When the value of a configuration parameter is altered, either directly or by loading a configuration file, a configuration event is generated. Handlers can be registered to be notified when a configuration event occurs. A configuration event is associated with the group and key that had its value modified.
If a configuration event is emitted by a configuration context, it propagates to all configuration contexts which inherit that group and key from it.
A configuration event handler must be a closure that accepts three arguments:
handler cfg group key
cfg is always the configuration context that received the event, and the group and key identify the configuration parameter that changed.