Cannot understand the organisation of children at all

Question

Can someone please explain some things to me?  I have two classes.  One is the document class:
package {
    import flash.display.*;
    import flash.events.*;
    import fl.motion.*;
    import fl.transitions.*;
    import fl.transitions.easing.*;
    public class SlideShow extends Sprite {
        private var viewer:ImageLoader;
        private var images:Array=new Array("images/seats.jpg","images/office.jpg","images/wall.jpg","images/light.jpg","images /elephant.jpg");
        private var count:int=0;
        public function SlideShow() {
            viewer=new ImageLoader();
            viewer.x=100;
            viewer.y=100;
            viewer.setUpTextField(this);
            addChild(viewer);
            loadImage();
            next_btn.addEventListener(MouseEvent.CLICK,nextPicture)
            prev_btn.addEventListener(MouseEvent.CLICK, prevPicture)
        private function loadImage():void{
            if(count>images.length-1 || count<0){
            count=0;
            viewer.loadImage(images[count]);
        private function nextPicture(e:Event):void{
            viewer.removeImage()
            count++;
            loadImage()
            trace(viewer)
        private function prevPicture(e:Event):void{
            viewer.removeImage()
            count--;
            loadImage()
The next is my ImageLoader class:
package {
    import flash.display.*;
    import flash.net.URLRequest;
    import flash.events.*;
    import flash.text.*;
    import fl.motion.*;
    import fl.transitions.*;
    import fl.transitions.easing.*;
    public class ImageLoader extends Sprite {
    private var loader:Loader;
    public var progress_txt:TextField;
    var img:Bitmap;
    var parent_mc:SlideShow;
    public function ImageLoader() {
        loader=new Loader();
        loader.contentLoaderInfo.addEventListener(Event.INIT, showImage);
        loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, showProgress);
        public function loadImage(url:String) {
            var URLObject:URLRequest=new URLRequest(url);
            loader.load(URLObject);
        private function showImage(e:Event) {
            progress_txt.visible=false;
            img=Bitmap(loader.content);
            img.alpha=0;
            loader.unload();
            addChild(img);
            var myTween:Tween = new Tween(img, "alpha", Strong.easeOut, 0, 1,100)
            trace(next_btn.x)
        public function removeImage():void{
            removeChild(img);
        public function setUpTextField(obj:Object):void{
            parent_mc=SlideShow(this.parent);
            progress_txt=new TextField();
            progress_txt.text="Loading";
            progress_txt.x=0;
            progress_txt.y=0;
            var tf:TextFormat=new TextFormat();
            tf.font="Arial";
            tf.size="18";
            tf.align="center";
            progress_txt.defaultTextFormat=tf;
            obj.addChild(progress_txt);
            //couldn't work out why I couldn't say parent_mc.addChild(progress_txt);
        private function showProgress(e:ProgressEvent){
            progress_txt.text=String(Math.round(e.bytesLoaded/e.bytesTotal*100))+"%"
Problem is with accessing the buttons.
I manually placed two buttons on the stage and called them next_btn and prev_button.  How on earth do I access these buttons from inside the ImageLoader class?  I have no clue as to the hierarchy.  When I say inside imageLoader.as trace(this.parent) I get NULL, which confuses me, because something has added it to the stage, I thought it was the document class SlideShow.
I tried trace(this.next_btn), (next_btn), (this.parent.next_btn), (stage.next_btn)....where on earth is the next_btn???

Answer 1

pass a reference to the stage from your document class to you imageloader class.