onChange abstract method
Listens for Metadata changes.
This method returns a StreamSubscription that allows you to stop listening for changes when it's no longer needed by calling StreamSubscription.cancel.
handler is a callback that will be called whenever the metadata changes.
If during the method call current metadata is available,
the handler will be called right away.
Implementation
StreamSubscription<Metadata> onChange(
void Function(Metadata metadata) handler,
);