I am working on a library to allow Lua (5.2) scripting of games in iOS 5.x. I have created a class and added bindings to allow it to be created and accessed form Lua. The C initializer method called from Lua is given below:
static int newGeminiObject(lua_State *L){
GeminiObject *go = [[GeminiObject alloc] initWithLuaState:L];
GeminiObject **lgo = (GeminiObject **)lua_newuserdata(L, sizeof(GeminiObject *));
*lgo = go;
luaL_getmetatable(L, GEMINI_OBJECT_LUA_KEY);
lua_setmetatable(L, -2);
lua_newtable(L);
lua_setuservalue(L, -2);
NSLog(@"New GeminiObject created");
// add this new object to the globall list of objects
[[Gemini shared].geminiObjects addObject:go];
return 1;
}
This assigns a metatable which is set up elsewhere to provide access to various methods. Additionally, it attaches a table as a uservalue to allow script code to assign attributes to the objects.
I can create these objects in Lua scripts with no problem:
require "gemini"
x = gemini.new()
x:addEventListener("touch", objectTouched)
Here objectTouched is a a Lua method defined elsewhere that handles a touch event. Here addEventListener binds it to touch events.
These objects work just fine. When I attempt to create one from C, however, I am running into problems. I can create the object, but trying to assign it to a global and then invoke it in a script fails.
The following C code runs
-(void) addRuntimeObject {
GeminiObject *rt = [[GeminiObject alloc] initWithLuaState:L];
GeminiObject **lruntime = (GeminiObject **)lua_newuserdata(L, sizeof(GeminiObject *));
*lruntime = rt;
// set the metatable - effectively declaring the type for this object
luaL_getmetatable(L, GEMINI_OBJECT_LUA_KEY);
lua_setmetatable(L, -2);
// add a table to hold anything the user wants to add
lua_newtable(L);
lua_setuservalue(L, -2);
// create an entry in the global table
lua_setglobal(L, "Runtime");
// empty the stack
lua_pop(L, lua_gettop(L));
}
This should define a global named "Runtime". Trying to access this variable from a script like this
Runtime:addEventListener("enterFrame", enterFrame)
Results in the following error:
attempt to index global 'Runtime' (a userdata value)
It is a userdata value, but this doesn't seem to matter when I create one in Lua directly. The metatable binding provides access to the methods and metamethods. Again, this works fine if the object is created from Lua, just not when it is created in C.
Any ideas as to what I'm doing wrong here, or what the correct way to make a global from userdata is?
EDIT
Based on comments below regarding confusion about GEMINI_OBJECT_LUA_KEY, I thought I would list the code that is actually used in the binding:
static const struct luaL_Reg geminiObjectLib_f [] = {
{"new", newGeminiObject},
{NULL, NULL}
};
static const struct luaL_Reg geminiObjectLib_m [] = {
{"addEventListener", addEventListener},
{"__gc", geminiObjectGC},
{"__index", l_irc_index},
{"__newindex", l_irc_newindex},
{NULL, NULL}
};
int luaopen_geminiObjectLib (lua_State *L){
// create the metatable and put it into the registry
luaL_newmetatable(L, GEMINI_OBJECT_LUA_KEY);
lua_pushvalue(L, -1); // duplicates the metatable
luaL_setfuncs(L, geminiObjectLib_m, 0);
// create a table/library to hold the functions
luaL_newlib(L, geminiObjectLib_f);
NSLog(@"gemini lib opened");
return 1;
}
This code registers the library of functions (not show here) that provide the methods and metamethods for the GeminiObjects. The call to luaL_newmetatable creates a new metatable and associates it in the registry with the key GEMINI_OBJECT_LUA_KEY. GEMINI_OBJECT_LUA_KEY is just a unique string defined in the header. luaL_setfuncs actually adds the function pointers to the metatable, making them available as methods of the objects.
