onFuture method
Sets value after future
finishes.
Via ValueConverter is possible to convert object from input Stream type to own stream value.
Returns Future to await and register other callbacks.
Implementation
Future onFuture(Future future, {ValueConverter? converter}) => future
.then(
(value) => setValue(converter == null ? value : converter(value)))
.catchError((err) {
printDebug(err);
});