Flutter：使用 Sliver 小部件时如何隐藏滚动时的 BottomAppBar？

当用户向下滚动列表时，我试图隐藏底部应用程序栏，就像材料设计文档中行为部分中显示的那样：

https://material.io/design/components/app-bars-bottom.html#behavior

我正在寻找的效果如下所示：https://storage.googleapis.com/spec-host-backup/mio-design%2Fassets%2F1gaSrddolFHd8BwOioeYz0ODiyEhCevtn%2Fbehavior-scroll.mp4

到目前为止，我向脚手架添加了底部应用栏，但它没有滚动行为。

StackOverflow 上有一个类似的问题得到了回答，但它显示了一种解决方法，并且不久前已经得到了回答。

Flutter 中是否有合法的方法来实现这种效果？

我的代码：

import 'package:fitness_fatality_flutter/data/entities/exercise.dart';
import 'package:fitness_fatality_flutter/data/entities/workout.dart';
import 'package:flutter/material.dart';

class WorkoutDetailsPage extends StatelessWidget {
  Workout _workout = Workout();

  final List<Exercise> exercises = [
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
  ];

  WorkoutDetailsPage(this._workout);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButtonAnimator: FloatingActionButtonAnimator.scaling,
      floatingActionButtonLocation: FloatingActionButtonLocation.endDocked,
      floatingActionButton: FloatingActionButton(
        backgroundColor: Colors.blue,
        elevation: 12,
        onPressed: () {},
      ),
      bottomNavigationBar: BottomAppBar(
        elevation: 8,
        shape: CircularNotchedRectangle(),
        color: Colors.blue,
        child: Container(
          height: 50,
          child: Row(
            children: <Widget>[Text("data")],
          ),
        ),
      ),
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            bottom:BottomAppBar(), //<-- This would throw an error
            expandedHeight: 150,
            pinned: true,
            floating: true,
            snap: true,
            flexibleSpace: FlexibleSpaceBar(
              title: Text(_workout.name),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(buildSliverListItem,
                childCount: exercises.length),
          ),
        ],
      ),
    );
  }

  Widget buildSliverListItem(BuildContext context, int index) {
    return Center(
      child: ListTile(
        title: Text(exercises[index].name),
      ),
    );
  }
}

BottomAppBar是其一部分Scaffold小部件，不是Sliver。有一种方法可以实现您所描述的目标，但是有点复杂并且需要StatefulWiget and ScrollController, AnimatedContainer。请参阅下面的完整示例代码：

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: WorkoutDetailsPage(Workout()),
    );
  }
}

class Exercise {
  String name;
  Exercise({@required name}) {
    this.name = name;
  }
}

class Workout {
  String name = "my name";
}

class WorkoutDetailsPage extends StatefulWidget {
  Workout _workout = Workout();

  WorkoutDetailsPage(this._workout);

  @override
  _WorkoutDetailsPageState createState() => _WorkoutDetailsPageState();
}

class _WorkoutDetailsPageState extends State<WorkoutDetailsPage> {
  final List<Exercise> exercises = [
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
    Exercise(name: "Push Ups"),
    Exercise(name: "Bench press"),
    Exercise(name: "Pull ups"),
    Exercise(name: "Press ups"),
    Exercise(name: "Crunches"),
    Exercise(name: "Sit ups"),
    Exercise(name: "BIceps curl"),
    Exercise(name: "Something else"),
  ];

  ScrollController _hideButtonController;

  bool _isVisible = true;
  @override
  void initState() {
    super.initState();
    _isVisible = true;
    _hideButtonController = new ScrollController();
    _hideButtonController.addListener(() {
      print("listener");
      if (_hideButtonController.position.userScrollDirection ==
          ScrollDirection.reverse) {
        setState(() {
          _isVisible = false;
          print("**** $_isVisible up");
        });
      }
      if (_hideButtonController.position.userScrollDirection ==
          ScrollDirection.forward) {
        setState(() {
          _isVisible = true;
          print("**** $_isVisible down");
        });
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButtonAnimator: FloatingActionButtonAnimator.scaling,
      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
      floatingActionButton: _isVisible
          ? FloatingActionButton(
              backgroundColor: Colors.blue,
              elevation: 12,
              onPressed: () {},
            )
          : null,
      bottomNavigationBar: AnimatedContainer(
        duration: Duration(milliseconds: 200),
        height: _isVisible ? 60 : 0.0,
        child: BottomAppBar(
          elevation: 8,
          shape: CircularNotchedRectangle(),
          color: Colors.blue,
          child: Container(
            height: 60,
            child: Row(
              children: <Widget>[Text("data")],
            ),
          ),
        ),
      ),
      body: CustomScrollView(
        controller: _hideButtonController,
        slivers: <Widget>[
          SliverAppBar(
            expandedHeight: 150,
            pinned: true,
            floating: true,
            snap: true,
            flexibleSpace: FlexibleSpaceBar(
              title: Text(widget._workout.name),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(buildSliverListItem,
                childCount: exercises.length),
          ),
        ],
      ),
    );
  }

  Widget buildSliverListItem(BuildContext context, int index) {
    return Center(
      child: ListTile(
        title: Text(exercises[index].name),
      ),
    );
  }
}