Preloader for an external swf

1

i`m loading an external swf file as shown below.



var mySWF:MovieClip;
var url = new URLRequest("external.swf");
var loader:Loader = new Loader();

loader.load(url);
loader.contentLoaderInfo.addEventListener(Event.INIT, loaded);


function loaded(e):void{
     mySWF = e.target.content;
	  addChild(mySWF);
 }

now, how can I add preloader for this?
help me please.