What is the HG2 equivelant to 'ObjectChildAdded' event in axis?

Hi,

I wrote a rather large GUI in R2013b relying heavily on callbacks to keep different axes up to date and to ensure recalculation of data. Curiously, many of these do not work in HG2. Whilst I've managed to understand this is due to a fundamental change in the underlying mechanism, I can't seem to be able to find an equivalent for the 'ChildObjectAdded' event in HG1, eg

addlistener(o.fig.input.hh.ax(1,1), 'ObjectChildAdded', @o.updateSpectrum);

works fine in HG1, but does not trigger in HG2. What it is trying to achieve is to trigger a calculation if a rectangle is added to an axes. Does anyone know how to achieve this in HG2?

Cheers, kris