Armor (#4934)

* Armor

* radiation tweaks

* asphyxiation ignore resistances

Content.Shared/Damage/DamageableSystem.cs

@@ -118,13 +118,18 @@ namespace Content.Shared.Damage
             }
 
             // Apply resistances
-            if (!ignoreResistances && damageable.DamageModifierSetId != null)
+            if (!ignoreResistances)
             {
-                if (_prototypeManager.TryIndex<DamageModifierSetPrototype>(damageable.DamageModifierSetId, out var modifierSet))
+                if (damageable.DamageModifierSetId != null &&
+                    _prototypeManager.TryIndex<DamageModifierSetPrototype>(damageable.DamageModifierSetId, out var modifierSet))
                 {
                     damage = DamageSpecifier.ApplyModifierSet(damage, modifierSet);
                 }
 
+                var ev = new DamageModifyEvent(damage);
+                RaiseLocalEvent(uid, ev, false);
+                damage = ev.Damage;
+
                 if (damage.Empty)
                 {
                     return damage;
@@ -199,6 +204,23 @@ namespace Content.Shared.Damage
         }
     }
 
+    /// <summary>
+    ///     Raised on an entity when damage is about to be dealt,
+    ///     in case anything else needs to modify it other than the base
+    ///     damageable component.
+    ///
+    ///     For example, armor.
+    /// </summary>
+    public class DamageModifyEvent : EntityEventArgs
+    {
+        public DamageSpecifier Damage;
+
+        public DamageModifyEvent(DamageSpecifier damage)
+        {
+            Damage = damage;
+        }
+    }
+
     public class DamageChangedEvent : EntityEventArgs
     {
         /// <summary>