向下滚动时隐藏底部导航栏，反之亦然

Question

我在正文和底部导航栏中有一个列表。我想在帖子列表向下滚动时隐藏带有向下滑动动画的底部导航栏，并在向上滚动时使用向上滑动动画显示。怎么做？

Answer 1

虽然 Naveen 的解决方案工作得很好，但我不喜欢使用setState来处理导航栏可见性的想法。每次用户更改滚动方向时，包括 appbar 和 body 在内的整个主页都会重建，这可能是一项昂贵的操作。我创建了一个单独的类来处理使用 aValueNotifier跟踪当前隐藏状态的可见性。

class HideNavbar {
  final ScrollController controller = ScrollController();
  final ValueNotifier<bool> visible = ValueNotifier<bool>(true);

  HideNavbar() {
    visible.value = true;
    controller.addListener(
      () {
        if (controller.position.userScrollDirection ==
            ScrollDirection.reverse) {
          if (visible.value) {
            visible.value = false;
          }
        }

        if (controller.position.userScrollDirection ==
            ScrollDirection.forward) {
          if (!visible.value) {
            visible.value = true;
          }
        }
      },
    );
  }

  void dispose() {
    controller.dispose();
    visible.dispose();
  }
}

现在您要做的就是HideNavbar在 HomePage 小部件中创建一个最终实例。

final HideNavbar hiding = HideNavbar();

现在，通过实例的ScrollController对ListView或CustomScrollView你的身体Scaffold。

body: CustomScrollView(
          controller: hiding.controller,
          ...

然后环绕你bottomNavigationBar用ValueListenableBuilder的是采取ValueNotifier从HideNavbar实例，然后设置bottomNavigationBar的高度属性为0或视的状态的任何其他值ValueNotifier。

bottomNavigationBar: ValueListenableBuilder(
        valueListenable: hiding.visible,
        builder: (context, bool value, child) => AnimatedContainer(
          duration: Duration(milliseconds: 500),
          height: value ? kBottomNavigationBarHeight : 0.0,
          child: Wrap(
            children: <Widget>[
              BottomNavigationBar(
                type: BottomNavigationBarType.fixed,
                backgroundColor: Colors.blue,
                fixedColor: Colors.white,
                unselectedItemColor: Colors.white,
                items: const [
                  BottomNavigationBarItem(
                    icon: Icon(Icons.home),
                    title: Text('Home'),
                  ),
                  BottomNavigationBarItem(
                    icon: Icon(Icons.card_giftcard),
                    title: Text('Offers'),
                  ),
                  BottomNavigationBarItem(
                    icon: Icon(Icons.account_box),
                    title: Text('Account'),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),

这种方法可以避免无数次重建，并且不需要任何外部库。您也可以将其实现为基于流的方法，但这将需要另一个库，例如dart:async并且不会更改任何内容。确保调用HideNavbarinsideHomePage的 dispose 函数的 dispose 函数来清除所有使用的资源。

Answer 2

使用BottomNavigationBar.

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(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  ScrollController _hideBottomNavController;

  bool _isVisible;

  @override
  initState() {
    super.initState();
    _isVisible = true;
    _hideBottomNavController = ScrollController();
    _hideBottomNavController.addListener(
      () {
        if (_hideBottomNavController.position.userScrollDirection ==
            ScrollDirection.reverse) {
          if (_isVisible)
            setState(() {
              _isVisible = false;
            });
        }
        if (_hideBottomNavController.position.userScrollDirection ==
            ScrollDirection.forward) {
          if (!_isVisible)
            setState(() {
              _isVisible = true;
            });
        }
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: CustomScrollView(
          controller: _hideBottomNavController,
          shrinkWrap: true,
          slivers: <Widget>[
            SliverPadding(
              padding: const EdgeInsets.all(10.0),
              sliver: SliverList(
                delegate: SliverChildBuilderDelegate(
                  (context, index) => _getItem(context),
                  childCount: 20,
                ),
              ),
            ),
          ],
        ),
      ),
      bottomNavigationBar: AnimatedContainer(
        duration: Duration(milliseconds: 500),
        height: _isVisible ? 56.0 : 0.0,
        child: Wrap(
          children: <Widget>[
            BottomNavigationBar(
              type: BottomNavigationBarType.fixed,
              backgroundColor: Colors.blue,
              fixedColor: Colors.white,
              unselectedItemColor: Colors.white,
              items: [
                BottomNavigationBarItem(
                  icon: Icon(Icons.home),
                  title: Text('Home'),
                ),
                BottomNavigationBarItem(
                  icon: Icon(Icons.card_giftcard),
                  title: Text('Offers'),
                ),
                BottomNavigationBarItem(
                  icon: Icon(Icons.account_box),
                  title: Text('Account'),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }

  _getItem(BuildContext context) {
    return Card(
      elevation: 3,
      margin: EdgeInsets.all(8),
      child: Row(
        children: <Widget>[
          Expanded(
            child: Container(
              padding: EdgeInsets.all(8),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Text(
                      'Item',
                      style:
                          TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
                    ),
                  )
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

工作模式

Answer 3

截图（空安全+优化）

代码：

class MyPage extends StatefulWidget {
  @override
  State<MyPage> createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
  late final ScrollListener _model;
  late final ScrollController _controller;
  final double _bottomNavBarHeight = 56;

  @override
  void initState() {
    super.initState();
    _controller = ScrollController();
    _model = ScrollListener.initialise(_controller);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: AnimatedBuilder(
        animation: _model,
        builder: (context, child) {
          return Stack(
            children: [
              ListView.builder(
                controller: _controller,
                itemCount: 20,
                itemBuilder: (_, i) => ListTile(title: Text('Item $i')),
              ),
              Positioned(
                left: 0,
                right: 0,
                bottom: _model.bottom,
                child: _bottomNavBar,
              ),
            ],
          );
        },
      ),
    );
  }

  Widget get _bottomNavBar {
    return SizedBox(
      height: _bottomNavBarHeight,
      child: BottomNavigationBar(
        backgroundColor: Colors.amber,
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.call), label: 'Call'),
          BottomNavigationBarItem(icon: Icon(Icons.message), label: 'Message'),
        ],
      ),
    );
  }
}

class ScrollListener extends ChangeNotifier {
  double bottom = 0;
  double _last = 0;

  ScrollListener.initialise(ScrollController controller, [double height = 56]) {
    controller.addListener(() {
      final current = controller.offset;
      bottom += _last - current;
      if (bottom <= -height) bottom = -height;
      if (bottom >= 0) bottom = 0;
      _last = current;
      if (bottom <= 0 && bottom >= -height) notifyListeners();
    });
  }
}