Latest updates (won't build yet)

src/NexusMods.EventSourcing.Abstractions/EntityIdDefinition.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Buffers;
+using System.Buffers.Binary;
+using NexusMods.EventSourcing.Abstractions.Serialization;
+
+namespace NexusMods.EventSourcing.Abstractions;
+
+public class EntityIdDefinition : IAttribute<EntityDefinitionAccumulator>, IIndexableAttribute<EntityId>
+{
+    /// <inheritdoc />
+    public Type Owner => typeof(IEntity);
+
+    /// <inheritdoc />
+    public string Name => "Id";
+    public EntityDefinitionAccumulator CreateAccumulator()
+    {
+        return new EntityDefinitionAccumulator();
+    }
+
+    IAccumulator IAttribute.CreateAccumulator()
+    {
+        return new EntityDefinitionAccumulator();
+    }
+
+    public void WriteTo(Span<byte> span, EntityId value)
+    {
+        throw new NotImplementedException();
+    }
+
+    /// <inheritdoc />
+    public UInt128 IndexedAttributeId { get; set; }
+
+    /// <inheritdoc />
+    public int SpanSize()
+    {
+        return 16;
+    }
+
+    /// <inheritdoc />
+    public void WriteTo(Span<byte> span, IAccumulator accumulator)
+    {
+
+    }
+}
+
+internal class EntityDefinitionAccumulator : IAccumulator
+{
+    internal EntityId Id;
+
+    public void WriteTo(IBufferWriter<byte> writer, ISerializationRegistry registry)
+    {
+        var span = writer.GetSpan(16);
+        BinaryPrimitives.WriteUInt128BigEndian(span, Id.Value);
+        writer.Advance(16);
+    }
+
+    public int ReadFrom(ref ReadOnlySpan<byte> data, ISerializationRegistry registry)
+    {
+        Id = EntityId.From(data);
+        return 16;
+    }
+}

src/NexusMods.EventSourcing.Abstractions/IEntity.cs

@@ -25,6 +25,9 @@ public interface IEntity : INotifyPropertyChanged
     /// </summary>
     public static readonly TypeAttributeDefinition TypeAttribute = new();
 
+
+    public static readonly ScalarAttribute<IEntity, EntityId> EntityIdAttribute = new("Id");
+
     /// <summary>
     /// Called when a property of the entity has changed.
     /// </summary>