Mousetraps (#8440)

Co-authored-by: Kara <lunarautomaton6@gmail.com>

Content.Server/Damage/Systems/DamageOnTriggerSystem.cs

@@ -0,0 +1,49 @@
+using Content.Server.Damage.Components;
+using Content.Shared.Damage;
+using Content.Shared.StepTrigger;
+
+namespace Content.Server.Damage.Systems;
+
+// System for damage that occurs on specific triggers.
+// This is originally meant for mousetraps, but could
+// probably be extended to fit other triggers as well.
+public sealed class DamageOnTriggerSystem : EntitySystem
+{
+    [Dependency] private readonly DamageableSystem _damageableSystem = default!;
+
+    public override void Initialize()
+    {
+        SubscribeLocalEvent<DamageOnTriggerComponent, StepTriggeredEvent>(OnStepTrigger);
+    }
+
+    private void OnStepTrigger(EntityUid uid, DamageOnTriggerComponent component, ref StepTriggeredEvent args)
+    {
+        OnDamageTrigger(uid, args.Tripper, component);
+    }
+
+    private void OnDamageTrigger(EntityUid source, EntityUid target, DamageOnTriggerComponent? component = null)
+    {
+        if (!Resolve(source, ref component))
+        {
+            return;
+        }
+
+        var damage = new DamageSpecifier(component.Damage);
+        var ev = new BeforeDamageOnTriggerEvent(damage, target);
+        RaiseLocalEvent(source, ev);
+
+        _damageableSystem.TryChangeDamage(target, ev.Damage, component.IgnoreResistances);
+    }
+}
+
+public sealed class BeforeDamageOnTriggerEvent : EntityEventArgs
+{
+    public DamageSpecifier Damage { get; set;  }
+    public EntityUid Tripper { get; }
+
+    public BeforeDamageOnTriggerEvent(DamageSpecifier damage, EntityUid target)
+    {
+        Damage = damage;
+        Tripper = target;
+    }
+}