Fix gravity shenanigans (#12866)

Content.Server/Gravity/GravitySystem.cs

@@ -0,0 +1,74 @@
+using Content.Shared.Gravity;
+using JetBrains.Annotations;
+using Robust.Shared.Map.Components;
+using Robust.Shared.Utility;
+
+namespace Content.Server.Gravity
+{
+    [UsedImplicitly]
+    public sealed class GravitySystem : SharedGravitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+            SubscribeLocalEvent<GravityComponent, ComponentInit>(OnGravityInit);
+        }
+
+        /// <summary>
+        /// Iterates gravity components and checks if this entity can have gravity applied.
+        /// </summary>
+        public void RefreshGravity(EntityUid uid, GravityComponent? gravity = null)
+        {
+            if (!Resolve(uid, ref gravity))
+                return;
+
+            var enabled = false;
+
+            foreach (var (comp, xform) in EntityQuery<GravityGeneratorComponent, TransformComponent>(true))
+            {
+                if (!comp.GravityActive || xform.ParentUid != uid)
+                    continue;
+
+                enabled = true;
+                break;
+            }
+
+            if (enabled != gravity.Enabled)
+            {
+                gravity.Enabled = enabled;
+                var ev = new GravityChangedEvent(uid, enabled);
+                RaiseLocalEvent(uid, ref ev, true);
+                Dirty(gravity);
+
+                if (HasComp<MapGridComponent>(uid))
+                {
+                    StartGridShake(uid);
+                }
+            }
+        }
+
+        private void OnGravityInit(EntityUid uid, GravityComponent component, ComponentInit args)
+        {
+            RefreshGravity(uid);
+        }
+
+        public void EnableGravity(EntityUid uid, GravityComponent? gravity = null)
+        {
+            if (!Resolve(uid, ref gravity))
+                return;
+
+            if (gravity.Enabled)
+                return;
+
+            gravity.Enabled = true;
+            var ev = new GravityChangedEvent(uid, true);
+            RaiseLocalEvent(uid, ref ev, true);
+            Dirty(gravity);
+
+            if (HasComp<MapGridComponent>(uid))
+            {
+                StartGridShake(uid);
+            }
+        }
+    }
+}