Add tile entity occlusion (#14626)

Content.Shared/Movement/Systems/SharedFloorOcclusionSystem.cs

@@ -0,0 +1,53 @@
+using Content.Shared.Movement.Components;
+using Robust.Shared.Physics.Events;
+
+namespace Content.Shared.Movement.Systems;
+
+/// <summary>
+/// Applies an occlusion shader for any relevant entities.
+/// </summary>
+public abstract class SharedFloorOcclusionSystem : EntitySystem
+{
+    public override void Initialize()
+    {
+        base.Initialize();
+        SubscribeLocalEvent<FloorOccluderComponent, StartCollideEvent>(OnStartCollide);
+        SubscribeLocalEvent<FloorOccluderComponent, EndCollideEvent>(OnEndCollide);
+    }
+
+    private void OnStartCollide(EntityUid uid, FloorOccluderComponent component, ref StartCollideEvent args)
+    {
+        var other = args.OtherEntity;
+
+        if (!TryComp<FloorOcclusionComponent>(other, out var occlusion) ||
+            occlusion.Colliding.Contains(uid))
+        {
+            return;
+        }
+
+        SetEnabled(other, occlusion, true);
+        occlusion.Colliding.Add(uid);
+    }
+
+    private void OnEndCollide(EntityUid uid, FloorOccluderComponent component, ref EndCollideEvent args)
+    {
+        var other = args.OtherEntity;
+
+        if (!TryComp<FloorOcclusionComponent>(other, out var occlusion))
+            return;
+
+        occlusion.Colliding.Remove(uid);
+
+        if (occlusion.Colliding.Count == 0)
+            SetEnabled(other, occlusion, false);
+    }
+
+    protected virtual void SetEnabled(EntityUid uid, FloorOcclusionComponent component, bool enabled)
+    {
+        if (component.Enabled == enabled)
+            return;
+
+        component.Enabled = enabled;
+        Dirty(uid, component);
+    }
+}