CallbackToken
A normalized identification of a callback's origin, agnostic to its instance.
This is meant to be used with callbacks that receive a parameter and return a value. For other kinds of callbacks, there are more specific versions:
InputCallbackToken: for callbacks that receive a parameter and return no value.
OutputCallbackToken: for callbacks that receive no parameters and return a value.
SimpleCallbackToken: for callbacks that receive no parameters and return no value.
Parameters
I
The type of parameter the callback receives.
O
The type of return of the callback.