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

Registers a service extension method with the given name (full name "ext.flutter.name"), which optionally takes a single argument with the name "value". If the argument is omitted, the value is to be read, otherwise it is to be set. Returns the current value.

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