#1462 extracted MapGrid's HasGravity property to separate Content com… (#4200)

* #1462 extracted MapGrid's HasGravity property to separate Content component

* #1462 - PR suggestions

* Merge to master

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Shared/Gravity/GravityChangedMessage.cs

@@ -1,4 +1,4 @@
-#nullable enable
+#nullable enable
 using Robust.Shared.GameObjects;
 using Robust.Shared.Map;
 
@@ -6,13 +6,14 @@ namespace Content.Shared.Gravity
 {
     public class GravityChangedMessage : EntityEventArgs
     {
-        public GravityChangedMessage(IMapGrid grid)
+        public GravityChangedMessage(GridId changedGridIndex, bool newGravityState)
         {
-            Grid = grid;
+            HasGravity = newGravityState;
+            ChangedGridIndex = changedGridIndex;
         }
 
-        public IMapGrid Grid { get; }
+        public GridId ChangedGridIndex { get; }
 
-        public bool HasGravity => Grid.HasGravity;
+        public bool HasGravity { get; }
     }
 }

Content.Shared/Gravity/GravityComponent.cs

@@ -0,0 +1,62 @@
+using System;
+using Content.Shared.NetIDs;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Log;
+using Robust.Shared.Players;
+using Robust.Shared.Serialization;
+using Robust.Shared.ViewVariables;
+
+namespace Content.Shared.Gravity
+{
+    [RegisterComponent]
+    public sealed class GravityComponent : Component
+    {
+        public override string Name => "Gravity";
+        public override uint? NetID => ContentNetIDs.GRAVITY;
+
+        [ViewVariables(VVAccess.ReadWrite)]
+        public bool Enabled
+        {
+            get => _enabled;
+            set
+            {
+                if (_enabled == value) return;
+                _enabled = value;
+                if (_enabled)
+                {
+                    Logger.Info($"Enabled gravity for {Owner}");
+                }
+                else
+                {
+                    Logger.Info($"Disabled gravity for {Owner}");
+                }
+                Dirty();
+            }
+        }
+
+        private bool _enabled;
+
+        public override ComponentState GetComponentState(ICommonSession player)
+        {
+            return new GravityComponentState(_enabled);
+        }
+
+        public override void HandleComponentState(ComponentState? curState, ComponentState? nextState)
+        {
+            base.HandleComponentState(curState, nextState);
+            if (curState is not GravityComponentState state) return;
+            Enabled = state.Enabled;
+        }
+
+        [Serializable, NetSerializable]
+        protected sealed class GravityComponentState : ComponentState
+        {
+            public bool Enabled { get; }
+
+            public GravityComponentState(bool enabled) : base(ContentNetIDs.GRAVITY)
+            {
+                Enabled = enabled;
+            }
+        }
+    }
+}

Content.Shared/Gravity/SharedGravitySystem.cs

@@ -0,0 +1,19 @@
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.Gravity
+{
+    public abstract class SharedGravitySystem : EntitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+            SubscribeLocalEvent<GridInitializedEvent>(HandleGridInitialize);
+        }
+
+        private void HandleGridInitialize(GridInitializedEvent ev)
+        {
+            var gridEnt = EntityManager.GetEntity(ev.EntityUid);
+            gridEnt.EnsureComponent<GravityComponent>();
+        }
+    }
+}