Using C++ w/ Lua in a runtime environment

Question

I have looked this question up so many times that I am convinced I am missing a huge piece of the puzzle when it comes to integrating LUA into my C++ Game Engine. What I want to do is run my game engine, then while its running I would like to click on my ui and click "add script" and then run the script. That part is easy enough to do but what I DON'T get is how a script that seemingly gets ran in place with lua_dofile could have code that gets mouse input or moves the character based on input. I don't see anyway to do this effectively. Am I supposed to allow the LUA state to be created and destroyed every frame or do I make the script fire every frame? In an application like this:

void init();

void update();
void render();

void end();

How would I use LUA to control the movement of an entity.

Lets say I set up the lua state so that you can write lua code like this:

entity1 = Entity.new()
entity1:setPosition(4,5)

How would I give the script some input from mouse to move the entity to the mouse position?

My overarching question is what is the best way to have a script control my entities in a way that if I supply the Lua State the ability to move my entities, then a scripter could write a "game" (ie a interactive runtime application)

Answer 1

To give a concrete example, suppose you wanted to teleport a ball to the player whenever they click the mouse. Here's what you could write to do that in Lua:

local function clickHandler(event)
    local x, y, z = event.player:getCoords()
    local ball = event.world:getBall()
    ball:teleportTo(x, y, z)
end

game.registerEventHandler('click', clickHandler)

To make that example actually work in your engine, here's what you'd need to do:

• Create a registerEventHandler function that saves the given callback somewhere C can access it
• Create a getCoords function that returns the coordinates of a given player
• Create a getBall function that gets a ball in your gameworld
• Create a teleportTo method that teleports an entity to a given location
• Whenever a player clicks the mouse, run all click event handlers that you saved, with an event object containing the player and the gameworld

And as for this question in particular:

Am I supposed to allow the LUA state to be created and destroyed every frame or do I make the script fire every frame?

No. You create a lua_State when your game starts, load the scripts in it, and then just keep using that state to call the event handlers.

Answer 2

Essentially the Lua scripts define functions to be called at each event. So the host C++ loads the scripts and then enters its main loop and calling back Lua.

See how LÖVE does it.