Core/AI: Finally move the "is creature engaged" flag to be a property of the creature AI, where it honestly always belonged.

Port From (https://github.com/TrinityCore/TrinityCore/commit/c13d83796f7b2111c5dcf8546bdd84eccd232ae3)

Source/Game/Entities/Creature/Creature.cs

@@ -1047,6 +1047,14 @@ namespace Game.Entities
                 && !GetCreatureTemplate().FlagsExtra.HasAnyFlag(CreatureFlagsExtra.NoXpAtKill);
         }
 
+        public override bool IsEngaged()
+        {
+            CreatureAI ai = GetAI();
+            if (ai != null)
+                return ai.IsEngaged();
+            return false;
+        }
+        
         public override void AtEngage(Unit target)
         {
             base.AtEngage(target);
@@ -1353,7 +1361,7 @@ namespace Game.Entities
             CreatureBaseStats stats = Global.ObjectMgr.GetCreatureBaseStats(level, cInfo.UnitClass);
 
             // health
-            float healthmod = _GetHealthMod(rank);
+            float healthmod = GetHealthMod(rank);
 
             uint basehp = (uint)GetMaxHealthByLevel(level);
             uint health = (uint)(basehp * healthmod);
@@ -1416,7 +1424,7 @@ namespace Game.Entities
             }
         }
 
-        float _GetHealthMod(CreatureEliteType Rank)
+        public float GetHealthMod(CreatureEliteType Rank)
         {
             switch (Rank)                                           // define rates for each elite rank
             {
@@ -1567,7 +1575,7 @@ namespace Game.Entities
                 curhealth = m_creatureData.curhealth;
                 if (curhealth != 0)
                 {
-                    curhealth = (uint)(curhealth * _GetHealthMod(GetCreatureTemplate().Rank));
+                    curhealth = (uint)(curhealth * GetHealthMod(GetCreatureTemplate().Rank));
                     if (curhealth < 1)
                         curhealth = 1;
                 }

Source/Game/Entities/Unit/Unit.Combat.cs

@@ -57,8 +57,9 @@ namespace Game.Entities
             RemoveAurasWithInterruptFlags(SpellAuraInterruptFlags.LeavingCombat);
         }
 
-        public virtual void AtEngage(Unit target) { m_isEngaged = true; }
-        public virtual void AtDisengage() { m_isEngaged = false; }
+        public virtual void AtEngage(Unit target) { }
+
+        public virtual void AtDisengage() { }
 
         public void CombatStop(bool includingCast = false, bool mutualPvP = true)
         {
@@ -147,10 +148,10 @@ namespace Game.Entities
 
         public bool CanHaveThreatList() { return m_threatManager.CanHaveThreatList(); }
 
-        // This value can be different from IsInCombat:
+        // This value can be different from IsInCombat, for example:
         // - when a projectile spell is midair against a creature (combat on launch - threat+aggro on impact)
         // - when the creature has no targets left, but the AI has not yet ceased engaged logic
-        public bool IsEngaged() { return m_isEngaged; }
+        public virtual bool IsEngaged() { return IsInCombat(); }
 
         public bool IsEngagedBy(Unit who) { return CanHaveThreatList() ? IsThreatenedBy(who) : IsInCombatWith(who); }
 
@@ -230,7 +231,6 @@ namespace Game.Entities
                 minion.StopAttackFaction(factionId);
         }
 
-
         public void HandleProcExtraAttackFor(Unit victim)
         {
             while (ExtraAttacks != 0)

Source/Game/Entities/Unit/Unit.Fields.cs

@@ -56,8 +56,6 @@ namespace Game.Entities
         protected uint[] m_attackTimer = new uint[(int)WeaponAttackType.Max];
 
         // Threat+combat management
-        bool m_isEngaged;
-
         CombatManager m_combatManager;
         ThreatManager m_threatManager;