Is it possible to execute a single lua statement from a host program?

Question

I am trying to embed a lua-based script system into my game engine. I would like the scripting to be able to have both blocking and non-blocking commands, for example:

character.walkTo(24, 359);  // Blocks until character arrives
c = 35; // Non blocking, execution goes on to the next statement

Since the "walkTo" needs to be "active" for more than 1 frame of execution, I would like to be able to run 1 statement at time from the Java host instead of the whole function. This is because it would be overkill to have real multithreading, which is not needed.

If I could execute just 1 statement, and keep the execution state "paused" until next statement execution, I would be able to implement blocking commands like "walkTo" by checking if the command is finished in the host, and if it is, go on to the next statement, otherwise, wait until the next frame iteration.

Is there any way to execute 1 statement a time from the Java host with LuaJ(or with any other Lua api), or am I forced to develop my own script engine with lex and yacc?

Any good idea is welcome, thanks!

Answer 1

Seems like you are missing asynchronous pattern. If c=35 has to be executed once character occurs at (24,359), then the correct way is to pass function() c=35 end as third argument to walk method and your engine (that performs actual 'walking') will call that callback when appropriate.

character.walkTo(24, 359, function ()
    c = 35
end)

Otherwise, walk may schedule walking to engine and yield immediately, resuming on correct event. In this case you have to setup script worker-coroutine (you cannot yield in main state).

script = coroutine.wrap(function ()
    character.walkTo(24, 359) -- will yield and leave callable global 'script'
    c = 35
end)
script() -- resume first time
-- here script is over

...

-- this wrapper may be implemented in Lua or host language

function character.walkTo(x, y)
    engine.startActualWalkingTo(x, y)
    coroutine.yield() -- yields to host
    -- coroutine.resume() will continue here
end

...

-- somewhere in engine code (pseudo-code here)

for event in eventLoop do
    if character.x == endPoint.x and character.y == endPoint.y then
        script() -- resume again at c=35
    end
end

You may cancel the script anytime with script=nil.

yield() has some limitations, consult the manual.

Answer 2

Bonus answer for everyone who is sticking with this problem.

Here is my exact solution:

-- test.lua --

onLookAt = function()
    character:walkTo(234, 35)
    print("Arrived!")
end

-- LuaTest.java --

public static void luaTest() {
    Globals g = JsePlatform.standardGlobals();
    g.load(new CoroutineLib());

    g.set("character", CoerceJavaToLua.coerce(new Character(g)));
    g.load(Gdx.files.internal("test.lua").reader(), "test.lua").call();
    g.load("co = coroutine.wrap(onLookAt)").call();
    g.get("co").call();
    try {
        // Simulate time that passes while the script is paused
        Thread.sleep(2000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    g.get("co").call();
}

public static class Character{
    Globals g;

    public Character(Globals g){
        this.g = g;
    }

    public void walkTo(int x, int y) {
        System.out.println("Started walking.");
        g.yield(LuaValue.NONE);
    }
}

-- Output --

Started walking.

(2 seconds later)

Arrived!

One thing you should be very careful about:

• Do NOT use java's ScriptEngine interface if you want to accomplish this. ScriptEngine interface doesn't provide API for obtaining the implicitly allocated Globals instance that you need for yielding, and making a new instance of Globals and using that for yielding is obviously pointless.