The more migrating I do to AS3 the more little nuances (at least nuances when I am migrating) I find. A lot of small little methods have been switched or moved into new classes and it's always a bit of a drag to find them. Such is the case with the setMask() command.
Although setMask() was a very simple command in AS2, it is even easier in AS3. The following is really a basic overview and there really isn't much more to it.
Let's look at the AS2 code for setting a mask dynamically with setMask():
As simple as that is, it is even simpler in AS3:
box_mc.mask = circle_mc;
This not only looks cleaner but makes a lot more sense. After fumbling around with trying to find where setMask() went, this was a great solution.