event machine for lua

event.lua

EventMachine = {--[[static members]]}

function EventMachine:new()
    local this = {
        current_actor = nil,
        receivers_list = {},
        event_register = {}
    }

    local on_mt = {
        __newindex = function(tbl, event_name, func)
            local actor_key = tostring(this.current_actor)
            this.event_register[actor_key] = this.event_register[actor_key] or {}
            if this.event_register[actor_key][event_name] then 
                print("duplicated receiver is not allowed: " .. event_name)
                return
            end
            this.receivers_list[event_name] = this.receivers_list[event_name] or {}
            this.receivers_list[event_name][actor_key] = func
            this.event_register[actor_key][event_name] = true
        end
    }

    function this.on(actor)
        this.current_actor = actor 
        return setmetatable({}, on_mt)
    end

    local emit_mt = {
        __index = function(tbl, event_name)
            if not this.receivers_list[event_name] then
                print("no receivers for event: " .. event_name)
                return function() end
            end
            return function(...)
                for _, func in pairs(this.receivers_list[event_name]) do
                    func(...)
                end
            end
        end}

    function this.emit(actor)
        this.current_actor = actor
        return setmetatable({}, emit_mt)
    end

    function this.detach(actor)
        local actor_key = tostring(actor)
        
        if not this.event_register[actor_key] then
            print("detaching unknown actor: " .. actor_key)
            return
        end
        for event_name, _ in pairs(this.event_register[actor_key]) do
            print("removed '" .. event_name .. "' for " .. actor_key)
            this.receivers_list[event_name][actor_key] = nil
        end
        this.event_register[actor_key] = nil
    end

    return this
end

local event = EventMachine:new()

local character = {name = "foo"}
event.on(character).revive = function(...)
    print(character.name .. " revives", ...)
end

event.on(character).gainHP = function(...)
    print(character.name .. " gainHP", ...)
end

event.on(character).gainHP = function(...)
    -- not allowed
end

local character2 = {name = "bar"}
event.on(character2).revive = function(...)
    print(character2.name .. " revives", ...)
end

event.on(character2).gainHP = function(...)
    print(character2.name .. " gainHP", ...)
end

local skill = {}
event.emit().revive("foo")
event.emit().foobar(1, 2, 3)

event.detach(character)
event.detach(character2)
event.detach(character)