Why do multiple setTimeout() calls cause so much lag?

问题

I have a complex animation sequence involving fades and transitions in JavaScript. During this sequence, which consists of four elements changing at once, a setTimeout is used on each element.

Tested in Internet Explorer 9, the animation works at realtime speed (it should take 1.6 seconds and it took exactly 1.6 seconds). ANY other browser will lag horribly, with animation times of 4 seconds (Firefox 3 and 4, Chrome, Opera) and something like 20 seconds in IE 8 and below.

How can IE9 go so fast while all other browsers are stuck in the mud?

I have tried to find ways of merging the elements into one, so as to one have one setTimeout at any given time, but unfortunately it wouldn't stand up to any interference (such as clicking a different link to start a new animation before the current one has finished).

EDIT: To elaborate in response to comments, here's the outline of the code:

link.onclick = function() {
    clearTimeout(colourFadeTimeout);
    colourFadeTimeout = setTimeout("colourFade(0);",25);

    clearTimeout(arrowScrollTimeout);
    arrowScrollTimeout = setTimeout("arrowScroll(0);",25);

    clearTimeout(pageFadeOutTimeout);
    pageFadeOutTimeout = setTimeout("pageFadeOut(0);",25);

    clearTimeout(pageFadeInTimeout);
    pageFadeInTimeout = setTimeout("pageFadeIn(0);",25);
}

Each of the four functions progress the fade by one frame, then set another timeout with the argument incremented, until the end of the animation.

You can see the page at http://adamhaskell.net/cw/index.html (Username: knockknock; Password: goaway) (it has sound and music, which can be disabled, but be warned!) - my JavaScript is very messy since I haven't really organised it properly, but it is commented a bit so hopefully you can see what the general idea is.

回答1:

Several things:

Your timeout is 25ms. This translates to 40fps which is a very high framerate to try to achieve via javascript. Especially for things involving DOM manipulation that may trigger reflows. Increase it to 50 or 60. 15fps should be more than fluid enough for the kinds of animation you're doing. You're not trying to display videos here, just move things around the page.
Don't use strings as the first parameter to setTimeout(). Especially if you care about performance. That will force javascript to recompile the string each frame of animation. Use a function instead. If you need to pass an argument use an anonymous function to wrap the function you want to execute:
```
setTimeout(function(){
    pageFadeIn(0)
},50);
```
this will only get compiled once when the script is loaded.
As mentioned by Ben, it is cheaper to use a single setTimeout to schedule the functions. For that matter, code clarity may improve by using setInterval instead (or it may not, depends on your coding style).

Additional answer:

Programming javascript animation is all about optimisation and compromise. It's possible to animate lots of things on the page with little slow-down but you need to know how to do it right and decide what to sacrifice. As an example of just how much can be animated at once is a demo real-time strategy game I wrote a couple of years ago.

Among the things I did to optimize the game are:

The walking soldiers are made up of only two frames of animation and I simply toggle between the two images. But the effect is very convincing nonetheless. You don't need perfect animation, just one that looks convincing.
I use a single setInterval for everything. It's cheaper CPU-wise and easier to manage. Just decide on a base frame rate and then schedule for different animation to start at different times.

回答2:

Well, that's a lot of javascript (despite the "quadruple-dose of awesomeness" :)

You're firing a lot of setTimeout sequence, I'm not sure how well JS engines are optimised for this.. particularly IE <= 8

Ok, maybe just a rough suggestion... You could maybe write a small timing engine.

Maintain a global object that stores your current running timed events with the function to run, and the delay...

Then have a single setTimeout handler that check against that global object, and decreases the delay at each iteration and call the function when the delay becomes < 0

you global event would looks something like that:

var events = {

        fade1 : {
            fn : func_name,
            delay : 25,
            params : {}
        }

        fadeArrow : {
            fn : func_name,
            delay : 500,
            params : {}
        }

        slideArrow : {
            fn : func_name,
            delay : 500,
            params : {
                arrow:some_value
            }
        }

    }

then create a function to loop through these at an interval (maybe 10 or 20 ms) and decrease your delays until they complete and fire the function with params as a paramer to the function (check Function.call for that).

Once down, fire setTimeout again with the same delay..

To cancel an event just unset the property from the events list..

Build a few method to add / remove queued events, update params and so on..

That would reduce everything to just one timeout handler..

(just an idea)

来源：https://stackoverflow.com/questions/4211046/why-do-multiple-settimeout-calls-cause-so-much-lag

标签

javascript