Variable monitoring from outside class

I am attempting to make a simple class that I use to debug some of my code at runtime. I use a function in my debug class to pass a variable from another class that I want to monitor in realtime.
For simplicity sake I am only passing Stings at the moment. I can pass the variable that I want successfully but I am stumpted as to how I to get my DebugClass to monitor that variable in realtime.
Since the variable is being altered in another class, what would I needto do to have the debugClass monitor that variable without having to call a new function everytime the varible changes, which could be 300+ times a minute.

This is the current method I use to add the variable to my displayList:

protected function addDebugData(stringToAdd:String):void{
var debugText1:TextField = new TextField();
debugText1.border = true;
debugText1.width = 300;
debugText1.text = stringToAdd;
debugText1.y = 20;

The problem with this method is that it is a one time display method. The current value passed is displayed, but that is it. My Class has no knowledge of any changes made to the parent variable. I have a method to update my panel that displays the variable(s) I have Passed, but those variable are still whatever I initialy passed via my
method. I am likely missing something simple in scoping, constructing or in retuning
something. Fairly new to this all. So any thoughts, tips, alternatives and/or
solutions very welcome!