Type of a function that is called when the number of subscribers changes from 0 to 1
(but not called when the number of subscribers changes from 1 to 2, ...).
If it returns a function, that function will be called when the number of subscribers changes from 1 to 0.
Type of a function that is called when the number of subscribers changes from 0 to 1 (but not called when the number of subscribers changes from 1 to 2, ...).
If it returns a function, that function will be called when the number of subscribers changes from 1 to 0.