Example of making a system for registering functions to call when flags are set, for Divinity: Original Sin 2 with the Script Extender.

EventManager.lua

EventManager = {
	Register = {},
}

---@alias EventManagerObjectFlagSetCallback fun(flag:string, targetGUID:GUID, dialogInstance:integer)

local _Callbacks = {
	ObjectFlagSet = {
		---@type table<string, EventManagerObjectFlagSetCallback[]>
		Active = {},
		Regions = {
			__ANY__ = {}, -- Any region,
		}
	},
}

local function _GetCurrentLevel()
	local current = Ext.Entity.GetCurrentLevel()
	if current then
		return current.LevelDesc.LevelName
	end
	return ""
end

--[[
This is configured to be 1 function per flag (using a key-value type table where the flag is the key),
but could be changed to support multiple callbacks for the same flags.
]]
---Register a function to call when the given flag(s) are set.  
---@param flag string|string[] Any single flag, or a table of flags.
---@param callback EventManagerObjectFlagSetCallback
---@param region string|nil
function EventManager.Register.ObjectFlagSet(flag, callback, region)
	if not region then
		region = "__ANY__"
	end
	if _Callbacks.ObjectFlagSet.Regions[region] == nil then
		_Callbacks.ObjectFlagSet.Regions[region] = {}
	end
	local t = type(flag)
	if t == "table" then
		for _,v in pairs(flag) do
			_Callbacks.ObjectFlagSet.Regions[region][v] = callback
		end
	elseif t == "string" then
		_Callbacks.ObjectFlagSet.Regions[region][flag] = callback
	else
		error("Wrong flag type: " .. t, 2)
	end
end

local function _BuildCallbacks(region)
	local tbl = {}
	local regionCallbacks = _Callbacks.ObjectFlagSet.Regions[region]
	if regionCallbacks then
		for flag,v in pairs(regionCallbacks) do
			if tbl[flag] == nil then tbl[flag] = {} end
			table.insert(tbl[flag], v)
		end
	end
	for flag,v in pairs(_Callbacks.ObjectFlagSet.Regions.__ANY__) do
		if tbl[flag] == nil then tbl[flag] = {} end
		table.insert(tbl[flag], v)
	end
	_Callbacks.ObjectFlagSet.Active = tbl
end

Ext.Osiris.RegisterListener("RegionStarted", 1, "after", _BuildCallbacks)
Ext.Events.ResetCompleted:Subscribe(function (e) _BuildCallbacks(_GetCurrentLevel()) end)

Ext.Osiris.RegisterListener("ObjectFlagSet", 3, "after", function (flag, targetGUID, inst)
	---@cast flag string
	local callbacks = _Callbacks.ObjectFlagSet.Active[flag]
	if callbacks then
		targetGUID = GetUUID(targetGUID)
		for _,v in pairs(callbacks) do
			local b,err = xpcall(v, debug.traceback, flag, targetGUID, inst)
			if not b then
				Ext.Utils.PrintError(err)
			end
		end
	end
end)

--[[ 
--Usage Example:

EventManager.Register.ObjectFlagSet({"QuestUpdate_TUT_ShipMurder_Fled", "QuestUpdate_TUT_ShipMurder_Rescued", "QuestUpdate_TUT_ShipMurder_Failed"},
function (flag, targetGUID, dialogInstance)
	for key, value in pairs(TUT_Char) do
		if CharacterIsDead(value) == 0 then
			Crew[value] = true
		else
			Crew[value] = nil
		end
	end
end, "TUT_Tutorial_A")

]]