Register a Lua function to be called when a server state changed during the health check process.
The function should not block for a long period of time, as it would otherwise delay the execution of the other functions registered for this hook, as well as the execution of the health check process.
- :param function callback: The function to be called. It returns no value and takes two parameters: the first parameter is a string identifying the server, formatted as if returned by :func:`Server:getNameWithAddr()`, the second parameter is a boolean value indicating whether the server is up.
+ :param function callback: The function to be called. It returns no value and takes two parameters: the first parameter is a string identifying the server, formatted as if returned by :meth:`Server.getNameWithAddr`, the second parameter is a boolean value indicating whether the server is up.
.. code-block:: lua