onData Help!

What am I missing here? The code below should run the trace command once I have received data into my movieClip “screen”. The movie clip is loaded with data but the onData function never runs. I also put the onData function in a loop but that did not work either.

photo = "http://www.domain.com/data_base/gallery/gallery0/"+photoNum+".jpg";

screen.onData = function() {

I would like to be able to run a command after my movie clip, “screen” has received data.