_coresetup.init.lua

 -- inside _coresetup/init.lua somewhere
 -- ...
 
 function hs.swiftloader(mod)
    local cmod = "hs."..mod..".internal"
    package.preload[cmod] = function()
      local dylib = package.searchpath(cmod, package.cpath)
      local cfunc = "luaopen_"..cmod:gsub("%.","_")
      local symbol = string.format("_TF%s%s%s%sFGSpVSC9lua_State_Vs5Int32", #mod, mod, #cfunc, cfunc)
      return package.loadlib(dylib,symbol)()
    end
  end

-- ...

console.lua

> a = require("hs.swiftdylib")

> b = a.new("asdsd")
> b
note: (0x61000004c208)

asdsd

> b:getAuthor()
default

> b:setAuthor("me")
note: (0x61000004c208)

asdsd

> b:getAuthor()
me

> c = a.new("qqqq")
> c
note: (0x6100000587d8)

qqqq

> b
note: (0x61000004c208)

asdsd

> i = a.image("~/Desktop/Untitled.png")
> i
hs.image:  (0x60000004a588)

> a.thing("hello")
hello!!!

extension.swift

import Foundation
import LuaSkin

typealias LuaState = UnsafeMutablePointer<lua_State>


// functions
func thing(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    let msg = skin.toNSObjectAtIndex(1) as! String
    skin.pushNSObject(msg + "!!!")
    return 1;
}


// automatically gives hs.image object using its helper
func image(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    let path: String = skin.toNSObjectAtIndex(1) as! String
    skin.pushNSObject(NSImage(byReferencingFile: path))
    return 1;
}


// userdata stuff
class Note {
    var contents: String = ""
    var author: String = "default"
}

func new(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    let note = Note()
    let contents = skin.toNSObjectAtIndex(1) as! String
    note.contents = contents
    
    let ptr = UnsafeMutablePointer<Note>(lua_newuserdata(skin.L, sizeof(Note)))
    ptr.initialize(note)
    lua_getfield(L, -LUAI_MAXSTACK-1000, "note") // aka luaL_getmetatable(L, "note"), macros not available in swift, could be defined in LuaSkin
    lua_setmetatable(L, -2);
    
    return 1;
}

func getAuthor(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    let note = UnsafeMutablePointer<Note>(lua_touserdata(L, 1)).memory
    skin.pushNSObject(note.author)
    return 1;
}

func setAuthor(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    let note = UnsafeMutablePointer<Note>(lua_touserdata(L, 1)).memory
    let author = skin.toNSObjectAtIndex(2) as! String
    note.author = author
    lua_pushvalue(L, 1);
    return 1;
}

func tostring(L: LuaState) -> Int32 {
    let note = UnsafeMutablePointer<Note>(lua_touserdata(L, 1)).memory
    lua_pushstring(L, String(format: "%@: (%p)\n\n%@", "note", lua_topointer(L, 1), note.contents))
    return 1;
}


var funcs: [luaL_Reg] = ([
    ("thing", thing),
    ("stuff", stuff),
    ("image", image),
    ("new", new),
    ] as [(String, lua_CFunction)]).map { luaL_Reg(name: strdup($0.0), func: $0.1) }

var metafuncs: [luaL_Reg] = [
    luaL_Reg(name: strdup("getAuthor"), func: getAuthor),
    luaL_Reg(name: strdup("setAuthor"), func: setAuthor),
    luaL_Reg(name: strdup("__tostring"), func: tostring),
    luaL_Reg(name: nil, func: nil),
]

func luaopen_hs_swiftdylib_internal(L: LuaState) -> Int32 {
    let skin = LuaSkin.shared()
    funcs.append(luaL_Reg(name: nil, func: nil))
    skin.registerLibrary(funcs, metaFunctions: nil)
    skin.registerObject("note", objectFunctions: metafuncs)
    return 1;
}

init.lua

--- === hs.swiftdylib ===
---
--- Swift module test

local mod_name = "swiftdylib"
hs.swiftloader(mod_name)

local module = require("hs."..mod_name..".internal")
return module