Add gravitycomponent support to maps (#9857)

* Add gravitycomponent support to maps

The alert still needs fixing if you want planets.

* move

Content.Shared/Gravity/SharedGravitySystem.cs

@@ -1,7 +1,50 @@
+using Content.Shared.Clothing;
+using Content.Shared.Inventory;
+using Content.Shared.Movement.Components;
+using Robust.Shared.Map;
+using Robust.Shared.Physics;
+
 namespace Content.Shared.Gravity
 {
     public abstract class SharedGravitySystem : EntitySystem
     {
+        [Dependency] private readonly IMapManager _mapManager = default!;
+        [Dependency] private readonly InventorySystem _inventory = default!;
+
+        public bool IsWeightless(EntityUid uid, PhysicsComponent? body = null, TransformComponent? xform = null)
+        {
+            Resolve(uid, ref body, false);
+
+            if ((body?.BodyType & (BodyType.Static | BodyType.Kinematic)) != 0)
+                return false;
+
+            if (TryComp<MovementIgnoreGravityComponent>(uid, out var ignoreGravityComponent))
+                return ignoreGravityComponent.Weightless;
+
+            if (!Resolve(uid, ref xform)) return true;
+
+            // If grid / map has gravity
+            if ((TryComp<GravityComponent>(xform.GridUid, out var gravity) ||
+                 TryComp(xform.MapUid, out gravity)) && gravity.Enabled)
+                return false;
+
+            // On the map then always weightless (unless it has gravity comp obv).
+            if (!_mapManager.TryGetGrid(xform.GridUid, out var grid))
+                return true;
+
+            // Something holding us down
+            if (_inventory.TryGetSlotEntity(uid, "shoes", out var ent))
+            {
+                if (TryComp<MagbootsComponent>(ent, out var boots) && boots.On)
+                    return false;
+            }
+
+            if (!xform.Coordinates.IsValid(EntityManager)) return true;
+
+            var tile = grid.GetTileRef(xform.Coordinates).Tile;
+            return tile.IsEmpty;
+        }
+
         public override void Initialize()
         {
             base.Initialize();