1. protected
void registerNumericServiceExtension({@required String name, @required ValueGetter<double> getter, @required ValueSetter<double> setter })

Registers a service extension method with the given name (full name "ext.flutter.name"), which takes a single argument with the same name as the method which, if present, must have a value that can be parsed by double.parse, and can be omitted to read the current value. (Other arguments are ignored.)

Calls the getter callback to obtain the value when responding to the service extension method being called.

Calls the setter callback with the new value when the service extension method is called with a new value.

Source 

@protected
void registerNumericServiceExtension({
  @required String name,
  @required ValueGetter<double> getter,
  @required ValueSetter<double> setter
}) {
  assert(name != null);
  assert(getter != null);
  assert(setter != null);
  registerServiceExtension(
    name: name,
    callback: (Map<String, String> parameters) async {
      if (parameters.containsKey(name))
        setter(double.parse(parameters[name]));
      return <String, dynamic>{ name: getter() };
    }
  );
}