It can be used with apache upload progress module, nginx upload progress module or lighttpd, read more here: http://drogomir.com/blog/2008/6/18/upload-progress-bar-with-mod_passenger-and-apache

Usage

some html:

  <div id="uploading">
    <div id="progress" class="bar">
      <div id="progressbar">&nbsp;</div>
      <div id="percents"></div>
    </div>
  </div>

then some css:

.bar {
  width: 300px;
}

#progress {
  background: #eee;
  border: 1px solid #222;
  margin-top: 20px;
}
#progressbar {
  width: 0px;
  height: 24px;
  background: #333;
}

and a bit of javascript:

$(function() {
  $('form').uploadProgress({
    /* scripts locations for safari */
    jqueryPath: "../lib/jquery.js",
    uploadProgressPath: "../jquery.uploadProgress.js",
    /* function called each time bar is updated */
    uploading: function(upload) {$('#percents').html(upload.percents+'%');},
    /* selector or element that will be updated */
    progressBar: "#progressbar",
    /* progress reports url */
    progressUrl: "/progress",
    /* how often will bar be updated */
    interval: 2000
  });
});

If you need to update the progress bar from a different domain or subdomain, liek if your upload server is different from your normal web server, you can try the JSON-P protocol, like this:

$(function() {
  $('form').uploadProgress({
    /* scripts locations for safari */
    jqueryPath: "../lib/jquery.js",
    uploadProgressPath: "../jquery.uploadProgress.js",
    /* function called each time bar is updated */
    uploading: function(upload) {$('#percents').html(upload.percents+'%');},
    /* selector or element that will be updated */
    progressBar: "#progressbar",
    /* progress reports url in a different domain or subdomain from caller */
    progressUrl: "uploads.somewhere.com/progress",
    /* how often will bar be updated */
    interval: 2000,
    /* use json-p for cross-domain call */
    dataType: 'jsonp'
  });
});

defaults:

interval: 2000
progressBar: "#progressbar"
progressUrl: "/progress"
start: function() {}
uploading: function() {}
complete: function() {}
success: function() {}
error: function() {}
uploadProgressPath: '/javascripts/jquery.js'
jqueryPath: '/javascripts/jquery.uploadProgress.js'
dataType: 'json'

License

Ruby on Rails is released under the MIT License.