RegisterCallbackHandler typedef Null safety
The type of a custom Zone.registerCallback implementation function.
Receives the Zone that the handler was registered on as
a delegate forwarding to the handlers of
self's parent zone as
and the current zone where the error was uncaught as
which will have
self as a parent zone.
f is the function which was passed to the
The handler should return either the function
or another function replacing
typically by wrapping
f in a function
which does something extra before and after invoking
typedef RegisterCallbackHandler = ZoneCallback<R> Function<R>( Zone self, ZoneDelegate parent, Zone zone, R Function() f);