Add mechanism events when being added/removed to/from body/parts (#2271)

* Add mechanism events when added/removed to/from body/parts

* Change old usages

* Add TODO

Content.Shared/GameObjects/Components/Body/Behavior/IMechanismBehavior.cs

@@ -15,5 +15,55 @@ namespace Content.Shared.GameObjects.Components.Body.Behavior
         IMechanism? Mechanism { get; }
 
         void Update(float frameTime);
+
+        /// <summary>
+        ///     Called when the containing <see cref="IBodyPart"/> is attached to a
+        ///     <see cref="IBody"/>.
+        ///     For instance, attaching a head with a brain inside to a body.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void AddedToBody();
+
+        /// <summary>
+        ///     Called when the parent <see cref="IMechanism"/> is
+        ///     added into a <see cref="IBodyPart"/> that is not attached to a
+        ///     <see cref="IBody"/>.
+        ///     For instance, adding a brain to a dismembered head.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void AddedToPart();
+
+        /// <summary>
+        ///     Called when the parent <see cref="IBodyPart"/> is removed from a
+        ///     <see cref="IBody"/>.
+        ///     For instance, removing a head with a brain inside from a body.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void RemovedFromBody(IBody old);
+
+        /// <summary>
+        ///     Called when the parent <see cref="IMechanism"/> is
+        ///     removed from a <see cref="IBodyPart"/> that is not attached to a
+        ///     <see cref="IBody"/>.
+        ///     For instance, removing a brain from a dismembered head.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void RemovedFromPart(IBodyPart old);
+
+        /// <summary>
+        ///     Called when the parent <see cref="IMechanism"/> is added to a
+        ///     <see cref="IBodyPart"/> that is attached to a <see cref="IBody"/>.
+        ///     For instance, adding a brain to a head that is attached to a body.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void AddedToPartInBody();
+
+        /// <summary>
+        ///     Called when the parent <see cref="IMechanism"/> is removed from a
+        ///     <see cref="IBodyPart"/> that is attached to a <see cref="IBody"/>.
+        ///     For instance, removing a brain from a head that is attached to a body.
+        ///     DO NOT CALL THIS DIRECTLY FROM OUTSIDE BODY SYSTEM CODE!
+        /// </summary>
+        void RemovedFromPartInBody(IBody? oldBody, IBodyPart? oldPart);
     }
 }