See https://github.com/Textualize/textual/pull/2442#issuecomment-1529512891
This changes the original PR so that, rather than calling a private watcher
instead of a public, as originally issued, we now call public and private,
if they're both there.
If they are both there private is called first.
This change allows for private watch methods. By convention they start with
an underscore. If a reactive or var has a private watch method, it will be
used in preference to a public watch method.
With this change it becomes easier to have a private reactive/var whose
watcher is also private. For example:
_counter = var(0)
"""This is a private counter, it won't appear in the docs."
...
def _watch__counter(self) -> None:
"""Watch _counter, but don't appear in the docs either."
...
See #2382.
