# 04.BottomNavigationBar

### 前言

开发一个应用，界面的开始大部分情况下都是一个`底部标签控制器`。而在Flutter中就是 `BottomNavigationBar` 。

### 一、 创建 BottomNavigationBar

这里我想创建一个类似微信的底部标签，Widget内我会这样写：

```
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        items: const [
          BottomNavigationBarItem(
            icon: Icon(Icons.chat),
            label: '微信',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.contact_mail),
            label: '通讯录',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.nature_people),
            label: '发现',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.settings),
            label: '我',
          ),
        ],
      ),
    );
  }
```

#### 显示问题

看看效果：

![1](/files/HDTIvzLpeWvU6GYA9j7k)

怎么什么都没有？仔细看下其实都是白色的，我们设置一下颜色看看：

```
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        selectedItemColor: Colors.green,
        unselectedItemColor: Colors.grey,
        ...
      ),
    );
  }
```

![2](/files/pHHAmwAJycSRNARJAntZ)

但是选中的标签的样式会比较突出，怎么改成我们熟悉的样式呢？

```
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        type: BottomNavigationBarType.fixed,
        selectedItemColor: Colors.green,
        unselectedItemColor: Colors.grey,
        ...
      ),
    );
  }
```

![3](/files/Qp78UBLViHhv5HylAMtG)

### 二、 选中标签

这里明显是需要 StatefulWidget 的。通过 `BottomNavigationBar: onTap` 修改选中的标签

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

  @override
  State<StatefulWidget> createState() {
    return _MyHomePageDataSource();
  }
}

class _MyHomePageDataSource extends State<MyHomePage> {
  int _selectedTab = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavigationBar(
        ...
        currentIndex: _selectedTab,
        onTap: (idx) {
          setState(() {
            _selectedTab = idx;
          });
        },
        ...
      ),
    );
  }
}
```

### 三、 页面切换

选中了标签我们也需要切换到对应的页面才行，而具体显示哪个页面是由 Scaffold 的 body 控制的。

```
class _MyHomePageDataSource extends State<MyHomePage> {
  int _selectedTab = 0;
  final List<Widget> _pages = [MessagePage(), ContactsPage(), MommentPage(), MinePage()];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _pages[_selectedTab],// 切换页面
      bottomNavigationBar: BottomNavigationBar(
        ...
        currentIndex: _selectedTab,
        onTap: (idx) {
          setState(() {
            _selectedTab = idx; // 切换选中标签
          });
        },
        ...
      ),
    );
  }
}
```


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