Tween + onMotionFinished

I’ve got the following code in my actionscript class. When the onMotionFinished is run, it is not referencing my class to execute the slide() method. How would I properly do this?

var tween = new mx.transitions.Tween(target, "_y", easeType, beginPos, endPos, slideSpeed);
tween.onMotionFinished = function() {

Of course I can always go through the instance of my class like the following example, but that is not good practice.