问如何知道床单已经完全消失了？

EN

showModalBottomSheet<void>(
        builder: (context) => Container(),
).whenComplete(() {
// Goes here
});

@override
void initState() {
  super.initState();
  _bottomSheetController = AnimationController(
      vsync: this, duration: const Duration(milliseconds: 200))
    ..addStatusListener(_onListenerAnimation);
}

void _onListenerAnimation(AnimationStatus status) async {
  switch (status) {
    case AnimationStatus.dismissed:
      print('finished closing animation');
      break;
    case AnimationStatus.forward:
      print('starts opening sheet');
      break;
    case AnimationStatus.reverse:
      print('start closing the modal');
      break;
    case AnimationStatus.completed:
      print('finished opening animation');
      break;
  }
}

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(primarySwatch: Colors.blue),
        home: const MyStateFullWidget());
  }
}

class MyStateFullWidget extends StatefulWidget {
  const MyStateFullWidget({Key? key}) : super(key: key);

  @override
  State<MyStateFullWidget> createState() => _MyStateFullWidgetState();
}

class _MyStateFullWidgetState extends State<MyStateFullWidget>
    with TickerProviderStateMixin {
  late AnimationController _bottomSheetController;

  @override
  void initState() {
    super.initState();
    _bottomSheetController = AnimationController(
        vsync: this, duration: const Duration(milliseconds: 200))
      ..addStatusListener(_onListenerAnimation);
  }

  @override
  void dispose() {
    super.dispose();
    _bottomSheetController
      ..removeStatusListener(_onListenerAnimation)
      ..dispose();
  }

  void _onListenerAnimation(AnimationStatus status) async {
    switch (status) {
      case AnimationStatus.dismissed:
        print('finished closing animation');
        break;
      case AnimationStatus.forward:
        print('starts opening sheet');
        break;
      case AnimationStatus.reverse:
        print('start closing the modal');
        break;
      case AnimationStatus.completed:
        print('finished opening animation');
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: const Text('Demo')),
        body: SizedBox.expand(
          child: Column(
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: [
              TextButton(
                  onPressed: _openBottomSheet, child: const Text('Open sheet'))
            ],
          ),
        ));
  }

  void _openBottomSheet() {
    showModalBottomSheet(
      shape: const RoundedRectangleBorder(
        borderRadius: BorderRadius.only(
          topLeft: Radius.circular(10),
          topRight: Radius.circular(10),
        ),
      ),
      backgroundColor: Colors.white,
      context: context,
      transitionAnimationController: _bottomSheetController,
      isScrollControlled: false,
      builder: (context) {
        return Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          mainAxisAlignment: MainAxisAlignment.center,
          mainAxisSize: MainAxisSize.max,
          children: [
            TextButton(
                onPressed: () => Navigator.of(context).pop(),
                child: const Text('Pop')),
          ],
        );
      },
    );
  }
}

await showModalBottomSheet(
  context: context,
  builder: (context) => MyBottomSheet(),
);

doSomethinAfterAwait();