feat(Core/Grids): Implement visibility notifier (#15919)

* Cherry-picked from TrinityCore (unable to find author)

src/server/game/Grids/ObjectGridLoader.cpp

@@ -26,6 +26,35 @@
 #include "Transport.h"
 #include "Vehicle.h"
 
+void ObjectGridEvacuator::Visit(CreatureMapType& m)
+{
+    // creature in unloading grid can have respawn point in another grid
+    // if it will be unloaded then it will not respawn in original grid until unload/load original grid
+    // move to respawn point to prevent this case. For player view in respawn grid this will be normal respawn.
+    for (CreatureMapType::iterator iter = m.begin(); iter != m.end();)
+    {
+        Creature* c = iter->GetSource();
+        ++iter;
+
+        ASSERT(!c->IsPet() && "ObjectGridRespawnMover must not be called for pets");
+        c->GetMap()->CreatureRespawnRelocation(c, true);
+    }
+}
+
+void ObjectGridEvacuator::Visit(GameObjectMapType& m)
+{
+    // gameobject in unloading grid can have respawn point in another grid
+    // if it will be unloaded then it will not respawn in original grid until unload/load original grid
+    // move to respawn point to prevent this case. For player view in respawn grid this will be normal respawn.
+    for (GameObjectMapType::iterator iter = m.begin(); iter != m.end();)
+    {
+        GameObject* go = iter->GetSource();
+        ++iter;
+
+        go->GetMap()->GameObjectRespawnRelocation(go, true);
+    }
+}
+
 // for loading world object at grid loading (Corpses)
 //TODO: to implement npc on transport, also need to load npcs at grid loading
 class ObjectWorldLoader
@@ -229,6 +258,17 @@ void ObjectGridUnloader::Visit(GridRefMgr<T>& m)
     }
 }
 
+void ObjectGridStoper::Visit(CreatureMapType& m)
+{
+    // stop any fights at grid de-activation and remove dynobjects created at cast by creatures
+    for (CreatureMapType::iterator iter = m.begin(); iter != m.end(); ++iter)
+    {
+        iter->GetSource()->RemoveAllDynObjects();
+        if (iter->GetSource()->IsInCombat())
+            iter->GetSource()->CombatStop();
+    }
+}
+
 template<class T>
 void ObjectGridCleaner::Visit(GridRefMgr<T>& m)
 {