[MIRROR] topmost turf crossed component (#12005)

Co-authored-by: Will <7099514+Willburd@users.noreply.github.com>

code/datums/elements/topturfcrossed.dm

@@ -0,0 +1,73 @@
+/datum/component/topturfcrossed
+	var/atom/movable/our_owner
+	var/turf/our_old_turf = null
+
+/datum/component/topturfcrossed/Initialize()
+	if(!ismovable(parent))
+		return COMPONENT_INCOMPATIBLE
+	our_owner = parent
+
+/datum/component/topturfcrossed/Destroy(force)
+	. = ..()
+	our_owner = null
+	our_old_turf = null
+
+/datum/component/topturfcrossed/RegisterWithParent()
+	our_owner.AddComponent(/datum/component/recursive_move) // Required if we want to be useful at all
+	RegisterSignal(our_owner, COMSIG_OBSERVER_MOVED, PROC_REF(handle_location_change))
+	update_turf_signals(get_turf(our_owner))
+
+/datum/component/topturfcrossed/UnregisterFromParent()
+	UnregisterSignal(our_owner, COMSIG_OBSERVER_MOVED)
+	update_turf_signals(null)
+
+/datum/component/topturfcrossed/proc/handle_location_change(datum/source, atom/old_loc, atom/new_loc)
+	SIGNAL_HANDLER
+	if(!our_owner)
+		return
+	update_turf_signals(new_loc)
+
+/// Updates the topmost turf we are registered to when we are moved.
+/datum/component/topturfcrossed/proc/update_turf_signals(atom/new_loc)
+	// Our turf is the exact same, don't change anything!
+	if(new_loc && our_old_turf)
+		var/turf/check_valid = isturf(new_loc) ? new_loc : get_turf(new_loc)
+		if(check_valid == our_old_turf)
+			return
+	// Always remove the signal from our old turf when registering the new one
+	if(our_old_turf)
+		UnregisterSignal(our_old_turf, COMSIG_OBSERVER_TURF_ENTERED)
+		our_old_turf = null
+	// Only register the turf signal if we are inside something, otherwise we'd get DOUBLECROSSED
+	if(new_loc && !isturf(our_owner.loc))
+		var/turf/find_new = isturf(new_loc) ? new_loc : get_turf(new_loc)
+		if(find_new)
+			RegisterSignal(find_new, COMSIG_OBSERVER_TURF_ENTERED, PROC_REF(handle_turf_entered))
+			our_old_turf = find_new
+
+/// Forwards the Cross() call from the turf to the object registered
+/datum/component/topturfcrossed/proc/handle_turf_entered(datum/source, datum/weakref/WF, oldloc)
+	SIGNAL_HANDLER
+	var/atom/movable/crosser = WF?.resolve()
+	if(QDELETED(crosser) || QDELETED(our_owner))
+		return
+	if(isturf(our_owner.loc))
+		return
+	if(crosser == our_owner)
+		return
+	our_owner.Crossed(crosser)
+
+
+
+//the bikehorn of testing
+/obj/item/bikehorn/topturf_testing
+	name = "bikehorn of testing"
+	desc = "honk if you're working correctly"
+
+/obj/item/bikehorn/topturf_testing/Initialize(mapload)
+	. = ..()
+	AddComponent(/datum/component/topturfcrossed)
+
+/obj/item/bikehorn/topturf_testing/Crossed(atom/movable/AM)
+	. = ..()
+	to_chat(world, "[src] crossed by [AM], was inside [loc]")