[matplotlib-devel] adding valid signals to a CallbackRegistry?

Hello all,

I am working to make mplot3d feature-parity with regular axes objects.  I
have come across a possible design flaw with the CallbackRegistry.

Many of the Axes3D methods are merely wrappers around Axes methods letting
it do the work for x and y axis and then let Axes3D do the same for the z
axis.  In Axes.cla(), self.callbacks gets a CallbackRegistry of signals
named "xlim_changed" and "ylim_changed".  However, once that is set, it does
not appear to be a way for me to add another signal of "zlim_changed" in
Axes3D.cla().  I could replace self.callbacks with a new CallbackRegistry,
but since there is a lot of interveaning code between that first
initialization of self.callbacks and coming back out of Axes.cla(), I worry
that some callbacks may have been registered by then and would get
eliminated in the process.

Is there a recommended way around this?  Or maybe this is more evidence that
we should move towards the use of a dictionary of axis objects and make Axes
functions more agnostic to the number of axis?

Thanks,
Ben Root