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Thanks for the replies, the tricky bit I am stuck on is illustrated with these 2 lines from the quoted example
    // and the registry to store the Lua object (this time a Lua table).
     lua_State * L = GetWindowLongPtr(hwnd, GWLP_USERDATA);
     lua_rawgetp(L, LUA_REGISTRYINDEX, (void*)hwnd);

In my example when the instance of the userdata was created I kept a copy of the pointer to the userdata instance on the C side.  So instead of the above example where it gets the object ptr from the registry and pushes it onto the stack I was thinking I just push my stashed ptr with something like a pushUserdata() API function.
Only snag with my idea is no such function exists :( So how do I get my instance ptr back on the stack so that I can see if it has got an associated action function defined in the Lua code ?
Maybe I should stash it in and retrieve it from the Registry anyway ?
Thanks for any further tips. For info, I am using Lua 5.1 
Regards Geoff
Date: Wed, 13 Nov 2013 21:32:10 +0800
Subject: Re: Another userdata question

    于 2013-11-13 4:48, Geoff Smith 写道:

      I am really hoping someone could help me with this
        question please as I am totally stuck on it for now.


        I am trying to implement a userdata via the C API,  to give me
        this type of functionality


        btn1 = graphics.newButton(x,y,width,ht, "pressMe")

        btn1:setText("Different Text")



        That's OK I think I have got that figured out and largely
        working, but the next thing I want to do is call a user defined
        Lua action function on the button instance. For example


        In Lua


        function btn1:action(someArbitrayParamBackFromC)

             print("ouch someone just poked btn1")



        How can I call that Lua button instance function from C when I
        have detected the key is pressed ?


        I have done something vaguely similar in the past when I have
        created a custom timer on the Lua side, but the difference was I
        had a timer create function in Lua such as 


        myNewTimer = createTimer( period, luaFunctionCallback)


        So I was passed the callback function into the C code which
        meant I could stash it away in the registry and then when the
        timer elapsed on the C side, I did this


         // push function indexed by registryRef from the registry back
        onto the stack

        // the lua callback function in this case

        lua_rawgeti(local_L, LUA_REGISTRYINDEX, registryRef);

        /* the first function argument is the timerID */ 

        lua_pushinteger(local_L, myTimerID); 

        /* call the function with 1 argument, return 0 result */ 

        pcallResult = lua_pcall(local_L,1,0,0);    // pops 2 params from


        I am guessing the solution is somewhat similar but cant figure
        it out.


        Many thanks for any assistance






    You are right. The solution is similar.


    In your Timer example, you retrieved the callback Lua function which
    was passed to the C side

    when the timer was created. While in your button press example, you
    should retrieve the button

    object first, then call the specified method on that object.


    Thus said, I assumed you had used some method (e.g.
    GetWindowLongPtr) to associate the

    button object (from Lua) with the *native* button object of the
    underlying GUI system.


    Following is an imaginary example to illustrate this idea to bind
    Lua to Win32 message system.

    Just as an illustration, can't claim the correctness.



    [inside the window procedure of the button]



    case WM_LBUTTONUP:

        // first retrive the associated object of the Lua side.

        // the following is just an demo method using GWLP_USERDATA to
    store L

        // and the registry to store the Lua object (this time a Lua

        lua_State * L = GetWindowLongPtr(hwnd, GWLP_USERDATA);

        lua_rawgetp(L, LUA_REGISTRYINDEX, (void*)hwnd);


        // then call the "action" method on the button object

        lua_getfield(L, -1, "action");


        // do some check

        if (!lua_isnil(L, -1)) {

            // push the self parameter

            lua_pushvalue(L, -2);

            lua_pcall(L, 1, 1, 0);

            // check the callback return value and do some processing



    case WM_XXXX: