/*
  Title: ImagesObserver
  
  About: Author
  Joe Gornick
*/

/*
  Class: ImagesObserver
  
  Allows one to observe an elements images loaded state.
  
  Example:
  (start code)
  ...
  <div id="test">
    <img src="http://www.google.com/intl/en_ALL/images/logo.gif" />
  </div>
  <img src="http://www.google.com/intl/en_ALL/images/logo.gif" />
  ...
  <script type="text/javascript">
    document.observe('images:loaded', function(e) {
      alert('document.body images ready');
    });  
    
    new ImagesObserver('test', { 
      onImagesLoaded: function() { alert('test images ready'); }
    });
    
    new ImagesObserver();
  </script> 
  ...
  (end)
*/

/*
  Group: Options
  
    Property: onImagesLoaded
    An event fired when all the images in the specified element are loaded.  If 
    this isn't specified, then we will fire the 'images:loaded' event on the document.
*/
ImagesObserver = Class.create({
  /*
    Group: Constructor
    
    Constructor: initialize
    
    Constructor. Should not be called directly.
    
    Parameters:
      el - (String|HTMLElement) Optional - Element to observe the images loaded state.
      If el isn't specified, then default to document.body.
      options - (Object) Optional - Options used to setup the ImagesObserver.
    
    Returns:
      ImagesObserver
  */
  initialize: function(el, options) {
    this.options = {
      onImagesLoaded: Prototype.emptyFunction
    };
    Object.extend(this.options, options || { });
    
    this.el = $(document.body);
    if (typeof el != 'undefined') this.el = $(el);
      
    this.pe = new PeriodicalExecuter(this._checkImagesState.bind(this), (1/10));
  },
  
  _checkImagesState: function() {
    if (this._areImagesLoaded(this.el)) {
      this.pe.stop();
      
      if (this.options.onImagesLoaded != Prototype.emptyFunction)
        this.options.onImagesLoaded(this.el);
      else
        document.fire('images:loaded', { el: this.el });
    }    
  },
  
  /**
   * _areImagesLoaded()
   *
   * Checks wether all the images in the document have loaded.
   * 
   * Please note that the naturalWidth property will return before the complete
   * property is set to true.  So, the image will not be completely downloaded,
   * but will be the correct dimensions in the document.
   *
   * @author John-David Dalton <john.david.dalton[at]gmail[dot]com>
   * @author Joe Gornick <joe[at]joegornick[dot]com>
   * @return boolean
   * @link http://www.bigbold.com/snippets/posts/show/89
   * @link http://talideon.com/weblog/2005/02/detecting-broken-images-js.cfm
   */  
  _areImagesLoaded: function(el) {
    return el.select('img').all(function(img) {
      return (img.readyState == 'complete' || img.complete || 
        !(typeof img.naturalWidth != 'undefined' && img.naturalWidth == 0));
    });
  }  
});