explosion minor rework + fix (#21718)

Content.Shared/Explosion/ExplosionEvents.cs

@@ -1,6 +1,5 @@
+using Content.Shared.Damage;
 using Content.Shared.Inventory;
-using Robust.Shared.Map;
-using Robust.Shared.Serialization;
 
 namespace Content.Shared.Explosion;
 
@@ -20,3 +19,34 @@ public record struct GetExplosionResistanceEvent(string ExplosionPrototype) : II
 
     SlotFlags IInventoryRelayEvent.TargetSlots =>  ~SlotFlags.POCKET;
 }
+
+/// <summary>
+/// This event is raised directed at an entity that is about to receive damage from an explosion. It can be used to
+/// recursively add contained/child entities that should also receive damage. E.g., entities in a player's inventory
+/// or backpack. This event will be raised recursively so a matchbox in a backpack in a player's inventory
+/// will also receive this event.
+/// </summary>
+[ByRefEvent]
+public record struct BeforeExplodeEvent(DamageSpecifier Damage, string Id, List<EntityUid> Contents)
+{
+    /// <summary>
+    /// The damage that will be received by this entity. Note that the entity's explosion resistance has already been
+    /// used to modify this damage.
+    /// </summary>
+    public readonly DamageSpecifier Damage = Damage;
+
+    /// <summary>
+    /// ID of the explosion prototype.
+    /// </summary>
+    public readonly string Id = Id;
+
+    /// <summary>
+    /// Damage multiplier for modifying the damage that will get dealt to contained entities.
+    /// </summary>
+    public float DamageCoefficient = 1;
+
+    /// <summary>
+    /// Contained/child entities that should receive recursive explosion damage.
+    /// </summary>
+    public readonly List<EntityUid> Contents = Contents;
+}