Please explain this code

Question

Trying to make sense of the code below, as I want to customise it a bit. As per the comments, the callback function is passed the last image to load as an argument. Can someone please explain how the callback is passed only to the last image (I understand why, I just don't know how)? From the way I'm reading the code it seems to be passed to all of them.

Cheers.

// $('img.photo',this).imagesLoaded(myFunction)
// execute a callback when all images have loaded.
// needed because .load() doesn't work on cached images

// mit license. paul irish. 2010.
// webkit fix from Oren Solomianik. thx!

// callback function is passed the last image to load
//   as an argument, and the collection as 'this'


$.fn.imagesLoaded = function(callback){
  var elems = this.filter('img'),
      len   = elems.length,
      blank = 'data:image/gif;base64,' +
          'R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==';

  elems.bind('load',function(){
      if (--len <= 0 && this.src !== blank){ callback.call(elems,this); }
  }).each(function(){
     // cached images don't fire load sometimes, so we reset src.
     if (this.complete || this.complete === undefined){
        var src = this.src;
        // webkit hack from
        // groups.google.com/group/jquery-dev/browse_thread/thread/eee6ab7b2da50e1f
        // data uri bypasses webkit log warning (thx doug jones)
        this.src = blank;
        this.src = src;
     }  
  }); 

  return this;
};

Answer 1

The callback function is only called if len (decremented) is less than or equal to zero. Since it starts off with the value of the length of the elements (or how many there are), and is decremented when each image is loaded, only the last image will make the expression true.

You can sort of read this part of the code:

   elems.bind('load',function(){
      if (--len <= 0 && this.src !== blank){ callback.call(elems,this); }
  })

as "bind this function to the load method for each element in elems." So, each element calls the function when it is loaded, so the function () {...} is called. The first image decrements len and then checks if it is less than or equal to 0, then the next element does the same when its done loading, and so on until the last image decrements len and then checks if it is less than 0. Since it's the last image, it returns true and if the src is not blank, it runs callback.call().