不能使用超过三个项目的导航栏

For 4 or more items, set the type to fixed.

bottomNavigationBar: BottomNavigationBar(
type: BottomNavigationBarType.fixed, // This is all you need!
items: // ...,
)

From https://github.com/flutter/flutter/issues/13642#issuecomment-371875044

When more than 3 BottomNavigationBar items are provided the type, if unspecified, changes to BottomNavigationBarType.shifting per https://docs.flutter.io/flutter/material/BottomNavigationBar/BottomNavigationBar.html. This bit of information should be highlighted in the class's doc. It's easy to overlook where it is (I overlooked it).

When the BottomNavigationBar's type is BottomNavigationBarType.shifting the items text and icons are rendered in white, via DefaultTextStyle and IconTheme. It's assumed that theirBottomNavigationBarItem.backgroundColor will be specified as a contrasting color. This is obviously confusing.

The overall idea with shifting type bottom navigation bars is that each item will have a different background color (that contrasts with white), since that color will become the color of the entire navigation bar, when the item is selected.

The doc for BottomNavigationBar, and NavigationBarItem needs to be improved.

Add type: BottomNavigationBarType.fixed on BottomNavigationBar

bottomNavigationBar: BottomNavigationBar(
type: BottomNavigationBarType.fixed,
items: <BottomNavigationBarItem>[
BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('A')),
BottomNavigationBarItem(icon: Icon(Icons.access_alarm), title: Text('B')),
BottomNavigationBarItem(icon: Icon(Icons.business), title: Text('C')),
BottomNavigationBarItem(icon: Icon(Icons.school), title: Text('D')),
],

child: BottomNavigationBar(
type: BottomNavigationBarType.fixed,
backgroundColor: Colors.blueGrey,
currentIndex: selectedIndex,
onTap: onItemTap,
// ignore: prefer_const_literals_to_create_immutables
items: [
BottomNavigationBarItem(
icon: Icon(
Icons.home,
),
label: "Home"),
BottomNavigationBarItem(
icon: Icon(
Icons.home,
),
label: "Home"),
BottomNavigationBarItem(
icon: Icon(Icons.upload), label: "Post"),
// BottomNavigationBarItem(icon: Icon(Icons.layers), label: "Item"),
BottomNavigationBarItem(
icon: Icon(Icons.person), label: "Accounts"),
],
),
)),
)

set the type to fixed

// Create a Function First then call it like i call Dashbord()
// Change the icon according to your requirements






import 'package:flutter/material.dart';
import 'package:pedometer/widgets/const.dart';
import 'package:pedometer/widgets/const.dart';
import 'package:pedometer/widgets/const.dart';
import 'package:pedometer/widgets/icon.dart';


import 'dashbord.dart';


class MyStatefulWidget extends StatefulWidget {
const MyStatefulWidget({Key? key}) : super(key: key);


@override
State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}


class _MyStatefulWidgetState extends State<MyStatefulWidget> {
int _selectedIndex = 0;
static const TextStyle optionStyle =
TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
static const List<Widget> _widgetOptions = <Widget>[
DashbordScreen(),
DashbordScreen(),
DashbordScreen(),
DashbordScreen(),
];


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


@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: _widgetOptions.elementAt(_selectedIndex),
),
bottomNavigationBar: BottomNavigationBar(
backgroundColor: const Color.fromARGB(255, 6, 17, 93),
type: BottomNavigationBarType.fixed,
items: const <BottomNavigationBarItem>[
BottomNavigationBarItem(
// iconFun(path:Icons.home,context: context )
icon: Icon(Icons.home,color: Colors.white,size: 35,),
label: 'Home',
//  backgroundColor: Colors.red,
),
BottomNavigationBarItem(
icon: Icon(Icons.auto_graph_outlined,color: Colors.white,size: 35),
label: 'Business',
backgroundColor: Colors.green,
),
BottomNavigationBarItem(
icon: Icon(Icons.health_and_safety,color: Colors.white,size: 35),
label: 'School',
// backgroundColor: Colors.purple,
),
BottomNavigationBarItem(
icon: Icon(Icons.settings,color: Colors.white,size: 35),
label: 'Settings',
//   backgroundColor: Colors.pink,
),
],
currentIndex: _selectedIndex,
selectedItemColor: Colors.white,
onTap: _onItemTapped,
),
);
}
}