在 GoRoute 中使用 NavigationBar

前言

在App 中通常会把主要的几个页面放在下方icon，让使用者能够方便操作，这个元件在flutter 中称为BottomNavigationBar。
而GoRouter则是Flutter 官方所提供的套件，可以用来整合整个专案的路由。
当这两个功能整合在一起的时候，一个不小心呈现出来的效果就会差很多。

准备：先创建一个新的项目 叫做my_app！

flutter create my_app
cd my_app

加入BottomNavigationBar

在MyHomePage元件中找到build的方法，在Scaffold 加上bottomNavigationBar的属性，加上两个有icon 的元件。
之后执行指令flutter run就可以看到：画面的下方有一个icon 的区块，显示刚刚所加入的search 和add。

@override
Widget build(BuildContext context) {
  return Scaffold(
    bottomNavigationBar: BottomNavigationBar(
      items: const [
        BottomNavigationBarItem(
          icon: Icon(Icons.search),
          label: 'search',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.add),
          label: 'add',
        ),
      ],
    ),
    appBar: AppBar(
      title: Text(widget.title),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          const Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '$_counter',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    ),
  );
}

加入GoRouter

接着要来加入GoRouter这个插件。

定义Router

定义两个route，会使用同一个元件，但是透过传入不同title 的内容来做识别。
找到MyApp 这个元件，在build 里面加上这段。

var router = GoRouter(
  initialLocation: '/page1',
  routes: [
    GoRoute(
      path: '/page1',
      name: 'page1',
      builder: (BuildContext context, GoRouterState state) =>
          const MyHomePage(
        title: 'search',
      ),
    ),
    GoRoute(
      path: '/page2',
      name: 'page2',
      builder: (BuildContext context, GoRouterState state) =>
          const MyHomePage(
        title: 'add',
      ),
    ),
  ],
);

接着要调整MyApp 的 return 的行为：原本是用MaterialApp，现在要来改用MaterialApp.router才能加上路由的设定。

return MaterialApp.router(
  title: 'Flutter Demo',
  theme: ThemeData(
    primarySwatch: Colors.blue,
  ),
  // 把原本的 home 属性刪除并加上这段
  routeInformationProvider: router.routeInformationProvider,
  routeInformationParser: router.routeInformationParser,
  routerDelegate: router.routerDelegate,
);

最后 回去调整BottomNavigationBar 的行为，监听onTap的事件，来达到切换页面的效果。

bottomNavigationBar: BottomNavigationBar(
items: const [
  BottomNavigationBarItem(
    icon: Icon(Icons.search),
    label: 'search',
  ),
  BottomNavigationBarItem(
    icon: Icon(Icons.add),
    label: 'add',
  ),
],
// 监听点击事件
onTap: (index) => context.go('/page${index + 1}'),

改好以后重新启动，即可看到效果，整个页面包含NavigationBar 随着导航的切换也都会跟着重新载入（请先忽略点选了第二页但是icon 还是停留在第一页的问题）。

使用ShellRoute

根据GoRouter 的介绍，当有需要BottomNavigationBar 的时候，应该要采用ShellRoute的架构，就能够只有内容重新载入。
接着就要动一个比较大的工程，要将Scaffold 整个拉出来放到ShellRoute 中。

建立一个新的组件，就叫它ScaffoldWithBottomNavBar，这里为方便 我就不摘取核心代码了，偷个懒直接一个main.dart 到底。

class ScaffoldWithBottomNavBar extends StatefulWidget {
  const ScaffoldWithBottomNavBar({Key? key, required this.child})
      : super(key: key);
  final Widget child;

  @override
  State<ScaffoldWithBottomNavBar> createState() =>
      _ScaffoldWithBottomNavBarState();
}

class _ScaffoldWithBottomNavBarState extends State<ScaffoldWithBottomNavBar> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        items: const [
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: 'search',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.add),
            label: 'add',
          ),
        ],
        onTap: (index) => context.go('/page${index + 1}'),
      ),
      // 內容由外面來決定
      body: widget.child,
    );
  }
}

然后 把这个元件加到路由的定义中。

var router = GoRouter(
  initialLocation: '/page1',
  routes: [
    // 在原本的路由前面加上 ShellRoute 并且回传刚刚所建立的元件
    ShellRoute(
      builder: ((context, state, child) =>
          ScaffoldWithBottomNavBar(child: child)),
      routes: [
        GoRoute(
          path: '/page1',
          name: 'page1',
          builder: (BuildContext context, GoRouterState state) =>
              const MyHomePage(
            title: 'search',
          ),
        ),
        GoRoute(
          path: '/page2',
          name: 'page2',
          builder: (BuildContext context, GoRouterState state) =>
              const MyHomePage(
            title: 'add',
          ),
        ),
      ],
    ),
  ],
);

最后 回到MyHomePage元件将原本加关于 BottomNavigationBar 代码移除掉（因为前面已经将其抽出去放到ShellRoute 中）。

@override
Widget build(BuildContext context) {
  return Scaffold(
  	// 移除 bottomNavigationBar 属性
    appBar: AppBar(
      title: Text(widget.title),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          const Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '$_counter',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    ),
  );
}

都改完后可以看到，BottomNavigationBar 的区块是固定的了，点击切换只有内容页是不同。

结论

在web 上会很习惯这种功能的存在，转到flutter 时，一时间没找到也没特别注意到问题，后来是测试的时候才被点出来😅。
一个元件使用上的小地方，用错方法就会让使用者看起来没有那么舒服！

最后附上完整的程式码。

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

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    var router = GoRouter(
      initialLocation: '/page1',
      routes: [
        ShellRoute(
          builder: ((context, state, child) =>
              ScaffoldWithBottomNavBar(child: child)),
          routes: [
            GoRoute(
              path: '/page1',
              name: 'page1',
              builder: (BuildContext context, GoRouterState state) =>
                  const MyHomePage(
                title: 'search',
              ),
            ),
            GoRoute(
              path: '/page2',
              name: 'page2',
              builder: (BuildContext context, GoRouterState state) =>
                  const MyHomePage(
                title: 'add',
              ),
            ),
          ],
        ),
      ],
    );

    return MaterialApp.router(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      routeInformationProvider: router.routeInformationProvider,
      routeInformationParser: router.routeInformationParser,
      routerDelegate: router.routerDelegate,
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class ScaffoldWithBottomNavBar extends StatefulWidget {
  const ScaffoldWithBottomNavBar({Key? key, required this.child})
      : super(key: key);
  final Widget child;

  @override
  State<ScaffoldWithBottomNavBar> createState() =>
      _ScaffoldWithBottomNavBarState();
}

class _ScaffoldWithBottomNavBarState extends State<ScaffoldWithBottomNavBar> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        items: const [
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: 'search',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.add),
            label: 'add',
          ),
        ],
        onTap: (index) => context.go('/page${index + 1}'),
      ),
      body: widget.child,
    );
  }
}

如果上边完整例子的你不喜欢，我再来一个更加通俗易懂的完整例子

最后附上完整的代码。
```dart
import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';

// 定义标签栏和标签页
var _barItems = <BottomNavigationBarItem>[
  const BottomNavigationBarItem(
    icon: Icon(Icons.home),
    label: '首页',
  ),
  const BottomNavigationBarItem(
    icon: Icon(Icons.account_circle),
    label: '我的',
  ),
];

// 定义路由路径
var _routes = <String>[
  '/home',
  '/about',
];

class ScaffoldWithNavBar extends StatefulWidget {
  const ScaffoldWithNavBar({super.key, required this.child});
  final Widget child;

  @override
  State<ScaffoldWithNavBar> createState() => _ScaffoldWithNavBarState();
}

class _ScaffoldWithNavBarState extends State<ScaffoldWithNavBar> {
  int currentIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: currentIndex,
        items: _barItems,
        onTap: (index) {
          setState(() {
            currentIndex = index;
          });

          context.go(_routes[index]);
        },
      ),
      body: widget.child, // 这里应该是从路由中传入的页面
    );
  }
}

// GoRouter配置
final GoRouter _router = GoRouter(
  initialLocation: '/home',
  routes: [
    ShellRoute(
      builder: (context, state, child) {
        return ScaffoldWithNavBar(child: child);
      },
      routes: [
        GoRoute(
          path: '/home',
          builder: (context, state) {
            return const HomeScreen();
          },
        ),
        GoRoute(
          path: '/about',
          builder: (context, state) {
            return const AboutScreen();
          },
        ),
      ],
    ),
  ],
);

void main() {
  runApp(MaterialApp.router(
    routerConfig: _router,
  ));
}

class HomeScreen extends StatelessWidget {
  const HomeScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('首页')),
      body: const Center(child: Text('这是首页 页面')),
    );
  }
}

class AboutScreen extends StatelessWidget {
  const AboutScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('关于')),
      body: const Center(child: Text('这是关于页 页面')),
    );
  }
}

注意： 如果切换底部导航切换页面的时候出现一瞬黑屏闪烁，那是官方bug，将flutter 升级到 v3.27 及其以上就好了。

---

参考
[flutter] 在GoRoute 中使用NavigationBar