Question

Basically I have created an online leaderboared here that returns the top 100 user scores

and if users hit refresh and the ranks have changed it will show new positions. What I would like to do is have the ranks updated without doing a page refresh, using AJAX, jQuery and JSON.

The leaderboard is populated via an associative sql query and stored in an sql variable, I have managed to get the output in JSON format seen here using

    //Shows user details and the current top 100 ranks
while ($row = mysql_fetch_assoc($result)) {

    print json_encode ($row);
  }

So far so good, using this animated sorting example from [Tinysort][3]

function sort() {
    var $Ul = $('ul#leaderboard');
    $Ul.css({position:'relative',height:$Ul.height(),display:'block'});
    var iLnH;
    var $Li = $('ul#leaderboard>li');
    $Li.each(function(i,el){
        var iY = $(el).position().top;
        $.data(el,'h',iY);
        if (i===1) iLnH = iY;
    });
    $Li.tsort('h1:eq(0)',{order:'desc'}).each(function(i,el){
        var $El = $(el);
        var iFr = $.data(el,'h');
        var iTo = i*iLnH;
        $El.css({position:'absolute',top:iFr}).animate({top:iTo},500);
    });
}

I believe I am most of the way there but I am getting stuck on the poll() function below

poll();

function poll() {

    $.ajax({
        url: 'http://test.bmx-tube.com/ajax.php', // needs to return a JSON array of items having the following properties: id, score, facebook_id, username
        dataType: 'json',
        success: function(o) {
            for(i=0;i<o.length;i++) {
                if ($('#row-'+o[i].player_id).length == 0) {
                    // this id doesn't exist, so add it to our list.
                    $("#leaderboard").append('<li><h1 style="display:inline" player_id="row-' + o[i].player_id + '">' + o[i].score + '</h1> <img style="height:50px" src="http://graph.facebook.com/' + o[i].facebook_id + '/picture"/> ' + o[i].name + '</li>');
                } else {
                    // this id does exist, so update 'score' count in the h1 tag in the list item.
                    $('#row-'+o[i].player_id).html(o[i].score);
                }
            }
            sort();
        },
    }); 

    // play it again, sam
    var t = setTimeout(poll,3000);
}

I have tried changing all the variables around but have had no luck so far.

Any help greatly appreciated

Answer 1

Well firstly your JSON string is invalid. Your server isn’t actually returning an array of objects.

Are you using echo json_encode($yourArray); to create the JSON server side?

Here is an example of a proper array of objects as a JSON string:

{"array":[
     {"object":"1","description":"this is an object"},
     {"object":"2","description":"this is an object"},
     {"object":"3","description":"this is an object"}
]}

NB: You can validate JSON strings with JSONLint: http://jsonlint.com/

Also you can use firebug or chrome’s inspector tools to see if the object is valid or not, and to view the JSON string being returned by your server in a more intuitive way.

As for accessing the objects once you have a valid object, you can use jQuery .each() to do this easily and elegantly.

For example:

$.ajax({
        url: 'http://test.bmx-tube.com/ajax.php',
        type: "get", cache:false,
        dataType: 'json',
        success: function(ranks) 
        {
            $(ranks).each(function(index, rank)
            {
                if($('#row-'+rank.player_id).length == 0) 
                {
                    // this id doesn't exist, so add it to our list.
                    $("#leaderboard").append('<li><h1 style="display:inline" id="row-' + rank.player_id + '">'+rank.score+'</h1> <img style="height:50px" src="http://graph.facebook.com/'+rank.facebook_id + '/picture"/> ' + rank.name + '</li>');
                } 
                else 
                {
                    // this id does exist, so update 'score' count in the h1 tag in the list item.
                    $('#row-'+rank.player_id).html(rank.score);
                }
            });
            //now sort
            sort();
        },
        error: function()
        {
            alert("Ajax error: could not access URL");
        }
    });

Some other things to note

Its a good idea to disable caching on data that changes regularly when using GET with AJAX.
That is what the type: "get", cache:false, is doing above.

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Let me know if you need any further help…this should point you in the right direction though.

Cheers,
Oisin

— Edit 1 —

I realise now what you intended to do with player_id="row-'+rank.player_id+'".
That should actually be id="row-'+rank.player_id+'"

I’ve modified the code above to reflect this.

jsFIddle

Here is a JSFiddle demonstrating how you might go about with what you’re trying to do.
I’ve simplified things by cutting out your sort function for now and just used a very simple sort instead.
I’ve also broken up the generated HTML into section so you can style them individually and modify the data in a more structured way.
http://jsfiddle.net/DigitalBiscuits/fKcKF/6/

NB: due to the way jsFiddle works I have had to generate a mock dataset and modify the AJAX function to load it. On your website you would of course be loading the JSON from your own server.

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if this helps you please upvote. If it solves your problem please mark this answer as correct.

Good luck!