Suitable EventListener to use?

1

what event listener should i use if i want to listen to a String that changes its value from time to time?
(is it even possible???)

var sampleString:String;
sampleString = "this is a message";

sampleString.addEventListener(????????.????????, runFunction);

function runFunction(??????:??????):void
{
trace("the string is now: " + sampleString);
}