move hot plate item placement stuff into its own system (#18923)

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Shared/Placeable/ItemPlacerSystem.cs

@@ -0,0 +1,72 @@
+using Robust.Shared.Physics.Events;
+using Robust.Shared.Physics.Systems;
+using System.Linq;
+
+namespace Content.Shared.Placeable;
+
+/// <summary>
+/// Tracks placed entities
+/// Subscribe to <see cref="ItemPlacedEvent"/> or <see cref="ItemRemovedEvent"/> to do things when items or placed or removed.
+/// </summary>
+public sealed class ItemPlacerSystem : EntitySystem
+{
+    [Dependency] private readonly PlaceableSurfaceSystem _placeableSurface = default!;
+    [Dependency] private readonly SharedPhysicsSystem _physics = default!;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<ItemPlacerComponent, StartCollideEvent>(OnStartCollide);
+        SubscribeLocalEvent<ItemPlacerComponent, EndCollideEvent>(OnEndCollide);
+    }
+
+    private void OnStartCollide(EntityUid uid, ItemPlacerComponent comp, ref StartCollideEvent args)
+    {
+        if (comp.Whitelist != null && !comp.Whitelist.IsValid(args.OtherEntity))
+            return;
+
+        // Disallow sleeping so we can detect when entity is removed from the heater.
+        _physics.SetSleepingAllowed(args.OtherEntity, args.OtherBody, false);
+
+        var count = comp.PlacedEntities.Count;
+        if (comp.MaxEntities == 0 || count < comp.MaxEntities)
+        {
+            comp.PlacedEntities.Add(args.OtherEntity);
+
+            var ev = new ItemPlacedEvent(args.OtherEntity);
+            RaiseLocalEvent(uid, ref ev);
+        }
+
+        if (comp.MaxEntities > 0 && count >= (comp.MaxEntities - 1))
+        {
+            // Don't let any more items be placed if it's reached its limit.
+            _placeableSurface.SetPlaceable(uid, false);
+        }
+    }
+
+    private void OnEndCollide(EntityUid uid, ItemPlacerComponent comp, ref EndCollideEvent args)
+    {
+        // Re-allow sleeping.
+        _physics.SetSleepingAllowed(args.OtherEntity, args.OtherBody, true);
+
+        comp.PlacedEntities.Remove(args.OtherEntity);
+
+        var ev = new ItemRemovedEvent(args.OtherEntity);
+        RaiseLocalEvent(uid, ref ev);
+
+        _placeableSurface.SetPlaceable(uid, true);
+    }
+}
+
+/// <summary>
+/// Raised on the <see cref="ItemPlacer"/> when an item is placed and it is under the item limit.
+/// </summary>
+[ByRefEvent]
+public record struct ItemPlacedEvent(EntityUid OtherEntity);
+
+/// <summary>
+/// Raised on the <see cref="ItemPlacer"/> when an item is removed from it.
+/// </summary>
+[ByRefEvent]
+public record struct ItemRemovedEvent(EntityUid OtherEntity);