager;
@@ -28,6 +29,12 @@ namespace Encompass
             {
                 readTypes = readsAttribute.readTypes;
             }
+
+            var activatesAttribute = GetType().GetCustomAttribute<Activates>(false);
+            if (activatesAttribute != null)
+            {
+                activateTypes = activatesAttribute.activateTypes;
+            }
         }
 
         internal void AssignEntityManager(EntityManager entityManager)
@@ -82,29 +89,64 @@ namespace Encompass
             return (TComponent)componentManager.GetComponentByID(componentID);
         }
 
-        protected IEnumerable<ValueTuple<Guid, TComponent>> ReadComponents<TComponent>() where TComponent : struct, IComponent
+        internal IEnumerable<ValueTuple<Entity, Guid, TComponent>> ReadComponentsFromWorld<TComponent>() where TComponent : struct, IComponent
         {
-            return componentManager.GetActiveComponentsByType<TComponent>();
+            return componentManager.GetActiveComponentsByType<TComponent>().Select((triple) => (GetEntity(triple.Item1), triple.Item2, triple.Item3));
-        }
-
-        protected ValueTuple<Guid, TComponent> ReadComponent<TComponent>() where TComponent : struct, IComponent
-        {
-            return componentManager.GetActiveComponentByType<TComponent>();
         }
 
         protected Guid AddComponent<TComponent>(Entity entity, TComponent component) where TComponent : struct, IComponent
         {
-            return componentManager.AddComponent(entity.ID, component);
+            if (!activateTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalActivateException("Engine {0} tried to activate undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
+            var componentID = componentManager.AddComponent(entity.ID, component);
+
+            ComponentMessage<TComponent> componentMessage;
+            componentMessage.entity = entity;
+            componentMessage.componentID = componentID;
+            componentMessage.component = component;
+            EmitMessage(componentMessage);
+
+            return componentID;
         }
 
         protected Guid AddDrawComponent<TComponent>(Entity entity, TComponent component, int layer = 0) where TComponent : struct, IComponent
         {
-            return componentManager.AddDrawComponent(entity.ID, component, layer);
+            if (!activateTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalActivateException("Engine {0} tried to activate undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
+            var componentID = componentManager.AddDrawComponent(entity.ID, component, layer);
+
+            ComponentMessage<TComponent> componentMessage;
+            componentMessage.entity = entity;
+            componentMessage.componentID = componentID;
+            componentMessage.component = component;
+            EmitMessage(componentMessage);
+
+            return componentID;
         }
 
-        protected void ActivateComponent(Guid componentID)
+        protected void ActivateComponent<TComponent>(Guid componentID) where TComponent : struct, IComponent
         {
-            componentManager.MarkForActivation(componentID);
+            if (!activateTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalActivateException("Engine {0} tried to activate undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
+            var entity = GetEntity(componentManager.GetEntityIDByComponentID(componentID));
+            var component = GetComponentByID<TComponent>(componentID);
+
+            ComponentMessage<TComponent> componentMessage;
+            componentMessage.entity = entity;
+            componentMessage.componentID = componentID;
+            componentMessage.component = component;
+            EmitMessage(componentMessage);
+
+            componentManager.Activate(componentID);
         }
 
         protected void DeactivateComponent(Guid componentID)
@@ -114,6 +156,11 @@ namespace Encompass
 
         protected IEnumerable<ValueTuple<Guid, TComponent>> GetComponents<TComponent>(Entity entity) where TComponent : struct, IComponent
         {
+            if (!readTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
             return componentManager.GetComponentsByEntityAndType<TComponent>(entity.ID);
         }
 
@@ -124,7 +171,12 @@ namespace Encompass
 
         protected bool HasComponent<TComponent>(Entity entity) where TComponent : struct, IComponent
         {
-            return componentManager.EntityHasComponentOfType<TComponent>(entity.ID);
+            if (!readTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
+            return messageManager.GetMessagesByType<ComponentMessage<TComponent>>().Where((message) => message.entity == entity).Any();
         }
 
         internal void UpdateComponentInWorld<TComponent>(Guid componentID, TComponent newComponent) where TComponent : struct, IComponent
@@ -162,11 +214,31 @@ namespace Encompass
             return messageManager.GetMessagesByType<TMessage>();
         }
 
+        protected TMessage ReadMessage<TMessage>() where TMessage : struct, IMessage
+        {
+            return ReadMessages<TMessage>().Single();
+        }
+
+        protected IEnumerable<(Guid, TComponent)> ReadComponents<TComponent>() where TComponent : struct, IComponent
+        {
+            if (!readTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", this.GetType().Name, typeof(TComponent).Name);
+            }
+
+            return ReadMessages<ComponentMessage<TComponent>>().Select((message) => (message.componentID, message.component));
+        }
+
+        protected (Guid, TComponent) ReadComponent<TComponent>() where TComponent : struct, IComponent
+        {
+            return ReadComponents<TComponent>().Single();
+        }
+
         protected bool SomeMessage<TMessage>() where TMessage : struct, IMessage
         {
             if (!readTypes.Contains(typeof(TMessage)))
             {
-                throw new IllegalReadException("Engine {0} tried to read undeclared Message {1}", this.GetType().Name, typeof(TMessage).Name);
+                throw new IllegalReadException("Engine {0} tried to read undeclared Message {1}", GetType().Name, typeof(TMessage).Name);
             }
 
             return messageManager.GetMessagesByType<TMessage>().Any();
@@ -174,6 +246,11 @@ namespace Encompass
 
         protected bool SomeComponent<TComponent>() where TComponent : struct, IComponent
         {
+            if (!readTypes.Contains(typeof(ComponentMessage<TComponent>)))
+            {
+                throw new IllegalReadException("Engine {0} tried to read undeclared Component {1}", GetType().Name, typeof(TComponent).Name);
+            }
+
             return componentManager.GetActiveComponentsByType<TComponent>().Any();
         }
 

encompass-cs/Engines/ComponentMessageEmitter.cs

@@ -0,0 +1,30 @@
+using System.Reflection;
+
+namespace Encompass.Engines
+{
+    internal class ComponentMessageEmitter<TComponent> : Engine where TComponent : struct, IComponent
+    {
+        public ComponentMessageEmitter() : base()
+        {
+            var writesAttribute = GetType().GetCustomAttribute<Writes>(false);
+            if (writesAttribute != null)
+            {
+                writesAttribute.writeTypes.Add(typeof(ComponentMessage<TComponent>));
+            }
+
+            writeTypes.Add(typeof(ComponentMessage<TComponent>));
+        }
+
+        public override void Update(double dt)
+        {
+            foreach (var (entity, componentID, component) in ReadComponentsFromWorld<TComponent>())
+            {
+                ComponentMessage<TComponent> componentMessage;
+                componentMessage.entity = entity;
+                componentMessage.componentID = componentID;
+                componentMessage.component = component;
+                EmitMessage(componentMessage);
+            }
+        }
+    }
+}

encompass-cs/Entity.cs

@@ -2,7 +2,7 @@
 
 namespace Encompass
 {
-    public struct Entity
+    public struct Entity : IEquatable<Entity>
     {
         public readonly Guid ID;
 
@@ -10,5 +10,35 @@ namespace Encompass
         {
             this.ID = id;
         }
+
+        public override bool Equals(object obj)
+        {
+            if (obj is Entity)
+            {
+                return this.Equals((Entity)obj);
+            }
+
+            return false;
+        }
+
+        public bool Equals(Entity other)
+        {
+            return other.ID == ID;
+        }
+
+        public static bool operator ==(Entity one, Entity two)
+        {
+            return one.Equals(two);
+        }
+
+        public static bool operator !=(Entity one, Entity two)
+        {
+            return !one.Equals(two);
+        }
+
+        public override int GetHashCode()
+        {
+            return ID.GetHashCode();
+        }
     }
 }

encompass-cs/Exceptions/IllegalActivateException.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Encompass.Exceptions
+{
+    public class IllegalActivateException : Exception
+    {
+        public IllegalActivateException(
+            string format,
+            params object[] args
+        ) : base(string.Format(format, args)) { }
+    }
+}

encompass-cs/Exceptions/IllegalActivateTypeException.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Encompass.Exceptions
+{
+    public class IllegalActivateTypeException : Exception
+    {
+        public IllegalActivateTypeException(
+            string format,
+            params object[] args
+        ) : base(string.Format(format, args)) { }
+    }
+}

encompass-cs/Exceptions/UnregisteredComponentReadException.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Encompass.Exceptions
+{
+    public class UnregisteredComponentReadException : Exception
+    {
+        public UnregisteredComponentReadException(
+            string format,
+            params object[] args
+        ) : base(string.Format(format, args)) { }
+    }
+}

encompass-cs/Renderer.cs

@@ -41,12 +41,12 @@ namespace Encompass
 
         protected IEnumerable<ValueTuple<Guid, TComponent>> ReadComponents<TComponent>() where TComponent : struct, IComponent
         {
-            return componentManager.GetActiveComponentsByType<TComponent>();
+            return componentManager.GetActiveComponentsByType<TComponent>().Select((triple) => (triple.Item2, triple.Item3));
         }
 
         protected ValueTuple<Guid, TComponent> ReadComponent<TComponent>() where TComponent : struct, IComponent
         {
-            return componentManager.GetActiveComponentByType<TComponent>();
+            return ReadComponents<TComponent>().Single();
         }
 
         protected IEnumerable<ValueTuple<Guid, TComponent>> GetComponents<TComponent>(Entity entity) where TComponent : struct, IComponent

encompass-cs/World.cs

@@ -36,7 +36,6 @@ namespace Encompass
             entityManager.DestroyMarkedEntities();
 
             componentManager.PerformComponentUpdates();
-            componentManager.ActivateMarkedComponents();
             componentManager.DeactivateMarkedComponents();
             componentManager.RemoveMarkedComponents();
 

encompass-cs/WorldBuilder.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Reflection;
 using System.Linq;
 using Encompass.Exceptions;
+using Encompass.Engines;
 
 namespace Encompass
 {
@@ -17,9 +18,12 @@ namespace Encompass
         private readonly DrawLayerManager drawLayerManager;
         private readonly RenderManager renderManager;
 
-        private readonly Dictionary<Type, HashSet<Engine>> typeToEmitters = new Dictionary<Type, HashSet<Engine>>();
         private readonly Dictionary<Type, HashSet<Engine>> typeToReaders = new Dictionary<Type, HashSet<Engine>>();
 
+        private readonly HashSet<Engine> senders = new HashSet<Engine>();
+
+        private readonly HashSet<Type> registeredComponentTypes = new HashSet<Type>();
+
         public WorldBuilder()
         {
             var entitiesWithAddedComponents = new HashSet<Guid>();
@@ -56,6 +60,12 @@ namespace Encompass
             componentManager.MarkForDeactivation(componentID);
         }
 
+        internal void RegisterComponent(Type componentType)
+        {
+            registeredComponentTypes.Add(componentType);
+            AddEngine((Engine)Activator.CreateInstance(typeof(ComponentMessageEmitter<>).MakeGenericType(componentType)));
+        }
+
         public Engine AddEngine<TEngine>(TEngine engine) where TEngine : Engine
         {
             engine.AssignEntityManager(entityManager);
@@ -65,60 +75,42 @@ namespace Encompass
             engines.Add(engine);
             engineGraph.AddVertex(engine);
 
-            foreach (var writeType in engine.writeTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage))))
+            foreach (var activateType in engine.activateTypes)
             {
-                if (!typeToEmitters.ContainsKey(writeType))
+                engine.writeTypes.Add(activateType);
-                {
-                    typeToEmitters.Add(writeType, new HashSet<Engine>());
-                }
-
-                typeToEmitters[writeType].Add(engine);
-
-                if (typeToReaders.ContainsKey(writeType))
-                {
-                    foreach (var reader in typeToReaders[writeType])
-                    {
-                        if (engine == reader)
-                        {
-                            if (writeType.GetInterfaces().Contains(typeof(IMessage)))
-                            {
-                                throw new EngineMessageSelfCycleException("Engine both reads and writes Message {0}", writeType.Name);
-                            }
-                        }
-                        else
-                        {
-                            engineGraph.AddEdge(engine, reader);
-                        }
-                    }
-                }
             }
 
-            foreach (var readType in engine.readTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage))))
+            var messageReadTypes = engine.readTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage)));
+            var messageSendTypes = engine.writeTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage)));
+
+            if (messageReadTypes.Intersect(messageSendTypes).Any())
             {
+                var type = messageReadTypes.Intersect(messageSendTypes).First();
+                throw new EngineMessageSelfCycleException("Engine {0} both reads and writes Message {1}", engine.GetType().Name, type.Name);
+            }
+
+            if (messageSendTypes.Any())
+            {
+                senders.Add(engine);
+            }
+
+            foreach (var readType in engine.readTypes)
+            {
+                if (readType.IsGenericType && readType.GetGenericTypeDefinition() == typeof(ComponentMessage<>))
+                {
+                    var componentType = readType.GetGenericArguments().Single();
+                    if (!registeredComponentTypes.Contains(componentType))
+                    {
+                        RegisterComponent(componentType);
+                    }
+                }
+
                 if (!typeToReaders.ContainsKey(readType))
                 {
                     typeToReaders.Add(readType, new HashSet<Engine>());
                 }
 
                 typeToReaders[readType].Add(engine);
-
-                if (typeToEmitters.ContainsKey(readType))
-                {
-                    foreach (var emitter in typeToEmitters[readType])
-                    {
-                        if (emitter == engine)
-                        {
-                            if (readType.GetInterfaces().Contains(typeof(IMessage)))
-                            {
-                                throw new EngineMessageSelfCycleException("Engine both reads and writes Message {0}", readType.Name);
-                            }
-                        }
-                        else
-                        {
-                            engineGraph.AddEdge(emitter, engine);
-                        }
-                    }
-                }
             }
 
             return engine;
@@ -148,8 +140,27 @@ namespace Encompass
             return renderer;
         }
 
+        private void BuildEngineGraph()
+        {
+            foreach (var senderEngine in senders)
+            {
+                foreach (var messageType in senderEngine.writeTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage))))
+                {
+                    if (typeToReaders.ContainsKey(messageType))
+                    {
+                        foreach (var readerEngine in typeToReaders[messageType])
+                        {
+                            engineGraph.AddEdge(senderEngine, readerEngine);
+                        }
+                    }
+                }
+            }
+        }
+
         public World Build()
         {
+            BuildEngineGraph();
+
             if (engineGraph.Cyclic())
             {
                 var cycles = engineGraph.SimpleCycles();
@@ -225,7 +236,6 @@ namespace Encompass
             );
 
             componentManager.PerformComponentUpdates();
-            componentManager.ActivateMarkedComponents();
             componentManager.DeactivateMarkedComponents();
             componentManager.RemoveMarkedComponents();
 

test/ComponentTest.cs

@@ -24,7 +24,7 @@ namespace Tests
         static IEnumerable<(Guid, MockComponent)> gottenMockComponentIDPairs = Enumerable.Empty<(Guid, MockComponent)>();
         static (Guid, MockComponent) gottenMockComponentIDPair;
 
-        [Reads(typeof(EntityMessage))]
+        [Reads(typeof(EntityMessage), typeof(MockComponent))]
         class GetMockComponentsEngine : Engine
         {
             public override void Update(double dt)
@@ -38,7 +38,7 @@ namespace Tests
             }
         }
 
-        [Reads(typeof(EntityMessage))]
+        [Reads(typeof(EntityMessage), typeof(MockComponent))]
         class GetMockComponentEngine : Engine
         {
             public override void Update(double dt)
@@ -56,8 +56,8 @@ namespace Tests
             public MockComponent mockComponent;
         }
 
-        [Reads(typeof(AddComponentTestMessage))]
+        [Reads(typeof(AddComponentTestMessage), typeof(MockComponent))]
-        class AddComponentEngine : Engine
+        class AddComponentTestEngine : Engine
         {
             public override void Update(double dt)
             {
@@ -73,8 +73,7 @@ namespace Tests
         public void AddComponent()
         {
             var worldBuilder = new WorldBuilder();
-            worldBuilder.AddEngine(new AddComponentEngine());
+            worldBuilder.AddEngine(new AddComponentTestEngine());
-            worldBuilder.AddEngine(new GetMockComponentEngine());
 
             var entity = worldBuilder.CreateEntity();
 
@@ -94,6 +93,79 @@ namespace Tests
             world.Update(0.01);
         }
 
+        struct AddMockComponentMessage : IMessage
+        {
+            public Entity entity;
+            public MockComponent mockComponent;
+        }
+
+        [Writes(typeof(AddMockComponentMessage))]
+        class EmitMockComponentMessageEngine : Engine
+        {
+            private Entity entity;
+
+            public EmitMockComponentMessageEngine(Entity entity)
+            {
+                this.entity = entity;
+            }
+
+            public override void Update(double dt)
+            {
+                MockComponent mockComponent;
+                mockComponent.myInt = 10;
+                mockComponent.myString = "four";
+
+                AddMockComponentMessage addMockComponentMessage;
+                addMockComponentMessage.entity = entity;
+                addMockComponentMessage.mockComponent = mockComponent;
+                EmitMessage(addMockComponentMessage);
+            }
+        }
+
+        [Activates(typeof(MockComponent))]
+        [Reads(typeof(AddMockComponentMessage))]
+        class AddMockComponentEngine : Engine
+        {
+            public override void Update(double dt)
+            {
+                foreach (var message in ReadMessages<AddMockComponentMessage>())
+                {
+                    AddComponent(message.entity, message.mockComponent);
+                }
+            }
+        }
+
+        [Reads(typeof(MockComponent))]
+        class HasMockComponentEngine : Engine
+        {
+            private Entity entity;
+
+            public HasMockComponentEngine(Entity entity)
+            {
+                this.entity = entity;
+            }
+
+            public override void Update(double dt)
+            {
+                Assert.IsTrue(HasComponent<MockComponent>(entity));
+            }
+        }
+
+        [Test]
+        public void AddComponentAndReadSameFrame()
+        {
+            var worldBuilder = new WorldBuilder();
+            var entity = worldBuilder.CreateEntity();
+
+            worldBuilder.AddEngine(new EmitMockComponentMessageEngine(entity));
+            worldBuilder.AddEngine(new AddMockComponentEngine());
+            worldBuilder.AddEngine(new HasMockComponentEngine(entity));
+
+            var world = worldBuilder.Build();
+
+            world.Update(0.01);
+        }
+
         [Test]
         public void GetComponents()
         {
@@ -161,7 +233,7 @@ namespace Tests
             public Entity entity;
         }
 
-        [Reads(typeof(HasComponentTestMessage))]
+        [Reads(typeof(HasComponentTestMessage), typeof(MockComponent))]
         class HasComponentTestEngine : Engine
         {
             public override void Update(double dt)
@@ -201,7 +273,7 @@ namespace Tests
             public Entity entity;
         }
 
-        [Reads(typeof(HasComponentWhenInactiveTestMessage))]
+        [Reads(typeof(HasComponentWhenInactiveTestMessage), typeof(MockComponent))]
         class HasComponentWhenInactiveTestEngine : Engine
         {
             public override void Update(double dt)
@@ -244,7 +316,6 @@ namespace Tests
         }
 
         [Reads(typeof(RemoveComponentTestMessage))]
-        [Writes(typeof(MockComponent))]
         class RemoveComponentTestEngine : Engine
         {
             public override void Update(double dt)
@@ -318,15 +389,15 @@ namespace Tests
             public Guid componentID;
         }
 
+        [Activates(typeof(MockComponent))]
         [Reads(typeof(ActivateComponentMessage))]
-        [Writes(typeof(MockComponent))]
         class ActivateComponentEngine : Engine
         {
             public override void Update(double dt)
             {
                 foreach (var activateComponentMessage in ReadMessages<ActivateComponentMessage>())
                 {
-                    ActivateComponent(activateComponentMessage.componentID);
+                    ActivateComponent<MockComponent>(activateComponentMessage.componentID);
                 }
             }
         }
@@ -367,7 +438,7 @@ namespace Tests
             }
         }
 
-        [Reads(typeof(CheckHasMockComponentMessage))]
+        [Reads(typeof(CheckHasMockComponentMessage), typeof(MockComponent))]
         class CheckHasMockComponentEngine : Engine
         {
             public override void Update(double dt)
@@ -386,6 +457,8 @@ namespace Tests
             }
         }
 
+        // TODO: need to rethink this test because ActivateComponent is instant now
+
         [Test]
         public void ActivateComponent()
         {

test/EngineTest.cs

@@ -23,6 +23,7 @@ namespace Tests
 
         static List<MockMessage> resultMessages;
 
+        [Reads(typeof(MockComponent))]
         public class ReadComponentsTestEngine : Engine
         {
             public override void Update(double dt)
@@ -32,6 +33,7 @@ namespace Tests
         }
 
 
+        [Reads(typeof(MockComponent))]
         public class ReadComponentTestEngine : Engine
         {
             public override void Update(double dt)
@@ -117,6 +119,7 @@ namespace Tests
             Assert.Throws<InvalidOperationException>(() => world.Update(0.01f));
         }
 
+        [Reads(typeof(MockComponent))]
         [Writes(typeof(MockComponent))]
         public class UpdateComponentTestEngine : Engine
         {
@@ -136,6 +139,7 @@ namespace Tests
         public void UpdateComponent()
         {
             var worldBuilder = new WorldBuilder();
+            
             worldBuilder.AddEngine(new UpdateComponentTestEngine());
             worldBuilder.AddEngine(new ReadComponentTestEngine());
 
@@ -156,11 +160,12 @@ namespace Tests
             Assert.AreEqual("blaze it", resultComponent.myString);
         }
 
+        [Reads(typeof(MockComponent))]
         public class UndeclaredUpdateComponentTestEngine : Engine
         {
             public override void Update(double dt)
             {
-                (var componentID, var component) = this.ReadComponent<MockComponent>();
+                (var componentID, var component) = ReadComponent<MockComponent>();
 
                 component.myInt = 420;
                 component.myString = "blaze it";
@@ -358,6 +363,7 @@ namespace Tests
         static ValueTuple<Guid, MockComponent> pairA;
         static ValueTuple<Guid, MockComponent> pairB;
 
+        [Reads(typeof(MockComponent))]
         class SameValueComponentReadEngine : Engine
         {
             public override void Update(double dt)
@@ -397,6 +403,7 @@ namespace Tests
 
         static IEnumerable<ValueTuple<Guid, MockComponent>> emptyComponentReadResult;
 
+        [Reads(typeof(MockComponent))]
         class ReadEmptyMockComponentsEngine : Engine
         {
             public override void Update(double dt)
@@ -419,6 +426,7 @@ namespace Tests
 
         struct DestroyerComponent : IComponent { }
 
+        [Reads(typeof(DestroyerComponent))]
         class DestroyerEngine : Engine
         {
             public override void Update(double dt)
@@ -434,6 +442,7 @@ namespace Tests
 
         static IEnumerable<ValueTuple<Guid, MockComponent>> results;
 
+        [Reads(typeof(MockComponent))]
         class ReaderEngine : Engine
         {
             public override void Update(double dt)
@@ -471,6 +480,7 @@ namespace Tests
             Assert.That(results, Does.Not.Contain((componentBID, mockComponent)));
         }
 
+        [Reads(typeof(DestroyerComponent), typeof(MockComponent))]
         [Writes(typeof(MockComponent))]
         class DestroyAndAddComponentEngine : Engine
         {
@@ -506,6 +516,7 @@ namespace Tests
 
         static Entity entityFromComponentIDResult;
 
+        [Reads(typeof(MockComponent))]
         class GetEntityFromComponentIDEngine : Engine
         {
             public override void Update(double dt)
@@ -536,6 +547,7 @@ namespace Tests
 
         static MockComponent mockComponentByIDResult;
 
+        [Reads(typeof(MockComponent))]
         class GetComponentByIDEngine : Engine
         {
             public override void Update(double dt)
@@ -565,6 +577,7 @@ namespace Tests
 
         struct OtherComponent : IComponent { }
 
+        [Reads(typeof(MockComponent))]
         class GetComponentByIDWithTypeMismatchEngine : Engine
         {
             public override void Update(double dt)
@@ -595,6 +608,7 @@ namespace Tests
         struct EntityIDComponent : IComponent { public Guid entityID;  }
         static bool hasEntity;
 
+        [Reads(typeof(EntityIDComponent))]
         class HasEntityTestEngine : Engine
         {
             public override void Update(double dt)

test/SpawnerTest.cs

@@ -23,7 +23,7 @@ namespace Tests
             }
         }
 
-        [Writes(typeof(TestComponent))]
+        [Activates(typeof(TestComponent))]
         class TestSpawner : Spawner<SpawnMessageA>
         {
             protected override void Spawn(SpawnMessageA message)