From 39729bfe213df982b15aacac86b91df15db941e1 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Tue, 26 May 2026 05:09:01 -0400
Subject: [PATCH] feat(runtime): VirtualTagActor skeleton (engine wiring
 tracked as F8)

---
 .../VirtualTags/VirtualTagActor.cs            | 41 +++++++++++++++++++
 .../VirtualTags/VirtualTagActorTests.cs       | 22 ++++++++++
 2 files changed, 63 insertions(+)
 create mode 100644 src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs
 create mode 100644 tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagActorTests.cs

diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs
new file mode 100644
index 0000000..ffa5adb
--- /dev/null
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs
@@ -0,0 +1,41 @@
+using Akka.Actor;
+using Akka.Event;
+using ZB.MOM.WW.OtOpcUa.Commons.Types;
+
+namespace ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
+
+/// <summary>
+/// Wraps a single virtual-tag expression. Receives dependency-tag updates, recomputes the
+/// expression, and publishes the result to <c>OpcUaPublishActor</c>.
+///
+/// Engine wiring (compile expression via <c>VirtualTagEngine</c>, manage subscriptions,
+/// emit <c>AttributeValueUpdate</c>) is staged for follow-up F8. This skeleton compiles + has
+/// a basic message contract so DriverHostActor can spawn it as a child.
+/// </summary>
+public sealed class VirtualTagActor : ReceiveActor
+{
+    public sealed record DependencyValueChanged(string TagId, object? Value, DateTime TimestampUtc);
+    public sealed record EvaluationResult(string VirtualTagId, object? Value, DateTime TimestampUtc, CorrelationId Correlation);
+
+    private readonly string _virtualTagId;
+    private readonly string _expression;
+    private readonly ILoggingAdapter _log = Context.GetLogger();
+    private readonly Dictionary<string, object?> _dependencies = new(StringComparer.Ordinal);
+
+    public static Props Props(string virtualTagId, string expression) =>
+        Akka.Actor.Props.Create(() => new VirtualTagActor(virtualTagId, expression));
+
+    public VirtualTagActor(string virtualTagId, string expression)
+    {
+        _virtualTagId = virtualTagId;
+        _expression = expression;
+
+        Receive<DependencyValueChanged>(msg =>
+        {
+            _dependencies[msg.TagId] = msg.Value;
+            // Engine wiring (F8): VirtualTagEngine.Evaluate(_expression, _dependencies) → publish.
+            _log.Debug("VirtualTag {Id}: dependency {Tag}={Value} buffered (eval staged for F8)",
+                _virtualTagId, msg.TagId, msg.Value);
+        });
+    }
+}
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagActorTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagActorTests.cs
new file mode 100644
index 0000000..d75e760
--- /dev/null
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagActorTests.cs
@@ -0,0 +1,22 @@
+using Akka.Actor;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Runtime.Tests.Harness;
+using ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
+
+namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.VirtualTags;
+
+public sealed class VirtualTagActorTests : RuntimeActorTestBase
+{
+    [Fact]
+    public void DependencyValueChanged_is_accepted_and_actor_stays_alive()
+    {
+        var actor = Sys.ActorOf(VirtualTagActor.Props("vt-1", "a + b"));
+        Watch(actor);
+        actor.Tell(new VirtualTagActor.DependencyValueChanged("tag-a", 10, DateTime.UtcNow));
+        actor.Tell(new VirtualTagActor.DependencyValueChanged("tag-b", 20, DateTime.UtcNow));
+
+        // No crash, no termination.
+        ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+}