Added lazy events (#27091)

* Added lazy events

* remember to compile before changing your code

* Converted on_z_transition and on_post_z_transition to lazy events

* add todo

* fix oops

* fix another oops

* and anotha one

* Moved unit tests to the proper place

code/_hooks/events.dm

@@ -51,3 +51,92 @@
 		args["event"] = src
 		if(call(objRef,procName)(args, holder)) //An intercept value so whatever code section knows we mean business
 			. = 1
+
+#define EVENT_HANDLER_OBJREF_INDEX 1
+#define EVENT_HANDLER_PROCNAME_INDEX 2
+
+/proc/CallAsync(datum/source, proctype, list/arguments)
+	set waitfor = FALSE
+	return call(source, proctype)(arglist(arguments))
+
+// Declare children of this type path to use as identifiers for the events.
+/lazy_event
+
+// TODO: Document here the arguments that need to be passed to the procs invoked by each event
+/lazy_event/on_z_transition
+/lazy_event/on_post_z_transition
+
+/datum
+	/// Associative list of type path -> list(),
+	/// where the type path is a descendant of /event_type.
+	/// The inner list is itself an associative list of string -> list(),
+	/// where string is the \ref of an object + the proc to be called.
+	/// The list associated with the string above contains the hard-ref
+	/// to an object and the proc to be called.
+	var/list/list/registered_events
+
+/**
+  * Calls all registered event handlers with the specified parameters, if any.
+  * Arguments:
+  * * lazy_event/event_type Required. The typepath of the event to invoke.
+  * * list/arguments Optional. List of parameters to be passed to the event handlers.
+  */
+/datum/proc/lazy_invoke_event(lazy_event/event_type, list/arguments)
+	SHOULD_NOT_OVERRIDE(TRUE)
+	if(!length(registered_events))
+		// No event at all is registered for this datum.
+		return
+	var/list/event_handlers = registered_events[event_type]
+	if(!length(event_handlers))
+		// This datum does not have any handler registered for this event_type.
+		return
+	. = NONE
+	for(var/key in event_handlers)
+		var/list/handler = event_handlers[key]
+		var/objRef = handler[EVENT_HANDLER_OBJREF_INDEX]
+		var/procName = handler[EVENT_HANDLER_PROCNAME_INDEX]
+		. |= CallAsync(objRef, procName, arguments)
+
+/**
+  * Registers a proc to be called on an object whenever the specified event_type
+  * is invoked on this datum.
+  * Arguments:
+  * * lazy_event/event_type Required. The typepath of the event to register.
+  * * datum/target Required. The object that the proc will be called on.
+  * * procname Required. The proc to be called.
+  */
+/datum/proc/lazy_register_event(lazy_event/event_type, datum/target, procname)
+	SHOULD_NOT_OVERRIDE(TRUE)
+	if(!registered_events)
+		registered_events = list()
+	if(!registered_events[event_type])
+		registered_events[event_type] = list()
+	var/key = "[ref(target)]:[procname]"
+	registered_events[event_type][key] = list(
+		EVENT_HANDLER_OBJREF_INDEX = target,
+		EVENT_HANDLER_PROCNAME_INDEX = procname
+	)
+
+/**
+  * Unregisters a proc so that it is no longer called when the specified
+  * event is invoked.
+  * Arguments:
+  * * lazy_event/event_type Required. The typepath of the event to unregister.
+  * * datum/target Required. The object that's been previously registered.
+  * * procname Required. The proc of the object.
+  */
+/datum/proc/lazy_unregister_event(lazy_event/event_type, datum/target, procname)
+	SHOULD_NOT_OVERRIDE(TRUE)
+	if(!registered_events)
+		return
+	if(!registered_events[event_type])
+		return
+	var/key = "[ref(target)]:[procname]"
+	registered_events[event_type] -= key
+	if(!registered_events[event_type].len)
+		registered_events -= event_type
+	if(!registered_events.len)
+		registered_events = null
+
+#undef EVENT_HANDLER_OBJREF_INDEX
+#undef EVENT_HANDLER_PROCNAME_INDEX