Event for PowerNetBattery CurrentSupply changing (#4570)

I re-used AmbientOnPowered but maybe that was lazy; I was doubtful anything would ever have ApcPowerReceiver and PowerNetBatteryComponent.

Content.Server/Power/EntitySystems/PowerNetSystem.cs

@@ -19,6 +19,8 @@ namespace Content.Server.Power.EntitySystems
         private readonly HashSet<PowerNet> _powerNetReconnectQueue = new();
         private readonly HashSet<ApcNet> _apcNetReconnectQueue = new();
 
+        private readonly Dictionary<PowerNetworkBatteryComponent, float> _lastSupply = new();
+
         private readonly BatteryRampPegSolver _solver = new();
 
         public override void Initialize()
@@ -150,6 +152,14 @@ namespace Content.Server.Power.EntitySystems
         {
             base.Update(frameTime);
 
+            // Setup for events.
+            {
+                foreach (var powerNetBattery in ComponentManager.EntityQuery<PowerNetworkBatteryComponent>())
+                {
+                    _lastSupply[powerNetBattery] = powerNetBattery.CurrentSupply;
+                }
+            }
+
             // Reconnect networks.
             {
                 foreach (var apcNet in _apcNetReconnectQueue)
@@ -207,6 +217,27 @@ namespace Content.Server.Power.EntitySystems
                         RaiseLocalEvent(consumer.Owner.Uid, msg);
                     }
                 }
+
+                foreach (var powerNetBattery in ComponentManager.EntityQuery<PowerNetworkBatteryComponent>())
+                {
+                    if (!_lastSupply.TryGetValue(powerNetBattery, out var lastPowerSupply))
+                    {
+                        lastPowerSupply = 0f;
+                    }
+
+                    var currentSupply = powerNetBattery.CurrentSupply;
+
+                    if (lastPowerSupply == 0f && currentSupply != 0f)
+                    {
+                        RaiseLocalEvent(powerNetBattery.Owner.Uid, new PowerNetBatterySupplyEvent {Supply = true});
+                    }
+                    else if (lastPowerSupply > 0f && currentSupply == 0f)
+                    {
+                        RaiseLocalEvent(powerNetBattery.Owner.Uid, new PowerNetBatterySupplyEvent {Supply = false});
+                    }
+                }
+
+                _lastSupply.Clear();
             }
         }
 
@@ -324,6 +355,14 @@ namespace Content.Server.Power.EntitySystems
         }
     }
 
+    /// <summary>
+    /// Raised whenever a <see cref="PowerNetworkBatteryComponent"/> changes from / to 0 CurrentSupply.
+    /// </summary>
+    public sealed class PowerNetBatterySupplyEvent : EntityEventArgs
+    {
+        public bool Supply { get; init;  }
+    }
+
     public struct PowerStatistics
     {
         public int CountNetworks;