javascript - Synchronized scrolling using jQuery?

Question

I am trying to implement synchronized scrolling for two DIV with the following code.

DEMO

$(document).ready(function() {
    $("#div1").scroll(function () { 
        $("#div2").scrollTop($("#div1").scrollTop());
    });
    $("#div2").scroll(function () { 
        $("#div1").scrollTop($("#div2").scrollTop());
    });
});

#div1 and #div2 is having the very same content but different sizes, say

#div1 {
 height : 800px;
 width: 600px;
}
#div1 {
 height : 400px;
 width: 200px;
}

With this code, I am facing two issues.

1) Scrolling is not well synchronized, since the divs are of different sizes. I know, this is because, I am directly setting the scrollTop value. I need to find the percentage of scrolled content and calculate corresponding scrollTop value for the other div. I am not sure, how to find the actual height and current scroll position.

2) This issue is only found in firefox. In firefox, scrolling is not smooth as in other browsers. I think this because the above code is creating a infinite loop of scroll events. I am not sure, why this is only happening with firefox. Is there any way to find the source of scroll event, so that I can resolve this issue.

Any help would be greatly appreciated.

Answer 1

You can use element.scrollTop / (element.scrollHeight - element.offsetHeight) to get the percentage (it'll be a value between 0 and 1). So you can multiply the other element's (.scrollHeight - .offsetHeight) by this value for proportional scrolling.

To avoid triggering the listeners in a loop you could temporarily unbind the listener, set the scrollTop and rebind again.

var $divs = $('#div1, #div2');
var sync = function(e){
    var $other = $divs.not(this).off('scroll'), other = $other.get(0);
    var percentage = this.scrollTop / (this.scrollHeight - this.offsetHeight);
    other.scrollTop = percentage * (other.scrollHeight - other.offsetHeight);
    // Firefox workaround. Rebinding without delay isn't enough.
    setTimeout( function(){ $other.on('scroll', sync ); },10);
}
$divs.on( 'scroll', sync);

http://jsfiddle.net/b75KZ/5/