Question

What would be computationally-efficient ways to select elements touching the top edge of browser window viewport as the page is scrolled?

See attached image. Green elements are selected because they are touching the top edge.

UPDATE

An example of how I’ll use this is to fade elements that are going off-screen. There may be hundreds of them on the page. Imagine a page like Pinterest. Computing offset and scrollTop for hundreds of them at the rate of scroll event, even if throttled still feels really inefficient.

Answer 1

This is what I came up with. I think that it could be improved upon by caching the scrollTop values, but this is pretty good. I have included the framework for caching the boxtops, but not the implementation code. I have also only implemented scrolling down to hide divs. I have left reshowing them on upscroll as an exercise for you.

When the window is scrolled we get the last hidden div. We know that everything before this div is already hidden. Then use a ‘while next element is off the screen’ hide it. As soon as a div isn’t off the screen we abort. Thus saving time from iterating through the entire list.

http://jsfiddle.net/kkv3h/2/

//track whether user has scrolled up or down
var prevScrollTop = $(document).scrollTop();

$(document).scroll(function() {
    var currentScrollTop = $(this).scrollTop();
    if (currentScrollTop > prevScrollTop) {
        //down
        var lasthiddenbox = $('.fadeboxhidden:last');
        var nextbox = (lasthiddenbox.length > 0) ? lasthiddenbox.next('.fadebox') : $('.fadebox:first');
        while (nextbox.length) {
            console.log('box: ' + nextbox.offset().top + ' scroll: ' + currentScrollTop);
            if (nextbox.offset().top < currentScrollTop) {
                nextbox.animate({ opacity: 0 }, 3000).addClass('fadeboxhidden');
            }
            else { return; } 
            nextbox = nextbox.next('.fadebox:first');
        }        
    } else {
        //up          
    }
    prevScrollTop = currentScrollTop ;
});

//create an object to hold a list of box top locations
var boxtops = new Object;


//gather all boxes and store their top location
$('.fadebox').each( function(index) {    
    //you may want to dynamically generate div ids here based on index. I didn't do this
    //because i was already using the id for positioning.
    var divid = $(this).prop('id');
    boxtops[divid] = $(this).offset().top;
    //console.log(boxtops[divid]);    
});