make BoundUserInterfaceMessageAttempt by ref struct

RELEASE-NOTES.md

@@ -35,7 +35,7 @@ END TEMPLATE-->
 
 ### Breaking changes
 
-*None yet*
+* `BoundUserInterfaceMessageAttempt` is now a by-ref struct. It can no longer be assigned to `CancellableEntityEventArgs`.
 
 ### New features
 

Robust.Shared/GameObjects/Components/UserInterface/UserInterfaceComponent.cs

@@ -144,20 +144,23 @@ public InterfaceData(InterfaceData data)
     }
 
     /// <summary>
-    ///     Raised whenever the server receives a BUI message from a client relating to a UI that requires input
-    ///     validation.
+    /// Raised whenever the server receives a BUI message from a client relating to a UI that requires input validation.
+    /// This is first broadcast then, if not cancelled already, raised directly on the BUI's owner.
     /// </summary>
-    public sealed class BoundUserInterfaceMessageAttempt(
-        EntityUid actor,
-        EntityUid target,
-        Enum uiKey,
-        BoundUserInterfaceMessage message)
-        : CancellableEntityEventArgs
+    [ByRefEvent]
+    public record struct BoundUserInterfaceMessageAttempt(EntityUid Actor, EntityUid Target, Enum UiKey, BoundUserInterfaceMessage Message)
     {
-        public readonly EntityUid Actor = actor;
-        public readonly EntityUid Target = target;
-        public readonly Enum UiKey = uiKey;
-        public readonly BoundUserInterfaceMessage Message = message;
+        public bool Cancelled { get; private set; }
+
+        public void Cancel()
+        {
+            Cancelled = true;
+        }
+
+        public void Uncancel()
+        {
+            Cancelled = false;
+        }
     }
 
     /// <summary>

Robust.Shared/GameObjects/Systems/SharedUserInterfaceSystem.cs

@@ -123,11 +123,11 @@ private void OnMessageReceived(BoundUIWrapMessage msg, EntityUid sender)
         {
             var attempt = new BoundUserInterfaceMessageAttempt(sender, uid, msg.UiKey, msg.Message);
 
-            RaiseLocalEvent(attempt);
+            RaiseLocalEvent(ref attempt);
             if (attempt.Cancelled)
                 return;
 
-            RaiseLocalEvent(uid, attempt);
+            RaiseLocalEvent(uid, ref attempt);
             if (attempt.Cancelled)
                 return;
         }