Fix weightlessness status effect appearing only after something checks if an entity is weightless (#2434)

* Revert "Add weightlessness status effect. (#2384)"

This reverts commit 9b751fc079.

* Bring back the icon and status

* Make weightless status track gravity and parent

Content.Server/GameObjects/EntitySystems/WeightlessSystem.cs

@@ -0,0 +1,109 @@
+using System.Collections.Generic;
+using Content.Server.GameObjects.Components.Mobs;
+using Content.Shared.GameObjects.Components.Mobs;
+using Content.Shared.GameObjects.EntitySystemMessages.Gravity;
+using Content.Shared.GameTicking;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects.Components.Map;
+using Robust.Shared.GameObjects.EntitySystemMessages;
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Map;
+using Robust.Shared.Utility;
+
+namespace Content.Server.GameObjects.EntitySystems
+{
+    [UsedImplicitly]
+    public class WeightlessSystem : EntitySystem, IResettingEntitySystem
+    {
+        private readonly Dictionary<GridId, List<ServerStatusEffectsComponent>> _statuses = new Dictionary<GridId, List<ServerStatusEffectsComponent>>();
+
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            SubscribeLocalEvent<GravityChangedMessage>(GravityChanged);
+            SubscribeLocalEvent<EntParentChangedMessage>(EntParentChanged);
+        }
+
+        public void Reset()
+        {
+            _statuses.Clear();
+        }
+
+        public void AddStatus(ServerStatusEffectsComponent status)
+        {
+            var grid = status.Owner.Transform.GridID;
+            var statuses = _statuses.GetOrNew(grid);
+
+            statuses.Add(status);
+        }
+
+        public void RemoveStatus(ServerStatusEffectsComponent status)
+        {
+            var grid = status.Owner.Transform.GridID;
+            if (!_statuses.TryGetValue(grid, out var statuses))
+            {
+                return;
+            }
+
+            statuses.Remove(status);
+        }
+
+        private void GravityChanged(GravityChangedMessage ev)
+        {
+            if (!_statuses.TryGetValue(ev.Grid.Index, out var statuses))
+            {
+                return;
+            }
+
+            if (ev.HasGravity)
+            {
+                foreach (var status in statuses)
+                {
+                    RemoveWeightless(status);
+                }
+            }
+            else
+            {
+                foreach (var status in statuses)
+                {
+                    AddWeightless(status);
+                }
+            }
+        }
+
+        private void AddWeightless(ServerStatusEffectsComponent status)
+        {
+            status.ChangeStatusEffect(StatusEffect.Weightless, "/Textures/Interface/StatusEffects/Weightless/weightless.png", null);
+        }
+
+        private void RemoveWeightless(ServerStatusEffectsComponent status)
+        {
+            status.RemoveStatusEffect(StatusEffect.Weightless);
+        }
+
+        private void EntParentChanged(EntParentChangedMessage ev)
+        {
+            if (!ev.Entity.TryGetComponent(out ServerStatusEffectsComponent status))
+            {
+                return;
+            }
+
+            if (ev.OldParent != null &&
+                ev.OldParent.TryGetComponent(out IMapGridComponent mapGrid))
+            {
+                var oldGrid = mapGrid.GridIndex;
+
+                if (_statuses.TryGetValue(oldGrid, out var oldStatuses))
+                {
+                    oldStatuses.Remove(status);
+                }
+            }
+
+            var newGrid = ev.Entity.Transform.GridID;
+            var newStatuses = _statuses.GetOrNew(newGrid);
+
+            newStatuses.Add(status);
+        }
+    }
+}