AS3 - Access class functions from inside an instance

programming flash
1

I am attempting to create a fluid layout which will house a rather wide animation. I want to be able to set variables from inside the timeline of the instance of the class.

Class for creating a fluid object on the stage

package FluidLayout{
	
	/* Add import classes here */
	import flash.events.Event;
	import flash.display.*;
	
	public class FluidObject{  

        /* Declare instance variables here */ 
		protected var _param:Object;
		protected var _target:DisplayObject;
		protected var _stage:Stage;
		

        public function set param(value:Object):void
		{
			_param = value;
			this.reposition();
		}

		/* Constructor of the class */  
        public function FluidObject(target:DisplayObject,paramObj:Object)  
        {
			_target = target;
			_param = paramObj;
			_stage = target.stage;
			
			_stage.addEventListener(Event.RESIZE, onStageResize);
			this.reposition();
        }  

        /* Function that repositions the monitored object */  
        protected function reposition():void  
        {  
			var stageW = _stage.stageWidth;
			var stageH = _stage.stageHeight;
			
			trace(_param.offsetX); 

			_target.x = (stageW * _param.x) + _param.offsetX;
			_target.y = (stageH * _param.y) + _param.offsetY;
        }  

        /* Function that is called when the RESIZE event is fired */  
        protected function onStageResize(e):void  
        { 
			this.reposition();
        }
    }  
}

This is the structure.as that is linked to structure.fla

package {  
  	  
    import flash.display.*;
    import FluidLayout.*;

    public class Structure extends MovieClip{  

        public function Structure()  
        {  
            /* Set the Scale Mode of the Stage */  
            stage.scaleMode = StageScaleMode.NO_SCALE;  
            stage.align = StageAlign.TOP_LEFT;  
			 
			var viewer = new Viewer();
			addChild(viewer);

			var viewerParam = {  
				x:.5,
				y:.5,
				offsetX: -viewer.width/2,
				offsetY: -viewer.height/2
			}  
			new FluidObject(viewer,viewerParam);           
        }  
    }  
}

Inside the structure.fla there is movieclip called viewer that gets placed with structure.as.
Viewer contains animations. Inside the viewer movieclip on frame one is the following actionscript

this.param = {offsetX: -870};

What I am trying to accomplish is to create a fluid layout that runs an animation sequence.
The animation sequence is quite wide so it’s X position shifts during the course of it’s run.
I need to adjust that offsetX to match the animations x position otherwise when the stage
gets resized it snaps the animation x position back to 0.

The problem is I get no errors but the offsetX is definitely not changing. Am I going about this the wrong way? Where am I going wrong here?