Port new entity event feature

Author: BeanCheeseBurrito

src/Flecs.NET.Examples/Cpp/Observers/EntityEvent.cs

@@ -0,0 +1,38 @@
+// Entity events are events that are emitted and observed for a specific entity.
+// They are a thin wrapper around regular observers, which match against queries
+// instead of single entities. While they work similarly under the hood, entity
+// events provide a much simpler API.
+//
+// An entity event only needs two pieces of data:
+// - The entity on which to emit the event
+// - The event to emit
+
+#if Cpp_Observers_EntityEvent
+
+using Flecs.NET.Core;
+
+{
+    using World world = World.Create();
+
+    // Create a widget entity
+    Entity widget = world.Entity("MyWidget");
+
+    // Observe the OnClick event on the widget entity
+    widget.Observe<OnClick>((Iter it) =>
+    {
+        // The event source can be obtained with it.Src(1). This allows the same
+        // event function to be used for different entities.
+        Console.WriteLine($"Clicked on {it.Src(1).Path()}!");
+    });
+
+    // Emit the OnClick event for the widget
+    widget.Emit<OnClick>();
+}
+
+// The event to emit.
+public struct OnClick { }
+
+#endif
+
+// Output:
+// Clicked on MyWidget!

src/Flecs.NET/Core/Entity.cs

@@ -1043,6 +1043,27 @@ public TEnum ToConstant<TEnum>() where TEnum : unmanaged, Enum
             return *ptr;
         }
 
+        /// <summary>
+        ///     Emits an event for this entity.
+        /// </summary>
+        /// <param name="evt"></param>
+        public void Emit(ulong evt)
+        {
+            new World(World)
+                .Event(evt)
+                .Entity(Id)
+                .Emit();
+        }
+
+        /// <summary>
+        ///     Emits an event for this entity.
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        public void Emit<T>()
+        {
+            Emit(Type<T>.Id(World));
+        }
+
         /// <summary>
         ///     Short for Has(EcsChildOf, entity).
         /// </summary>
@@ -2212,6 +2233,38 @@ public ref Entity SetJsonSecond<TSecond>(ulong first, string json, ecs_from_json
             return ref SetJson(Macros.PairSecond<TSecond>(first, World), json, desc);
         }
 
+        /// <summary>
+        ///     Observe an event on this entity.
+        /// </summary>
+        /// <param name="evt"></param>
+        /// <param name="callback"></param>
+        /// <returns></returns>
+        public ref Entity Observe(ulong evt, Ecs.IterCallback callback)
+        {
+            World world = new World(World);
+
+            Observer observer = world.Observer(
+                filter: world.FilterBuilder().With(EcsAny).Src(Id),
+                observer: world.ObserverBuilder().Event(evt),
+                callback: callback
+            );
+
+            observer.Entity.ChildOf(Id);
+
+            return ref this;
+        }
+
+        /// <summary>
+        ///     Observe an event on this entity.
+        /// </summary>
+        /// <param name="callback"></param>
+        /// <typeparam name="T"></typeparam>
+        /// <returns></returns>
+        public ref Entity Observe<T>(Ecs.IterCallback callback)
+        {
+            return ref Observe(Type<T>.Id(World), callback);
+        }
+
         /// <summary>
         ///     Get mutable component value (untyped).
         /// </summary>

src/Flecs.NET/Core/EventBuilder.cs

@@ -104,14 +104,7 @@ public ref EventBuilder Id<TFirst, TSecond>()
         /// <returns></returns>
         public ref EventBuilder Entity(ulong entity)
         {
-            ecs_record_t* r = ecs_record_find(World, entity);
-
-            Ecs.Assert(r != null, nameof(ECS_INVALID_PARAMETER));
-            Ecs.Assert(r->table != null, nameof(ECS_INVALID_PARAMETER));
-
-            _desc.table = r->table;
-            _desc.offset = Macros.RecordToRow(r->row);
-            _desc.count = 1;
+            Desc.entity = entity;
             return ref this;
         }
 
@@ -158,9 +151,6 @@ public ref EventBuilder Ctx<T>(T* data) where T : unmanaged
         /// </summary>
         public void Emit()
         {
-            Ecs.Assert(_ids.count != 0, nameof(ECS_INVALID_PARAMETER));
-            Ecs.Assert(_desc.table != null, nameof(ECS_INVALID_PARAMETER));
-
             fixed (EventBuilder* self = &this)
             {
                 _ids.array = self->_idsArray;