RobustToolbox/Robust.Shared/GameObjects/Systems/SharedGridTraversalSystem.cs

using Robust.Shared.Containers;
using Robust.Shared.IoC;
using Robust.Shared.Map;

namespace Robust.Shared.GameObjects
{
    /// <summary>
    ///     Handles moving entities between grids as they move around.
    /// </summary>
    internal sealed class SharedGridTraversalSystem : EntitySystem
    {
        [Dependency] private readonly IMapManager _mapManager = default!;

        public override void Initialize()
        {
            base.Initialize();
            SubscribeLocalEvent<MoveEvent>(HandleMove);
        }

        private void HandleMove(MoveEvent moveEvent)
        {
            var entity = moveEvent.Sender;

            if (entity.Deleted ||
                entity.HasComponent<IMapComponent>() ||
                entity.HasComponent<IMapGridComponent>() ||
                entity.IsInContainer())
            {
                return;
            }

            var transform = entity.Transform;

            // Change parent if necessary
            if (_mapManager.TryFindGridAt(transform.MapID, moveEvent.NewPosition.ToMapPos(EntityManager), out var grid) &&
                grid.GridEntityId.IsValid() &&
                grid.GridEntityId != entity.Uid)
            {
                // Some minor duplication here with AttachParent but only happens when going on/off grid so not a big deal ATM.
                if (grid.Index != transform.GridID)
                {
                    transform.AttachParent(EntityManager.GetEntity(grid.GridEntityId));
                    RaiseLocalEvent(entity.Uid, new ChangedGridMessage(entity, transform.GridID, grid.Index));
                }
            }
            else
            {
                var oldGridId = transform.GridID;

                // Attach them to map / they are on an invalid grid
                if (oldGridId != GridId.Invalid)
                {
                    transform.AttachParent(_mapManager.GetMapEntity(transform.MapID));
                    RaiseLocalEvent(entity.Uid, new ChangedGridMessage(entity, oldGridId, GridId.Invalid));
                }
            }
        }
    }

    public sealed class ChangedGridMessage : EntityEventArgs
    {
        public IEntity Entity;
        public GridId OldGrid;
        public GridId NewGrid;

        public ChangedGridMessage(IEntity entity, GridId oldGrid, GridId newGrid)
        {
            Entity = entity;
            OldGrid = oldGrid;
            NewGrid = newGrid;
        }
    }
}