Events in Lua. That is, something like the multicast delegates (synchronous) of C#... well, sort of.

event.lua

local _G = _G

module('Event')

local ERROR_FUN_NOT_FUNCTION = 'Wrong parameter type: must be a function'

local function is_function(fun)
    if fun and _G.type(fun) == 'function' then 
        return true 
    else 
        return false
    end
end

local function check_function(fun)
    if not is_function(fun) then _G.error(ERROR_FUN_NOT_FUNCTION) end
end

local function add_subscriber(event, fun)
    check_function(fun)
    event.__subscribers[fun] = true
end

local function remove_subscriber(event, fun)
    check_function(fun)
    event.__subscribers[fun] = nil
end

local function clear_subscribers(event, fun)
    event.__subscribers = {}
end


function new()
    return _G.setmetatable({
            __subscribers = {}, 
            add        = add_subscriber, 
            remove     = remove_subscriber,
            remove_all = clear_subscribers, 
        }, {
        __call = function(self, ...)
            for f, _ in _G.pairs(self.__subscribers) do f(...) end
        end, 
    })
end

test-event.lua

require 'event'

local separator = "---------------------------------------"

local my_event  = Event.new()

local function say_hello(name)
    print("Hello " .. name .. "!")
end

my_event:add(say_hello)
my_event:add(function(...) print("Arguments: ", ...) end)

print(separator)

my_event("NRK", true, 10)

print(separator)

my_event:remove(say_hello)
my_event("NRK", true, 10)

print(separator)

my_event:remove_all()
my_event("NRK", true, 10)

print(separator)


--[[
########### OUTPUT ###########
---------------------------------------
Hello NRK!
Arguments: 	NRK	true	10
---------------------------------------
Arguments: 	NRK	true	10
---------------------------------------
---------------------------------------
]]