/**
 * @projectDescription Monitor Font Size Changes with jQuery
 *
 * @version 1.0
 * @author Dave Cardwell
 *
 * jQuery-Em - $Revision: 24 $ ($Date: 2007-08-19 11:24:56 +0100 (Sun, 19 Aug 2007) $)
 * http://davecardwell.co.uk/javascript/jquery/plugins/jquery-em/
 *
 * Copyright ©2007 Dave Cardwell <http://davecardwell.co.uk/>
 *
 * Released under the MIT licence:
 * http://www.opensource.org/licenses/mit-license.php
 */

// Upon $(document).ready()…
jQuery(function($) {
  // Configuration…
  var eventName = 'emchange';
  
  
  // Set up default options.
  $.em = $.extend({
    /**
     * The jQuery-Em version string.
     *
     * @example $.em.version;
     * @desc '1.0a'
     *
     * @property
     * @name version
     * @type String
     * @cat Plugins/Em
     */
    version: '1.0',
    
    /**
     * The number of milliseconds to wait when polling for changes to the
     * font size.
     *
     * @example $.em.delay = 400;
     * @desc Defaults to 200.
     *
     * @property
     * @name delay
     * @type Number
     * @cat Plugins/Em
     */
    delay: 200,
    
    /**
     * The element used to detect changes to the font size.
     *
     * @example $.em.element = $('<div />')[0];
     * @desc Default is an empty, absolutely positioned, 100em-wide <div>.
     *
     * @private
     * @property
     * @name element
     * @type Element
     * @cat Plugins/Em
     */
    element: $('<div />').css({ left:   '-100em',
                  position: 'absolute',
                  width:  '100em' })
               .prependTo('body')[0],
    
    /**
     * The action to perform when a change in the font size is detected.
     *
     * @example $.em.action = function() { ... }
     * @desc The default action is to trigger a global “emchange” event.
     * You probably shouldn’t change this behaviour as other plugins may
     * rely on it, but the option is here for completion.
     *
     * @example $(document).bind('emchange', function(e, cur, prev) {...})
     * @desc Any functions triggered on this event are passed the current
     * font size, and last known font size as additional parameters.
     *
     * @private
     * @property
     * @name action
     * @type Function
     * @cat Plugins/Em
     * @see current
     * @see previous
     */
    action: function() {
      var currentWidth = $.em.element.offsetWidth / 100;
      
      // If the font size has changed since we last checked…
      if ( currentWidth != $.em.current ) {
        /**
         * The previous pixel value of the user agent’s font size. See
         * $.em.current for caveats. Will initially be undefined until
         * the “emchange” event is triggered.
         *
         * @example $.em.previous;
         * @result 16
         *
         * @property
         * @name previous
         * @type Number
         * @cat Plugins/Em
         * @see current
         */
        $.em.previous = $.em.current;
        
        /**
         * The current pixel value of the user agent’s font size. As
         * with $.em.previous, this value *may* be subject to minor
         * browser rounding errors that mean you might not want to
         * rely upon it as an absolute value.
         *
         * @example $.em.current;
         * @result 14
         *
         * @property
         * @name current
         * @type Number
         * @cat Plugins/Em
         * @see previous
         */
        $.em.current = currentWidth;
        
        $.event.trigger(eventName, [$.em.current, $.em.previous]);
      }
    }
  }, $.em );
  
  
  /**
   * Bind a function to the emchange event of each matched element.
   *
   * @example $("p").emchange( function() { alert("Hello"); } );
   *
   * @name emchange
   * @type jQuery
   * @param Function fn A function to bind to the emchange event.
   * @cat Plugins/Em
   */

  /**
   * Trigger the emchange event of each matched element.
   *
   * @example $("p").emchange()
   *
   * @name emchange
   * @type jQuery
   * @cat Plugins/Em
   */
  $.fn[eventName] = function(fn) { return fn ? this.bind(eventName, fn)
                        : this.trigger(eventName); };
  
  
  // Store the initial pixel value of the user agent’s font size.
  $.em.current = $.em.element.offsetWidth / 100;
  
  /**
   * While polling for font-size changes, $.em.iid stores the intervalID in
   * case you should want to cancel with clearInterval().
   *
   * @example window.clearInterval( $.em.iid );
   * 
   * @property
   * @name iid
   * @type Number
   * @cat Plugins/Em
   */
  $.em.iid = setInterval( $.em.action, $.em.delay );
});