DefaultWritePriority attribute for engines

encompass-cs/Attributes/DefaultWritePriority.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Encompass
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    public class DefaultWritePriority : Attribute
+    {
+        public int writePriority;
+
+        public DefaultWritePriority(int writePriority)
+        {
+            this.writePriority = writePriority;
+        }
+    }
+}

encompass-cs/WorldBuilder.cs

@@ -192,16 +192,26 @@ namespace Encompass
 
             foreach (var engine in engines)
             {
+                var defaultWritePriorityAttribute = engine.GetType().GetCustomAttribute<DefaultWritePriority>(false);
+
                 var writeTypes = engine.sendTypes.Where((type) => type.IsGenericType && type.GetGenericTypeDefinition() == typeof(ComponentWriteMessage<>));
 
                 foreach (var writeType in writeTypes)
                 {
                     var componentType = writeType.GetGenericArguments()[0];
 
+                    int? priority = null;
                     if (engine.writePriorities.ContainsKey(componentType))
                     {
-                        var priority = engine.writePriorities[componentType];
+                        priority = engine.writePriorities[componentType];
+                    }
+                    else if (defaultWritePriorityAttribute != null)
+                    {
+                        priority = defaultWritePriorityAttribute.writePriority;
+                    }
 
+                    if (priority.HasValue)
+                    {
                         writtenComponentTypesWithPriority.Add(componentType);
 
                         if (!writePriorities.ContainsKey(componentType))
@@ -209,7 +219,7 @@ namespace Encompass
                             writePriorities[componentType] = new HashSet<int>();
                         }
 
-                        if (writePriorities[componentType].Contains(priority))
+                        if (writePriorities[componentType].Contains(priority.Value))
                         {
                             duplicateWritesWithSamePriority.Add(componentType);
                         }
@@ -219,7 +229,7 @@ namespace Encompass
                         }
                         else
                         {
-                            writePriorities[componentType].Add(priority);
+                            writePriorities[componentType].Add(priority.Value);
                         }
                     }
                     else
@@ -252,7 +262,7 @@ namespace Encompass
                     componentType.Name + " written by: " +
                     string.Join(", ", writeMessageToEngines[componentType].Select((engine) => engine.GetType().Name));
                 }
-                errorString += "\nTo resolve the conflict, add priority arguments to the Writes declarations.";
+                errorString += "\nTo resolve the conflict, add priority arguments to the Writes declarations or use a DefaultWritePriority attribute.";
 
                 throw new EngineWriteConflictException(errorString);
             }
@@ -266,7 +276,7 @@ namespace Encompass
                     componentType.Name + " written by: " +
                     string.Join(", ", writeMessageToEngines[componentType].Select(engine => engine.GetType().Name));
                 }
-                errorString += "\nTo resolve the conflict, add priority arguments to the Writes declarations.";
+                errorString += "\nTo resolve the conflict, add priority arguments to the Writes declarations or use a DefaultWritePriority attribute.";
 
                 throw new EngineWriteConflictException(errorString);
             }

test/WorldBuilderTest.cs

@@ -208,6 +208,77 @@ namespace Tests
             }
         }
 
+        public class DefaultWritePriority
+        {
+            struct SetMessage : IMessage
+            {
+                public Entity entity;
+            }
+
+            struct AComponent : IComponent
+            {
+                public int myInt;
+            }
+
+            [Receives(typeof(SetMessage))]
+            [Writes(typeof(AComponent))]
+            [WritesPending(typeof(AComponent))]
+            [Encompass.DefaultWritePriority(1)]
+            class AEngine : Engine
+            {
+                public override void Update(double dt)
+                {
+                    foreach (var setMessage in ReadMessages<SetMessage>())
+                    {
+                        SetComponent(setMessage.entity, new AComponent { myInt = 0 });
+                    }
+                }
+            }
+
+
+            [Receives(typeof(SetMessage))]
+            [Writes(typeof(AComponent), 3)]
+            [WritesPending(typeof(AComponent))]
+            class BEngine : Engine
+            {
+                public override void Update(double dt)
+                {
+                    foreach (var setMessage in ReadMessages<SetMessage>())
+                    {
+                        SetComponent(setMessage.entity, new AComponent { myInt = 1 });
+                    }
+                }
+            }
+
+            static AComponent resultComponent;
+
+            [ReadsPending(typeof(AComponent))]
+            class ReadComponentEngine : Engine
+            {
+                public override void Update(double dt)
+                {
+                    resultComponent = ReadComponent<AComponent>().Item2;
+                }
+            }
+
+            [Test]
+            public void LowerPriorityWrites()
+            {
+                var worldBuilder = new WorldBuilder();
+                worldBuilder.AddEngine(new AEngine());
+                worldBuilder.AddEngine(new BEngine());
+
+                var entity = worldBuilder.CreateEntity();
+                worldBuilder.SendMessage(new SetMessage { entity = entity });
+
+                var world = worldBuilder.Build();
+
+                world.Update(0.01);
+
+                Assert.That(resultComponent.myInt, Is.EqualTo(0));
+            }
+        }
+
         public class EngineMessageSelfCycle
         {
             struct AMessage : IMessage { }