Ports event system from Bay

code/datums/observation/dir_set.dm

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+//	Observer Pattern Implementation: Direction Set
+//		Registration type: /atom
+//
+//		Raised when: An /atom changes dir using the set_dir() proc.
+//
+//		Arguments that the called proc should expect:
+//			/atom/dir_changer: The instance that changed direction
+//			/old_dir: The dir before the change.
+//			/new_dir: The dir after the change.
+
+var/decl/observ/dir_set/dir_set_event = new()
+
+/decl/observ/dir_set
+	name = "Direction Set"
+	expected_type = /atom
+
+/decl/observ/dir_set/register(var/atom/dir_changer, var/datum/listener, var/proc_call)
+	. = ..()
+
+	// Listen to the parent if possible.
+	if(. && istype(dir_changer.loc, /atom/movable))	// We don't care about registering to turfs.
+		register(dir_changer.loc, dir_changer, /atom/proc/recursive_dir_set)
+
+/*********************
+* Direction Handling *
+*********************/
+
+/atom/movable/Entered(var/atom/movable/am, atom/old_loc)
+	. = ..()
+	if(. != CANCEL_MOVE_EVENT && dir_set_event.has_listeners(am))
+		dir_set_event.register(src, am, /atom/proc/recursive_dir_set)
+
+/atom/movable/Exited(var/atom/movable/am, atom/old_loc)
+	. = ..()
+	dir_set_event.unregister(src, am, /atom/proc/recursive_dir_set)