thread.

.. note::

   This method can safely be called from the observer thread. In this
   case it is equivalent to :meth:`send_stop()`.

Send a stop signal to the backgroud (see :meth:`send_stop`), and waits
for the background thread to exit (see :meth:`~threading.Thread.join`)
if the current thread is *not* the observer thread.

After this method returns in a thread *that is not the observer
thread*, the ``callback`` is guaranteed to not be invoked again
anymore.

.. note::

   The underlying :attr:`monitor` is *not* stopped.

.. versionchanged:: 0.16
   This method can be called from the observer thread.
N)