How to listen to specific function call in Lua from C++?

Question

Let's say I have the following Lua code.

function touched(x, y)
end
function moved(x, y)
end
function released(x, y)
end

These functions are called from C++ with lua_pcall so I can also listen to these events in C++.

But I wonder if it's possible to add a listener that listens to specific Lua function based on the name of that function in C++.

For example, it can be something like the following in C++

lua_addlistener(L, "touched", this, &MyClass::touchedFromLua);

And then it can listen to the touched function in Lua code. (if the function "touched" exists)

Is this possible to do something similar?

Maybe you can utilize an observer pattern? Check this [SO](https://stackoverflow.com/questions/27249195/implementing-c-to-lua-observer-pattern) post for more details. — kingJulian, Jul 10 '18 at 10:32

score 1 · Accepted Answer · answered Jul 10 '18 at 15:22

You can replace the function with your own and then in that function call the original after you handled the listener:

lua_getglobal(L, "touched");
lua_pushlightuserdata(L, this);
lua_pushcclosure(L, &MyClass::touchedFromLua, 2); 
//add  original function and this as upvalues
lua_setglobal(L, "touched");

touchedFromLua would have to be static and look something like:

int MyClass::touchedFromLua(Lua_State *L){
    int args = lua_gettop(L);
    MyClass* thiz = std::reinterpret_cast<MyClass*>(lua_touserdata(lua_upvalueindex(2)));
    thiz->touchedFromLua_nonstatic(L);

    lua_pushvalue(lua_upvalueindex(1));
    lua_insert(L, 1);
    lua_call(L, args , LUA_MULTRET);
    int rets = lua_gettop(L);
    return rets;
}

`lua_touserdata` returns `void *`, so `static_cast` should be sufficient. No need for `reinterpret_cast` (which is, by the way, not in the `std` namespace). — Henri Menke, Jul 10 '18 at 21:18

How to listen to specific function call in Lua from C++?

1 Answers1