Passing elapsed time to the update function from the game loop

Question

I want to pass the time elapsed to the update() method as this would make easy to implement the animations and time related concepts.

Here's my game-loop.

public void gameLoop(){
    boolean running = true;

    long gameTime = getCurrentTime();

    long elapsedTime = 0;
    long lastUpdateTime = 0;

    int loops;

    while (running){

        loops = 0;

        while(getCurrentTime()>gameTime && loops<Global.MAX_FRAMESKIP){
            elapsedTime = getCurrentTime() - lastUpdateTime;
            lastUpdateTime = getCurrentTime();

            update(elapsedTime);

            gameTime += SKIP_STEPS;
            loops++;
        }

        displayGame();
    }
}

getCurrentTime() method

public long getCurrentTime(){
    return (System.nanoTime()/1000000);
}

update() method

long time = 0;

public void update(long elapsedTime){
    time += elapsedTime;
    if (time>=1000){
        System.out.println("A second elapsed");
        time -= 1000;
    }
}

But this is printing the message for 3 seconds.

Thanks.

Answer 1

First, you should avoid calling getCurrentTime() multiple times per loop. You will potentially be getting different values at different times. Call it once at the start of the loop and use that value as the current time for the rest of the loop. Also, you are advancing gameTime by a fixed amount each time rather than the length of the frame, you are sort of mixing a fixed step and variable length frame concept here. To have variable length frames, you probably want something more like:

long elapsedTime = 0;
long lastUpdateTime = getCurrentTime();
long gameTime = 0;

while (running){
    gameTime = getCurrentTime();
    elapsedTime = lastUpdateTime - gameTime;

    // This is where you can apply a minimum frame time if you want, at the
    // very least, you should make sure elapsedTime > 0, since
    // you don't want to run 0 length frames
    if (elapsedTime < Global.MIN_FRAME_LENGTH)
        continue;

    update(elapsedTime);

    displayGame();

    lastUpdateTime = gameTime;
}

That inner loop is only necessary if you want to have a maximum frame time, in which case you could do something like:

long gameTime = getCurrentTime();

long elapsedTime = 0;
long lastUpdateTime = getCurrentTime();
long currentTime = lastUpdateTime;
long frameTime = 0;

while (running){

    currentTime = getCurrentTime();
    elapsedTime = lastUpdateTime - currentTime;

    if (elapsedTime < Global.MIN_FRAME_LENGTH)
        continue;

    while (elapsedTime > 0) {
        frameTime = elapsedTime;
        if (frameTime > Global.MAX_FRAME_LENGTH)
            frameTime = Global.MAX_FRAME_LENGTH;

        update(frameTime);
        elapsedTime -= frameTime;
    }

    displayGame();

    lastUpdateTime = currentTime;
}