Add support for singleton entities

Author: halgari

src/NexusMods.EventSourcing.Abstractions/IEntityContext.cs

@@ -17,6 +17,14 @@ public interface IEntityContext
     public TEntity Get<TEntity>(EntityId<TEntity> id) where TEntity : IEntity;
 
 
+    /// <summary>
+    /// Gets the singleton entity of the specified type.
+    /// </summary>
+    /// <typeparam name="TEntity"></typeparam>
+    /// <returns></returns>
+    public TEntity Get<TEntity>() where TEntity : ISingletonEntity;
+
+
     /// <summary>
     /// Transacts a new event into the context.
     /// </summary>

src/NexusMods.EventSourcing.Abstractions/ISingletonEntity.cs

@@ -3,7 +3,7 @@ namespace NexusMods.EventSourcing.Abstractions;
 /// <summary>
 /// Marks this entity as a singleton entity, the singleton id is used to retrieve the entity from the cache.
 /// </summary>
-public interface ISingletonEntity
+public interface ISingletonEntity : IEntity
 {
     public static virtual EntityId SingletonId { get; }
 }

src/NexusMods.EventSourcing/EntityContext.cs

@@ -11,6 +11,11 @@ public TEntity Get<TEntity>(EntityId<TEntity> id) where TEntity : IEntity
         throw new System.NotImplementedException();
     }
 
+    public TEntity Get<TEntity>() where TEntity : ISingletonEntity
+    {
+        throw new System.NotImplementedException();
+    }
+
     public ValueTask Add<TEvent>(TEvent entity) where TEvent : IEvent
     {
         throw new System.NotImplementedException();

src/NexusMods.EventSourcing/EventFormatter.cs

@@ -7,7 +7,7 @@
 
 namespace NexusMods.EventSourcing;
 
-public class EventFormatter : MemoryPackFormatter<IEvent>
+internal class EventFormatter : MemoryPackFormatter<IEvent>
 {
     private static Guid _zeroGuid = Guid.Empty;
     private readonly Dictionary<Guid,Type> _eventByGuid;

tests/NexusMods.EventSourcing.TestModel/Events/CreateLoadout.cs

@@ -18,6 +18,7 @@ public async ValueTask Apply<T>(T context) where T : IEventContext
     {
         context.New(Id);
         context.Emit(Id, Loadout._name, Name);
+        context.Emit(LoadoutRegistry.SingletonId, LoadoutRegistry._loadouts, Id);
     }
 
     public static CreateLoadout Create(string name) => new() { Name = name, Id = EntityId<Loadout>.NewId() };

tests/NexusMods.EventSourcing.TestModel/Model/LoadoutRegistry.cs

@@ -4,15 +4,9 @@
 
 namespace NexusMods.EventSourcing.TestModel.Model;
 
-public class LoadoutRegistry(IEntityContext context, EntityId id) : AEntity(context, id)
+public class LoadoutRegistry(IEntityContext context) : AEntity(context, SingletonId.Value), ISingletonEntity
 {
-    /// <summary>
-    /// Gets the instance of the loadout registry from the entity context.
-    /// </summary>
-    /// <param name="context"></param>
-    /// <returns></returns>
-    public static LoadoutRegistry GetInstance(IEntityContext context) =>
-        context.Get(EntityId<LoadoutRegistry>.From("10BAE6BA-D5F9-40F4-AF7F-CCA1417C3BB0"));
+    public static EntityId<LoadoutRegistry> SingletonId => EntityId<LoadoutRegistry>.From("10BAE6BA-D5F9-40F4-AF7F-CCA1417C3BB0");
 
     /// <summary>
     /// The loadouts in the registry.

tests/NexusMods.EventSourcing.Tests/BasicFunctionalityTests.cs

@@ -1,4 +1,5 @@
 using NexusMods.EventSourcing.TestModel.Events;
+using NexusMods.EventSourcing.TestModel.Model;
 using NexusMods.EventSourcing.Tests.Contexts;
 
 namespace NexusMods.EventSourcing.Tests;
@@ -73,4 +74,11 @@ public async void CanDeleteEntities()
 
         loadout.Mods.First().Name.Should().Be("Second Mod");
     }
+
+    [Fact]
+    public async void CanGetSingletonEntities()
+    {
+        var entity = _ctx.Get<LoadoutRegistry>();
+        entity.Should().NotBeNull();
+    }
 }

tests/NexusMods.EventSourcing.Tests/Contexts/TestContext.cs

@@ -29,6 +29,19 @@ public TEntity Get<TEntity>(EntityId<TEntity> id) where TEntity : IEntity
         return createdEntity;
     }
 
+    public TEntity Get<TEntity>() where TEntity : ISingletonEntity
+    {
+        var id = TEntity.SingletonId;
+        if (_entities.TryGetValue(id, out var entity))
+        {
+            return (TEntity)entity;
+        }
+
+        var instance = (TEntity)Activator.CreateInstance(typeof(TEntity), this)!;
+        _entities.Add(id, instance);
+        return instance;
+    }
+
     private Dictionary<IAttribute, IAccumulator> LoadValues(EntityId id)
     {
         var ingester = new Ingester(id);