Future<Null> show()

Show the refresh indicator and run the refresh callback as if it had been started interactively. If this method is called while the refresh callback is running, it quietly does nothing.

Creating the RefreshIndicator with a GlobalKey<RefreshIndicatorState> makes it possible to refer to the RefreshIndicatorState.

Source 

Future<Null> show() async {
  if (_mode != _RefreshIndicatorMode.refresh) {
    _sizeController.value = 0.0;
    _scaleController.value = 0.0;
    await _show();
  }
}