Question

So I have been recently developing a site, The problem is the backgrounds for each page are images, and as a result, on slower connections (which is the case of some of the target audience) the images load progressivly as they are downloaded, to resolve this I am trying to make a preloading page that does the following :

• Loads the Images

• Once the loading is done, redirects the user to the requested page

  <script type="text/javascript">
  <!--//--><![CDATA[//><!--
      var images = new Array()
      var count=0;
      function preload() {
          for (i = 0; i < preload.arguments.length; i++) {
              images[i] = new Image()
              images[i].src = preload.arguments[i]
          }
      if(count==4) {
      window.location = "index.html";
      }
      }
      preload(
          "backgrounds/bg1.jpg",
          "backgrounds/bg2.jpg",
          "backgrounds/bg3.jpg",
          "backgrounds/bg4.jpg"
      )
  //--><!]]>
  

The problem is it redirects directly (I assume that it just starts the download of the image then directly adds one to the counter variable, quickly reaching 4 and not giving the image the time to download.

Any ideas how I can either make it signal me when the images have finished downloading, or only execute the redirect after it has done downloading the images ?

Answer 1

You need to wait for the load event. It’s quite simple:

function preload(images, timeout, cb) {
  var cbFired = false,
      remaining = images.length,
      timer = null;

  function imageDone() {
    remaining--;

    if(remaining === 0 && !cbFired) {
      cbFired = true;
      clearTimeout(timer);
      cb();
    }
  }

  function timerExpired() {
    if(cbFired)
      return;

    cbFired = true;
    cb();
  }

  for(var i = 0; i < images.length; i++) {
    var img = new Image();
    img.onload = imageDone;
    img.onerror = imageDone;
    img.src = images[i];
  }

  timer = setTimeout(timerExpired, timeout);
}

You need to check a few things so that users don’t get stuck:

• You need to wait for both load and error so that the page doesn’t get stuck if an image fails to load.
• You should set a maximum timeout.
• Also, in your code, i was a global variable (no var declaration).

Here’s how to use it:

var images = [ "backgrounds/bg1.jpg",
    "backgrounds/bg2.jpg",
    "backgrounds/bg3.jpg",
    "backgrounds/bg4.jpg"];

preload(images, 10000 /* 10s */, function () {
  window.location = 'next_page';
});