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

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(

        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
 }
class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

 class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  ScrollController _hideButtonController;
  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }
  var _isVisible;
  @override
  initState(){
    super.initState();
    _isVisible = true;
    _hideButtonController = new ScrollController();
    _hideButtonController.addListener((){
      if(_hideButtonController.position.userScrollDirection == ScrollDirection.reverse){
        if(_isVisible == true) {
            /* only set when the previous state is false
             * Less widget rebuilds 
             */
            print("**** ${_isVisible} up"); //Move IO away from setState
            setState((){
              _isVisible = false;
            });
        }
      } else {
        if(_hideButtonController.position.userScrollDirection == ScrollDirection.forward){
          if(_isVisible == false) {
              /* only set when the previous state is false
               * Less widget rebuilds 
               */
               print("**** ${_isVisible} down"); //Move IO away from setState
               setState((){
                 _isVisible = true;
               });
           }
        }
    }});
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new CustomScrollView(
          controller: _hideButtonController,
          shrinkWrap: true,
          slivers: <Widget>[
            new SliverPadding(
              padding: const EdgeInsets.all(20.0),
              sliver: new SliverList(
                delegate: new SliverChildListDelegate(
                  <Widget>[
                    const Text('I\'m dedicating every day to you'),
                    const Text('Domestic life was never quite my style'),
                    const Text('When you smile, you knock me out, I fall apart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('And I thought I was so smart'),
                    const Text('I realize I am crazy'),   
                  ],
                ),
              ),
            ),
          ],
        )
      ),
      floatingActionButton: new Visibility( 
        visible: _isVisible,
        child: new FloatingActionButton(
          onPressed: _incrementCounter,
          tooltip: 'Increment',
          child: new Icon(Icons.add),
        ),     
      ),
    );
  }
}

我很抱歉，如果我没有使用列表视图，因为我不知道如何滚动列表视图。我会回答你的问题的其他部分。
首先，您需要创建一个侦听scrollPostion事件的scrollcontroller
如果scrollcontroller设法找到scrolldirection正向或反向，则添加一个状态，将其设置为visible。
在绘制按钮时，将按钮 Package 在visibility类中，设置visible标志，小部件将忽略输入命令。
编辑：我似乎不能添加到ScrollController，ScrollerPosition，ScrollDirection和Opacity的链接。我想你可以自己搜索或者其他人编辑链接
Edit 2：使用CopsonRoad或使用可见性小部件，除非您希望布局树中有未绘制的小部件
Edit 3：考虑到新手使用代码的现状，我会更新代码以鼓励更好的实践。使用可见性代替不透明性。从setState中删除io。在Flutter 1.5.4-hotfix.2上测试

无动画：

• 使用**Visibility**小工具：

floatingActionButton: Visibility(
  visible: false, // Set it to false
  child: FloatingActionButton(...),
)

• 使用**Opacity**小工具：

floatingActionButton: Opacity(
  opacity: 0, // Set it to 0
  child: FloatingActionButton(...),
)

• 使用三元运算符：

floatingActionButton: shouldShow ? FloatingActionButton() : null,

• 使用**if**条件：

floatingActionButton: Column(
  children: <Widget>[
    if (shouldShow) FloatingActionButton(...), // Visible if condition is true
  ],
)

带动画：

这只是使用动画的一个例子，你可以使用这种方法创建不同类型的UI。

bool _showFab = true;
  
@override
Widget build(BuildContext context) {
  const duration = Duration(milliseconds: 300);
  return Scaffold(
    floatingActionButton: AnimatedSlide(
      duration: duration,
      offset: _showFab ? Offset.zero : Offset(0, 2),
      child: AnimatedOpacity(
        duration: duration,
        opacity: _showFab ? 1 : 0,
        child: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: () {},
        ),
      ),
    ),
    body: NotificationListener<UserScrollNotification>(
      onNotification: (notification) {
        final ScrollDirection direction = notification.direction;
        setState(() {
          if (direction == ScrollDirection.reverse) {
            _showFab = false;
          } else if (direction == ScrollDirection.forward) {
            _showFab = true;
          }
        });
        return true;
      },
      child: ListView.builder(
        itemCount: 100,
        itemBuilder: (_, i) => ListTile(title: Text('$i')),
      ),
    ),
  );
}

这是一个很老的问题，但在我看来，随着最新的波动，有一个更好（更短）的解决方案。
其他解决方案确实有效，但如果你想要一个漂亮的动画（与Android中的默认动画相当），这里可以：
每当用户滚动（上/下）时，NotificationListener会通知您。使用AnimationController，您可以控制FAB的动画。
下面是一个完整的例子：

class WidgetState extends State<Widget> with TickerProviderStateMixin<Widget> {
  AnimationController _hideFabAnimation;

  @override
  initState() {
    super.initState();
    _hideFabAnimation = AnimationController(vsync: this, duration: kThemeAnimationDuration);
  }

  @override
  void dispose() {
    _hideFabAnimation.dispose();
    super.dispose();
  }

  bool _handleScrollNotification(ScrollNotification notification) {
    if (notification.depth == 0) {
      if (notification is UserScrollNotification) {
        final UserScrollNotification userScroll = notification;
        switch (userScroll.direction) {
          case ScrollDirection.forward:
            if (userScroll.metrics.maxScrollExtent !=
                userScroll.metrics.minScrollExtent) {
              _hideFabAnimation.forward();
            }
            break;
          case ScrollDirection.reverse:
           if (userScroll.metrics.maxScrollExtent !=
                userScroll.metrics.minScrollExtent) {
              _hideFabAnimation.reverse();
            }
            break;
          case ScrollDirection.idle:
            break;
        }
      }
    }
    return false;
  }

  @override
  Widget build(BuildContext context) {
    return NotificationListener<ScrollNotification>(
      onNotification: _handleScrollNotification,
      child: Scaffold(
        appBar: AppBar(
          title: Text('Fabulous FAB Animation')
        ),
        body: Container(),
        floatingActionButton: ScaleTransition(
          scale: _hideFabAnimation,
          alignment: Alignment.bottomCenter,
          child: FloatingActionButton(
            elevation: 8,
            onPressed: () {},
            child: Icon(Icons.code),
          ),
        ),
      ),
    );
  }
}

一个好办法是...

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

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  ScrollController controller;
  bool fabIsVisible = true;

  @override
  void initState() {
    super.initState();
    controller = ScrollController();
    controller.addListener(() {
      setState(() {
        fabIsVisible =
            controller.position.userScrollDirection == ScrollDirection.forward;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView(
        controller: controller,
        children: List.generate(
            100,
            (index) => ListTile(
                  title: Text("Text $index"),
                )),
      ),
      floatingActionButton: AnimatedOpacity(
        child: FloatingActionButton(
          child: Icon(Icons.add),
          tooltip: "Increment",
          onPressed: !fabIsVisible ? null: () {
            print("Pressed");
          },
        ),
        duration: Duration(milliseconds: 100),
        opacity: fabIsVisible ? 1 : 0,
      ),
    );
  }
}

你可以使用下面的代码来保持默认动画

floatingActionButton: _isVisible
        ? FloatingActionButton(...)
        : null,

您可以使用Visibility微件来处理子微件的可见性
样品：

floatingActionButton:
            Visibility(visible: _visibilityFlag , child: _buildFAB(context)),

另一个非常好的方法是动画不透明度

AnimatedOpacity(
          opacity: isEnabled ? 0.0 : 1.0,
          duration: Duration(milliseconds: 1000),
          child: FloatingActionButton(
             onPressed: your_method,
             tooltip: 'Increment',
             child: new Icon(Icons.add),
          ),
        )

对于任何使用Rxdart的人来说，有一种简洁的方法可以做到这一点，它附带了额外的方便工具。
首先，将滚动位置转换为流，您也可以在以后重用此方法。

extension ScrollControllerX on ScrollController {
  Stream<double> positionAsStream() {
    late StreamController<double> controller;

    void addListener() => controller.add(position.pixels);
    void onListen() => this.addListener(addListener);
    void onCancel() {
      removeListener(addListener);
      controller.close();
    }

    controller = StreamController<double>(onListen: onListen, onCancel: onCancel);

    return controller.stream;
  }
}

像这样使用它。
x一个一个一个一个x一个一个二个x
别忘了取消订阅！

@override
      void dispose() {
        subscription.cancel();
      }

你可以做其他的事情，比如当用户滚动太快的时候节流。

@Josteve的答案是正确的，但是每次用户滚动时都调用setState()并不是一个好主意，更好的方法如下所示：

import 'package:flutter/material.dart';

class Home extends StatefulWidget {
  const Home({super.key});

  @override
  State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  late ScrollController controller;
  bool _isFabVisible = true;

  @override
  void initState() {
    super.initState();
    controller = ScrollController();
    controller.addListener(() {
      // FAB should be visible if and only if user has not scrolled to bottom
      var userHasScrolledToBottom = controller.position.atEdge && controller.position.pixels > 0;

      if(_isFabVisible == userHasScrolledToBottom) {
        setState(() => _isFabVisible = !userHasScrolledToBottom);
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView(
        controller: controller,
        children: List.generate(
          100, 
          (index) => ListTile(
            title: Text("Text $index"),
          )),
      ),
      floatingActionButton: AnimatedOpacity(
        duration: const Duration(milliseconds: 100),
        opacity: _isFabVisible? 1 : 0,
        child: FloatingActionButton(
          tooltip: "Increment",
          onPressed: () {
            debugPrint('Pressed');
          },
          child: const Icon(Icons.add),
        ),
      ),
    );
  }
}