Add puddles / reagent spills (#743)

Content.Server/GameObjects/EntitySystems/PuddleSystem.cs

@@ -0,0 +1,47 @@
+using Content.Server.GameObjects.Components.Fluids;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Components.Transform;
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Interfaces.Map;
+using Robust.Shared.IoC;
+using Robust.Shared.Map;
+
+namespace Content.Server.GameObjects.EntitySystems
+{
+    public class PuddleSystem : EntitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+            EntityQuery = new TypeEntityQuery(typeof(PuddleComponent));
+            var mapManager = IoCManager.Resolve<IMapManager>();
+            mapManager.TileChanged += HandleTileChanged;
+        }
+
+        public override void Shutdown()
+        {
+            base.Shutdown();
+            var mapManager = IoCManager.Resolve<IMapManager>();
+            mapManager.TileChanged -= HandleTileChanged;
+        }
+
+        private void HandleTileChanged(object sender, TileChangedEventArgs eventArgs)
+        {
+            // If this gets hammered you could probably queue up all the tile changes every tick but I doubt that would ever happen.
+            var entities = EntityManager.GetEntities(EntityQuery);
+
+            foreach (var entity in entities)
+            {
+                // If the tile becomes space then delete it (potentially change by design)
+                if (eventArgs.NewTile.GridIndex == entity.Transform.GridID &&
+                    entity.TryGetComponent(out SnapGridComponent snapGridComponent) &&
+                    snapGridComponent.Position == eventArgs.NewTile.GridIndices &&
+                    eventArgs.NewTile.Tile.IsEmpty)
+                {
+                    entity.Delete();
+                    break; // Currently it's one puddle per tile, if that changes remove this
+                }
+            }
+        }
+    }
+}