Flutter | 超实用简单菜单弹出框 PopupMenuButton

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。

PopupMenuButton

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: [
PopupMenuButton(
onSelected: (WhyFarther result) {
setState(() {
_selection = result;
});
},
icon: Icon(Icons.more_vert),
itemBuilder: (BuildContext context) => <PopupMenuEntry>[
const PopupMenuItem(
value: WhyFarther.harder,
child: Text(‘Working a lot harder’),
),
const PopupMenuItem(
value: WhyFarther.smarter,
child: Text(‘Being a lot smarter’),
),
const PopupMenuItem(
value: WhyFarther.selfStarter,
child: Text(‘Being a self-starter’),
),
const PopupMenuItem(
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  PopupMenuButton 3. 设置 icon 为  Icon(Icons.more_vert) 4. itemBuilder  需返回一个  List<PopupMenuEntry> 5. 这里传入的值就是  PopupMenuItem 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：

总结

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

完整代码已经传至GitHub：https://github.com/wanglu1209/WFlutterDemo

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