Combined delayed function calls in Javascript

Question

I am not quite sure what the technical term for this is. I have a GUI with interactive graphics. After the user has interacted with the GUI, I need to perform some CPU intensive action. However, user input is very frequent, so I only want to call the function after e.g. 1000ms of no userinput. Below the pattern that I use:

scheduler = (function(){
    var timer;
    function exec(call, delay){
        clearTimeout(timer);
        timer = setTimeout(call, delay);
    };
    return exec;
})()

I.e. if the 3 calls to scheduler are done right after each other, only the final one will actually be executed:

scheduler(function(){alert('foo')}, 1000);
scheduler(function(){alert('bar')}, 1000);
scheduler(function(){alert('zoo')}, 1000);

It seems to work, but it feels a bit hacky I am a little worried about any caveats of Javascript setTimeout, especially the scoping problems. Does this seem like a reliable pattern I could use on a larger scale? Will the inline function that I pass to scheduler be able to lookup all objects in its lexical scope as usual, when it is called by settimeout? What about if I have several of these scheduler instances? Could they interfere with each other? Is there an alternative way of accomplishing this?

Answer 1

You could opt for using web worker threads instead:

https://developer.mozilla.org/en-US/docs/DOM/Using_web_workers http://www.html5rocks.com/en/tutorials/workers/basics/

Answer 2

What I would do:

http://jsfiddle.net/gunderson/4XXQ4/1/

    var severQueue = [];
    var delay;

    $("#inputSquare").mousemove(onMouseMove);

    function onMouseMove(){
        if (delay){
           clearTimeout(delay);
        }
        serverQueue.push("doSomething")
        delay = setTimeout(sendToServer, 1000);
    }

    function sendToServer(){
        console.log(serverQueue.length);
        delay = null;
        $("#inputSquare").addClass("activated");
        // do some ajax using serverQueue
        // we'll just simulate it with another timeout
        for (var i in serverQueue){
            serverQueue.pop();
        }
        onComplete = setTimeout(onAjaxComplete, 1000);
    }

    function onAjaxComplete(){
        $("#inputSquare").removeClass("activated");
    }

​

Answer 3

In theory, your solution looks like it will work. There are no scoping problems related to you passing a callback function to your scheduler function; the callback will close over whatever environment it was created in, just like any other function in JavaScript. That being said, scoping rules can be a bit tricky in JavaScript, so make sure that you read up on it.

In practice, there may be some browser-specific issues related to setTimeout that may make this solution unworkable. For example, the frequency at which certain browsers execute setTimeout callbacks may vary such that you'll be waiting longer than you expect for a callback to be executed. All setTimeout callbacks will be executed sequentially; they'll never be executed in parallel. However, you have guarantees as to what order they will be executed in.

All that being said, any major gotcha in your solution will likely have more to do with the callbacks that your registering rather than the way in which you're registering them.