Question

I have static image 500×640 sitting in folder catted by 20×20 pieces with css sprites, I am setting background-position to display each piece, I need such kind of display to be able to manipulate with each piece later.

css:

   .piece
        {
            width: 20px;
            height: 20px;
            display: inline-block;
            //display: inline;
            //zoom:1;
        }        

    .ob { background-image: url("/Images/ob.jpg");}

js:

<script id="flipTemplate" type="text/html">
    <div class="piece ob" data-bind="style: { backgroundPosition: viewModel.getLeftValue($index) + ' ' + viewModel.getTopValue($index) }, attr: {cond: Cond, id: Id }, click: viewModel.setClick ">
                </div>
</script>
<script type="text/javascript">
    viewModel = {
        flips: ko.observableArray([]),      

        setClick: function (data, e) {
            e.preventDefault();            
            //doing click
        },

        getLeftValue: function (index) {

            var position = 0;

            var currentLine = div(index(), 25);

            if (currentLine > 0)
                return '-' + (index() - (currentLine * 25)) * 20 + 'px';
            else
                return '-' + index() * 20 + 'px';
        },

        getTopValue: function (index) {

            return '-' + (div(index(), 25)) * 20 + 'px';
        }
    };    

    ko.applyBindings(viewModel);
</script>
function div(val, by){
    return (val - val % by) / by;
}

So am having some performance issues.
For example in Opera and FF images loading very quickly about 1 sec, in IE about 3 sec, but in Chrome it is loading very slow

it is taking about 17 sec to display all pieces in Chrome…

Browser doing just one request to get the image and than cutting small pieces from it, why it may take so long in Chrome?

Is there any way i can improve performance?

just did CTRL+Refresh and here strange loading result:

UPDATE:
I just placed a sample here: http://bit.ly/TrcCdp

UPDATE:
In my sample there is JSON array, it is contains 800 elements, so I just find out if I make it less, eg 600-700 elements the performance is getting better, but I need 800 elements anyway.

e.g When there is only 600 elements it is reducing the load in Chrome to about 6 sec….

So probably may be the problem somewhere at the point where knockout iterating template?

Answer 1

The problem is not the image. The image can be fixed by placing a preload at the top, before any of the stylesheet or script tags:

<meta name="viewport" content="width=device-width">

<script type="text/javascript">
    var img = new Image();
    img.src = 'TestApp_files/obm000.jpg';
</script>

<link href="TestApp_files/jquery00.css" rel="stylesheet">
<link href="TestApp_files/jquery01.css" rel="stylesheet">
<!-- ad nauseum -->

After this, the image loads in 170ms (locally). However, the page still mucks about for another 10-15 seconds afterwards trying to decide what to do.

The root issue is that the javascript is an absolute mess. Image/file/function names are cryptic. Things in the middle of the page depend on code at the end depends on code at the beginning depends on code at the end. Controller/view/model logic is all over the map. Global variables and multi-file coupling… hokay, </soapbox>, now onto treating the symptom.

Problem 1: binding knockout before the DOM loads

Put applyBindings into a domready callback:

jQuery(function($) {
   ko.applyBindings(viewModel);
});

Problem 2: foreach is slow

The knockout foreach binding is incredibly slow with large data sets. You can try jQuery templates and move the foreach inside the template, as described in this SO question. It seems to drop the time down to about 3 seconds.

I don’t really understand why this is necessary as it seems to render fine with your current foreach, it simply hangs forever while knockout does some magic in the background that, as far as I can tell, takes place after foreach completes.

Side note: is it necessary to put flips into an observable array? I assume you intend to use that later as nothing in the current code needs it. If not, take it out and it will help performance (though it won’t solve this issue).

Cheers, I hope this helps.