How do I abort image <img> load requests without using window.stop()

I have a very long page that dynamically loads images as users scroll through.

However, if a user quickly scrolls away from a certain part of the page, I don't want the images to continue loading in that now out-of-view part of the page.

There are lots of other requests happening on the page simultaneously apart from image loading, so a blunt window.stop() firing on the scroll event is not acceptable.

I have tried removing & clearing the img src attributes for images that are no longer in view, however, since the request was already started, the image continues to load.

Remember that the image src was filled in as the user briefly scrolled past that part of the page. Once past though, I couldn't get that image from stop loading without using window.stop(). Clearing src didn't work. (Chrome & FF)

Similar posts I found that get close, but don't seem to solve this problem:

Answers:

Answer

What you are trying to do is the wrong approach, as mentioned by nrabinowitz. You can't just "cancel" the loading process of an image (setting the src attribute to an empty string is not a good idea). In fact, even if you could, doing so would only make things worst, as your server would continually send data that would get cancelled, increasing it's load factor and slow it down. Also, consider this:

  1. if your user scroll frenetically up and down the page, he/she will expect some loading delays.
  2. having a timeout delay (ex: 200 ms) before starting to load a portion of the page is pretty acceptable, and how many times will one stop and jump after 200 ms interval on your page? Even it it happens, it comes back to point 1
  3. how big are your images? Even a slow server can serve about a few tens of 3Kb thunbnails per second. If your site has bigger images, consider using low and hi resolution images with some components like lightBox

Often, computer problems are simply design problems.

** EDIT **

Here's an idea :

  1. your page should display DIV containers with the width and height of the expected image size (use CSS to style). Inside of each DIV, add an link. For example :

    <div class="img-wrapper thumbnail">
       <a href="http://www.domain.com/path/to/image">Loading...</a>
    </div>
    
  2. Add this Javascript (untested, the idea is self describing)

    $(function() {
    
    var imgStack;
    var loadTimeout;
    
    $(window).scroll(function() {
       imgStack = null;
       if (loadTimeout) clearTimeout(loadTimeout);
    
       loadTimeout = setTimeout(function() {
    
          // get all links visible in the view port
          // should be an array or jQuery object
          imgStack = ...
    
          loadNextImage();
       }, 200);                // 200 ms delay
    });
    
    function loadNextImage() {
       if (imgStack && imgStack.length) {
          var nextLink = $(imgStack.pop());   // get next image element
    
          $('<img />').attr('src', nextLink.attr('href'))
            .appendTo(nextLink.parent())
            .load(function() {
               loadNextImage();
            });
    
          // remove link from container (so we don't precess it twice)
          nextLink.remove();
       }
    };
    
    });
    
Answer

Well, my idea:

1) initiate an AJAX request for the image, if it succeeds, the image goes to the browser cache, and once you set the 'src' attribute, the image is shown from the cache

2) you can abort the XHR

I wrote a tiny server with express emulating the huge image download (it actually just waits 20 seconds, then returns an image). Then I have this in my HTML:

<!DOCTYPE html>
<html>
    <head>
        <style>
            img {
                width: 469px;
                height: 428px;
                background-color: #CCC;
                border: 1px solid #999;
            }
        </style>
    </head>

    <body>
        <img data-src="./img" src="" />
        <br />
        <a id="cancel" href="javascript:void(0)">CANCEL</a>
        <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
        <script>
            $(function () {
                var xhr, img = $('img'), src = img.data('src');

                xhr = $.ajax(src, {
                    success: function (data) { img.attr('src', src) }
                });

                $('#cancel').click(function (){
                    xhr.abort();
                })
            });
        </script>
    </body>
</html>
Answer

You can load your images using ajax calls, and in case that the uses scrolls-out, you can abort the calls.
In jQuery pseudo-code it would be something like that (forgive me mistakes in syntax, it is just an example):

1) tag images that you want to load

$(".image").each( function(){
    if ( is_in_visible_area(this) ) { // check [1] for example
        $(this).addClass("load_me");
    } else {
        $(this).addClass("dont_load");
    }
});

2) load images

ajax_requests = {};

$(".image.load_me").each( function(){
    // load image
    var a = $.ajax({
        url: 'give_me_photos.php',
        data: {img: photo_id},
        success: function(html){
            photo_by_id(photo_id), img.append(html);
        }
    });
    ajax_requests[photo_id] = a;

});

3) cancel loading those out of the screen

for( id in ajax_requests ) {
    if ( ! is_in_visible_area(id) ) {
        ajax_requests[id].abort();
    }
}

Of course, add also some checking if the image is already loaded (e.g. class "loaded")

[1]. Check if element is visible after scrolling

[2]. Abort Ajax requests using jQuery

Answer

BTW, another idea that might work:

1) create a new iframe

2) inside of the iframe have the script that starts loading the image, and once it's loaded, call the .parent's method

3) when in need, stop the iframe content loading using .stop on the iframe object

Answer
  1. Use a stack to manage ajax requests (means you will have serial loading instead of parallel but it is worth it)

  2. On scroll stop, wait for 300ms and then push all images inside view-area into stack

  3. Every time a user scrolls check if a stack is running. (fyi - you can stop all requests to a particular url instead of killing all ajax calls. also you can use regex so it should not stop any other requests on the page)

  4. If an existing stack is running - pop all the images that are in it except for the top most one.

  5. On all ajax calls - bind beforeSend() event to remove that particular image from the stack

It is late right now, but we have done something very similar at work - if you need the detailed code let me know.

Cheers!

Answer

Maybe you could serve the image through a php script which would check a field in the the db (or better yet a memcached) that would indicate stop loading. the script would portion up the image into chunks and pause in between each chunk and check if the stop flag for the particular request is. If it is set you send the header with A 204 no content which as soon as the browser gets it will stop receiving.

This may be a bit over kill though.

Answer

The solution could be a webworker. a webworker can be terminated and with him the connection. But there is a small problem that the webworker uses the limited connections of the browser so the application will be blocked.

Right now I'm working on a solution with serviceWorkers - they don't have a connection limit (I hope so)

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