/**
 * @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 );
});