BottomNavigationBar

重点：

bottomNavigationBar: BottomAppBar(
        shape: CircularNotchedRectangle(),//这个就是设置floatingactionbutton嵌入底部导航的设置；

下面是完整示例：
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
   return Scaffold(
      appBar: AppBar(
        elevation: 0.0,
        leading: Icon(Icons.airplay),
        backgroundColor: Colors.white70,
        centerTitle: true,
        title: Text(
          '勇往直前',
          style: TextStyle(
            color: Colors.orange,
            fontWeight: FontWeight.bold,
            fontSize: 18,
            fontStyle: FontStyle.italic,
          ),
        ),
      ),
      bottomNavigationBar: BottomAppBar(
        color: Colors.black12,
        shape: CircularNotchedRectangle(),
        child: Row(
          mainAxisSize: MainAxisSize.max,
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: <Widget>[
            IconButton(icon: Icon(Icons.airplay,color: Colors.orange,),highlightColor: Colors.lightGreen, onPressed: (){}),
            IconButton(icon: Icon(Icons.email,color: Colors.orange,),highlightColor: Colors.lightGreen, onPressed: (){}),
            SizedBox(width: 55,),
            IconButton(icon: Icon(Icons.save,color: Colors.orange,),highlightColor: Colors.lightGreen, onPressed: (){}),
            IconButton(icon: Icon(Icons.add_a_photo,color: Colors.orange,),highlightColor: Colors.lightGreen, onPressed: (){}),
          ],
        ),
      ),
      body:Container(),
     floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
     floatingActionButton: FloatingActionButton(
       backgroundColor: Colors.white,
       child: Icon(Icons.airplay,color: Colors.orange,),
       onPressed: (){
           setState(() { });
         },
     ),

    );
  }
}