需要一点阴影，

bottomNavigationBar: Container(                                             
  decoration: BoxDecoration(                                                   
    borderRadius: BorderRadius.only(                                           
      topRight: Radius.circular(30), topLeft: Radius.circular(30)),            
    boxShadow: [                                                               
      BoxShadow(color: Colors.black38, spreadRadius: 0, blurRadius: 10),       
    ],                                                                         
  ),                                                                           
  child: ClipRRect(                                                            
    borderRadius: BorderRadius.only(                                           
    topLeft: Radius.circular(30.0),                                            
    topRight: Radius.circular(30.0),                                           
    ),                                                                         
    child: BottomNavigationBar(                                                
      items: <BottomNavigationBarItem>[                                        
        BottomNavigationBarItem(                                               
          icon: Icon(Icons.favorite), title: Text('Favourite')),               
        BottomNavigationBarItem(                                               
          icon: Icon(Icons.favorite), title: Text('Favourite'))                
      ],                                                                       
    ),                                                                         
  )                                                                            
)

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没有影子：

的数据
影子：

的

在Scaffold中，将extendBody属性设置为true。这将使body扩展到Scaffold的底部，而不是仅扩展到bottomNavigationBar的顶部。这应该可以解决您的问题。
范例：

Widget build(BuildContext context) {
    return Scaffold(
      body: bodyOfApp(),
      extendBody: true,
      backgroundColor: Colors.transparent,
      bottomNavigationBar: BottomNavBar(),
    );
   }

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上述所有答案都以某种方式使用ClipRRect，这是非常昂贵的计算方式。因此，作为替代使用Material Widget，以下是如何：

bottomNavigationBar: Container(
      decoration: BoxDecoration(
        color: Colors.white,
        borderRadius: BorderRadius.only(
            topRight: Radius.circular(30), topLeft: Radius.circular(30)),
      ),
      child: Material(
        elevation: 0.0,
        shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(30.0)),
        child: BottomNavigationBar(
          elevation: 0,
          backgroundColor: Colors.transparent,
          items: <BottomNavigationBarItem>[
            BottomNavigationBarItem(
                icon: Icon(Icons.favorite), title: Text('Favourite')),
            BottomNavigationBarItem(
                icon: Icon(Icons.favorite), title: Text('Favourite'))
          ],
        ),
      )),
)

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将Theme中的canvasColor更改为透明。

return MaterialApp(
  theme: ThemeData(
    canvasColor: Colors.transparent
  ),
);

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的数据

class _DashBoardActivityState extends State<DashBoardActivity> {
  int _selectedIndex = 0;
  static const TextStyle optionStyle =
  TextStyle(fontSize: 30, fontWeight: FontWeight.bold);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("BottomNavigationBar"),
      backgroundColor: PrimaryColor,),
      body: Container(child: Center(child: _widgetOptions.elementAt(_selectedIndex),),),
      bottomNavigationBar: CreateBottombar(context),
    );
  }

  final _widgetOptions = [
    new ListFragScreen(),
    new ConsultationFragScreen(),
    new LabFragScreen(),
  ];

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  Container CreateBottombar(BuildContext context) {
    return Container(
      //add ClipRRect widget for Round Corner
      child: ClipRRect(
        borderRadius: const BorderRadius.only(
          topRight: Radius.circular(24),
          topLeft: Radius.circular(24),
        ),
        child: BottomNavigationBar(
          //add background color
          backgroundColor: PrimaryColor,
          items: const <BottomNavigationBarItem>[
            BottomNavigationBarItem(
              icon: Icon(Icons.home),
              label: 'Home',
            ),
            BottomNavigationBarItem(
              icon: Icon(Icons.business),
              label: 'Business',
            ),
            BottomNavigationBarItem(
              icon: Icon(Icons.school),
              label: 'School',
            ),
          ],
          currentIndex: _selectedIndex,
          selectedItemColor: Colors.white,
          onTap: _onItemTapped,
        ),
      ),
    );
  }
}

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@CoderMonk建议的method应该可以工作。如果它不工作，那么我认为你可能在你的Scaffold()的主体中使用了类似SafeArea()的小部件，如果是这样，你应该完全删除它，或者做一些类似的事情。

Scaffold(
  body: SafeArea(
    bottom: false,
    child: ...
  ),
);

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该解决方案与extendBody，有助于删除背景的酒吧：

@override
  Widget build(BuildContext context) {
    return Scaffold(
      extendBody: true, // Important: to remove background of bottom navigation (making the bar transparent doesn't help)
      bottomNavigationBar: Container(
        decoration: const BoxDecoration(
            borderRadius: BorderRadius.only(
              topLeft: Radius.circular(12.0), // adjust to your liking
              topRight: Radius.circular(12.0), // adjust to your liking
            ),
            color: your_bar_color, // put the color here
          ),
          child: BottomNavigationBar(backgroundColor: Colors.transparent), // don't forget to put it
      ),
      body: SingleChildScrollView(
        child: Column(
          children: [
            // widget
            // widget
            const SizedBox(height: kBottomNavigationBarHeight) // add space at the bottom due to extendBody property for proper scrolling
          ],
        ),
      ),
    );
  }

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