Adds callback select system

code/datums/callback.dm

@@ -100,3 +100,57 @@
 	if (object == GLOBAL_PROC)
 		return call(delegate)(arglist(calling_arguments))
 	return call(object, delegate)(arglist(calling_arguments))
+
+
+/datum/callback_select
+	var/list/finished
+	var/pendingcount
+	var/total
+
+/datum/callback_select/New(count, savereturns)
+	total = count
+	if (savereturns)
+		finished = new(count)
+
+
+/datum/callback_select/proc/invoke_callback(index, datum/callback/callback, list/callback_args, savereturn = TRUE)
+	set waitfor = FALSE
+	if (!callback || !istype(callback))
+		//This check only exists because the alternative is callback_select would block forever if given invalid data
+		CRASH("invalid callback passed to invoke_callback")
+	if (!length(callback_args))
+		callback_args = list()
+	pendingcount++
+	var/rtn = callback.Invoke(arglist(callback_args))
+	pendingcount--
+	if (savereturn)
+		finished[index] = rtn
+
+
+
+
+//runs a list of callbacks asynchronously, returning once all of them return.
+//callbacks can be repeated.
+//callbacks-args is a optional list of argument lists, in the same order as the callbacks,
+//	the inner lists will be sent to the callbacks when invoked() as additional args.
+//can optionly save and return a list of return values, in the same order as the original list of callbacks
+//resolution is the number of byond ticks between checks.
+/proc/callback_select(list/callbacks, list/callback_args, savereturns = TRUE, resolution = 1)
+	if (!callbacks)
+		return
+	var/count = length(callbacks)
+	if (!count)
+		return
+	if (!callback_args)
+		callback_args = list()
+
+	callback_args.len = count
+
+	var/datum/callback_select/CS = new(count, savereturns)
+	for (var/i in 1 to count)
+		CS.invoke_callback(i, callbacks[i], callback_args[i], savereturns)
+	
+	while(CS.pendingcount)
+		sleep(resolution*world.tick_lag)
+	return CS.finished
+