(function($){    
    $.fn.silder = function(settings){    
        settings = jQuery.extend({
        	speed : "normal",
			slideBy : 2 //一次滚动数量
    	}, settings);
		return this.each(function() {
		$.fn.silder.run( $( this ), settings );
    });
    }; 
	   
    $.fn.silder.run=function($this, settings){
		var ul = $( "ul:eq(0)", $this );
		var li = ul.children();
		if(li.length > settings.slideBy){
			var $next = $("#slider_next");
			var $back = $("#slider_back");
			var liWidth =  $(li[0]).width();
			var animating = false;
			ul.css( "width", liWidth * li.length);
			var Next = $next.click(function() {
				if(!animating){
					animating=true;
					offsetLeft = parseInt(ul.css("left"))-(liWidth* settings.slideBy);
					if ( offsetLeft + ul.width() > 0 )
					{
						$back.css( "display", "block" );
						ul.animate({left:offsetLeft},settings.speed,function()
						{
							if ( parseInt( ul.css( "left" ) ) + ul.width() <= liWidth * settings.slideBy ) 
							{
								$next.css( "display", "none" );
							}
							animating = false;
						});
					}
				}
				return false;
			});
			var Back = $back.click(function() {
				if(!animating){
					animating=true;
					offsetRight = parseInt(ul.css("left"))+(liWidth * settings.slideBy);
					if ( offsetLeft + ul.width() <= ul.width() ){
						$next.css( "display", "block" );
						ul.animate({left:offsetRight},settings.speed,function()
						{
							if ( parseInt( ul.css( "left" ) ) ==0) 
							{
								$back.css( "display", "none" );		
							}
							animating = false;
						});
					}					
				}
				return false;
			});	
		};
	}; 
})(jQuery);