Flutter | 超实用简单菜单弹出框 PopupMenuButton

相信在实际开发过程当中，肯定少不了这样的功能：

点击 AppBar 右上角的按钮，弹出一个菜单供用户选择。

幸运的是，Flutter 提供给我们了一个 Widget，直接就能实现如上的效果。

还是老规矩，先看官方的说明：

Displays a menu when pressed and calls onSelected[1] when the menu is dismissed because an item was selected. The value passed to onSelected[2] is the value of the selected menu item.

One of child[3] or icon[4] may be provided, but not both. If icon[5] is provided, then PopupMenuButton[6] behaves like an IconButton[7].

If both are null, then a standard overflow icon is created (depending on the platform).

大致意思为：

当按下的时候显示一个菜单，选择了一个项目的时候会回调 onSelected，传递的值是所选菜单的值。

可以提供 child or icon ，但是不能同时提供。

如果为空，则提供一个默认的图标，取决于平台。

构造函数

看完了官方说明，再来看构造函数：

const PopupMenuButton({
  Key key,
  @required this.itemBuilder,
  this.initialValue,
  this.onSelected,
  this.onCanceled,
  this.tooltip,
  this.elevation = 8.0,
  this.padding = const EdgeInsets.all(8.0),
  this.child,
  this.icon,
  this.offset = Offset.zero,
  this.enabled = true,
}) : assert(itemBuilder != null),
assert(offset != null),
assert(enabled != null),
assert(!(child != null && icon != null)), // fails if passed both parameters
super(key: key);

这里面每一个参数应该都很好理解，就不做过多的解释了，

唯一必传的参数就是 itemBuilder，也可以看到后面的断言：

assert(!(child != null && icon != null)) 判断了 child 、icon 是否同时不为空，如果是的话就报错了。

简单 Demo

构造函数理解了，官方也提供了一个 Demo，我们来看一下运行效果：

再来看一下代码：

/// 首先定义了一个枚举
enum WhyFarther {
  harder,
  smarter,
  selfStarter,
  tradingCharter,
}

/// ------------------------------------

/// build 方法
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('PopupMenuButtonPage'),
      actions: <Widget>[
        PopupMenuButton<WhyFarther>(
          onSelected: (WhyFarther result) {
            setState(() {
              _selection = result;
            });
          },
          icon: Icon(Icons.more_vert),
          itemBuilder: (BuildContext context) => <PopupMenuEntry<WhyFarther>>[
            const PopupMenuItem<WhyFarther>(
              value: WhyFarther.harder,
              child: Text('Working a lot harder'),
            ),
            const PopupMenuItem<WhyFarther>(
              value: WhyFarther.smarter,
              child: Text('Being a lot smarter'),
            ),
            const PopupMenuItem<WhyFarther>(
              value: WhyFarther.selfStarter,
              child: Text('Being a self-starter'),
            ),
            const PopupMenuItem<WhyFarther>(
              value: WhyFarther.tradingCharter,
              child: Text('Placed in charge of trading charter'),
            ),
          ],
        ),
      ],
    ),
    body: Container(),
  );
}

解释一下逻辑：

1.首先定义了一个枚举2.然后在 AppBar 的「actions」里定义了 PopupMenuButton3.设置 icon 为 Icon(Icons.more_vert)4.itemBuilder 需返回一个 List<PopupMenuEntry<T>>5.这里传入的值就是 PopupMenuItem<WhyFarther>6.然后定义 onSelected 参数接收点击回调

这样整体的逻辑就是定义好了，运行一下：

这样就完成了一个超级简单并且实用的菜单弹出框，

其实它的实现逻辑和 DropdownButton 差不多，都是使用了 PopupRoute，

有对这方面感兴趣的同学，可以查看我以前写的文章：Flutter 源码系列：DropdownButton 源码浅析

References

[1] onSelected: https://api.flutter.dev/flutter/material/PopupMenuButton/onSelected.html
[2] onSelected: https://api.flutter.dev/flutter/material/PopupMenuButton/onSelected.html
[3] child: https://api.flutter.dev/flutter/material/PopupMenuButton/child.html
[4] icon: https://api.flutter.dev/flutter/material/PopupMenuButton/icon.html
[5] icon: https://api.flutter.dev/flutter/material/PopupMenuButton/icon.html
[6] PopupMenuButton: https://api.flutter.dev/flutter/material/PopupMenuButton-class.html
[7] IconButton: https://api.flutter.dev/flutter/material/IconButton-class.html