requestTwoLevel method

Future<void> requestTwoLevel({
  1. Duration duration = const Duration(milliseconds: 300),
  2. Curve curve = Curves.linear,

make the header enter refreshing state,and callback onRefresh


Future<void> requestTwoLevel(
    {Duration duration: const Duration(milliseconds: 300),
    Curve curve: Curves.linear}) {
  assert(position != null,
      'Try not to call requestRefresh() before build,please call after the ui was rendered');
  headerMode!.value = RefreshStatus.twoLevelOpening;
  return Future.delayed(const Duration(milliseconds: 50)).then((_) async {
    await position?.animateTo(position!.minScrollExtent,
        duration: duration, curve: curve);