A resettable reference event.
NOTE: Although Cache Reference does not support
null as a valid value we are using it in
getValue
as a way to represent missing value. We have the following reasons for this:
- Firstly, some events inherently do not have value. For instance resetting does not have a value as the
corresponding element was just removed from the cached reference
- Secondly, in some implementations the information for the value and/or old value is not available at all.
- Thirdly, the listener can be added as a valueless listener