example.lua

local hooks = require("hooks") -- loads hooks.lua from same dir

hooks.add("test", "a", function(num) print("a", num) end)
hooks.add("test", "b", function(num) print("b", num + 2) end)
hooks.call("test", 4)
hooks.remove("test", "a")
hooks.call("test", 4)

hooks.lua

local hooks = {} -- create a table to define the module on
local registry= {} -- local table to keep track of the registered hooks

-- Adds a callback to a hook. The callback needs an associated key so you can remove it later again.
-- The hookname is a string that indicates the list to add the callback to, the list is used by hooks.call
function hooks.add(hookname, key, callback)
  registry[hookname] = registry[hookname] or {}
  registry[hookname][key] = callback;
end

-- Removes a hook by its key
function hooks.remove(hookname, key)
  if (registry[hookname]) then
    registry[hookname][key] = nil
  end
end

-- Invoke all callbacks on a specific hook
-- (...) is a shortcut for all remaining arguments
function hooks.call(hookname, ...)
  if (registry[hookname]) then
    for _, callback in pairs(registry[hookname]) do
      callback(...)
    end
  end
end

--Return the module, this is what is given back by the require() function when this file is included
--We return the module table which means that all local variables in this file are private to the module and only the functions on the module table are exposed.
return hooks