Unlocking XNA game update rate and preventing unnecessary Draw calls and heavy logic Updates

Question

I had some input problems which can be partially solved by unlocking game update rate. Now the input updates as fast as possible, but the game also calls the main Draw method a lot too. I believe there are some calls that may be omitted without causing any noticeable problems, and I'd like to know how I can do it.

I tried using code similar to frame rate counter code, but it just doesn't work. It either doesn't skip frames, or skips them all. Here it is:

double time = 0;
protected override void Draw(GameTime gameTime)
{
    double frameRate = 1.0 / 60;
    time += gameTime.ElapsedGameTime.TotalSeconds;
    if (time > frameRate)
    {
        time = time % frameRate;
    // the rest of the draw call

When the game window is active, only dark purple color is visible I guess that's application's default background color when nothing is being drawn. When the game window is not active, frame rate drops from infinity (though I get about 4000 fps) to around 50 (which is unusual, since the default frame rate is 60, but it's not an issue).

If I set frameRate value to lower than 1/50 (like 1/10) it becomes obvious that draw calls are skipped and the dark purple background is visible (and resources are being saved), but only when the window is inactive. So the game behaves as expected, but only when it is not in focus, which is not the way it is supposed to be played :)

How do I fix this?

I'd like to extend this to work on heavy logic updates, so they too are not done more than enough times per second. The game should only update input as fast as possible, and I believe, reducing the number of logic update and draw calls frees up resources for input update to use, increasing their frequency.

---

This is how I ended up doing this:

double interval = 1d / 60; // draw and do heavy updates only 60 frames per second
double time;

protected override void Update(GameTime gameTime)
{
    LightUpdate(gameTime);

    time += gameTime.ElapsedGameTime.TotalSeconds;

    if (time > interval)
        time = time % interval; // or while (time > interval) time -= interval;
    else
    {
        SuppressDraw();
        base.Update(gameTime);
        return;
    }

    HeavyUpdate(gameTime);
}

Saves frames, light update rate rose from 2k to 110k @100Hz which is a tremendous success!

Answer 1

If you know in your Update method that you don't need or want to render anything simply call SupressDraw() on your game object (http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.game.suppressdraw.aspx). You don't need to add any additional logic into your Draw method to handle this, and your frame skipping logic can be moved into your Update call.

Answer 2

For one I would not be using doubles in a modulus operation because 1/60th can't be represented exactly in a double variable and neither will the time accumulation so you will be getting a fair bit of round off error and due to that, some interesting results.

What I would do, if you simply have to do this, is to calculate your 1/60th constant outside of this loop, assign your time variable to that value initially, and then decrement it by elapsedTime in each draw call. When the time variable is equal to or below zero then do your draw and then add the 1/60th time to it. You will still get a minor amount of round off/up error but that should balance out between draw calls.