AS3 Preloader (without Flex SDK or Flash CS)

Question

I use FlashBuilder but not the Flex SDK, just pure Actionscript. I have a swf application that is too long to download, so I would like to display a progressbar while the swf is being downloaded from the host.
My main class is a Sprite. I tried to add a progressEvent.PROGRESS listener on root.loaderInfo in the constructor of my main Sprite, but then a PROGRESS event is fired only when all the swf is downloaded. Useless.
On a Flex project, you have a Preloader included in the main Application, that displays a progressBar while the application swf is being downloaded from the server and initializing.
That's what I want.
Does someone understand how the Flex Preloader works ?
Do you know how to do that without Flex SDK ?

Answer 1

Here is the Preloader class :
public class Preloader extends MovieClip
  public function Preloader()
    super();
    stop();
    if (root && root.loaderInfo)
    root.loaderInfo.addEventListener(Event.INIT, initHandler, false, 0, true);
  private function initHandler(e:Event):void
    root.loaderInfo.removeEventListener(Event.INIT, initHandler);
    displayProgressBar();
    root.loaderInfo.addEventListener(ProgressEvent.PROGRESS, progressHandler, false, 0, true);
    root.loaderInfo.addEventListener(Event.COMPLETE, completeHandler, false, 0, true);
  private function progressHandler(e:ProgressEvent):void
    displayProgressBar(e.bytesLoaded, e.bytesTotal);
  private function completeHandler(e:Event):void
    root.loaderInfo.removeEventListener(ProgressEvent.PROGRESS, progressHandler);
    root.loaderInfo.removeEventListener(Event.COMPLETE, completeHandler);
    removeProgressBar();
    nextFrame();
    var mainClass:Class = Class(getDefinitionByName("MainApplication"));
    if(mainClass)
      var app:Sprite = new mainClass();
      stage.addChild(app);
    stage.removeChild(this);
  private function displayProgressBar(bytesLoaded:int = -1, bytesTotal:int = -1):void
   // add a progressBar and display the progress
  private function removeProgressBar(bytesLoaded:int = -1, bytesTotal:int = -1):void
   // remove the progressBar