相信在實(shí)際開(kāi)發(fā)過(guò)程當(dāng)中，肯定少不了這樣的功能：

點(diǎn)擊 AppBar 右上角的按鈕，彈出一個(gè)菜單供用戶選擇。

幸運(yùn)的是，F(xiàn)lutter 提供給我們了一個(gè) Widget，直接就能實(shí)現(xiàn)如上的效果。

PopupMenuButton

還是老規(guī)矩，先看官方的說(shuō)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).

大致意思為：

當(dāng)按下的時(shí)候顯示一個(gè)菜單，選擇了一個(gè)項(xiàng)目的時(shí)候會(huì)回調(diào) onSelected ，傳遞的值是所選菜單的值。

可以提供 child or  icon ，但是不能同時(shí)提供。

如果為空，則提供一個(gè)默認(rèn)的圖標(biāo)，取決于平臺(tái)。

構(gòu)造函數(shù)

看完了官方說(shuō)明，再來(lái)看構(gòu)造函數(shù)：

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 parameterssuper(key: key);

這里面每一個(gè)參數(shù)應(yīng)該都很好理解，就不做過(guò)多的解釋了，

唯一必傳的參數(shù)就是 itemBuilder ，也可以看到后面的斷言：

assert(!(child != null && icon != null)) 判斷了 child 、icon 是否同時(shí)不為空，如果是的話就報(bào)錯(cuò)了。

簡(jiǎn)單 Demo

構(gòu)造函數(shù)理解了，官方也提供了一個(gè) Demo，我們來(lái)看一下運(yùn)行效果：

再來(lái)看一下代碼：

/// 首先定義了一個(gè)枚舉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(), );}

解釋一下邏輯：

1. 首先定義了一個(gè)枚舉

2. 然后在  AppBar  的「actions」里定義了  PopupMenuButton

3. 設(shè)置 icon 為  Icon(Icons.more_vert)

4. itemBuilder  需返回一個(gè)  List<PopupMenuEntry<T>>

5. 這里傳入的值就是  PopupMenuItem<WhyFarther>

6. 然后定義  onSelected  參數(shù)接收點(diǎn)擊回調(diào)

這樣整體的邏輯就是定義好了，運(yùn)行一下：

總結(jié)

這樣就完成了一個(gè)超級(jí)簡(jiǎn)單并且實(shí)用的菜單彈出框，

其實(shí)它的實(shí)現(xiàn)邏輯和 DropdownButton 差不多，都是使用了  PopupRoute ，

有對(duì)這方面感興趣的同學(xué)，可以查看我以前寫(xiě)的文章： Flutter 源碼系列：DropdownButton 源碼淺析

完整代碼已經(jīng)傳至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

總結(jié)

以上所述是小編給大家介紹的Flutter 超實(shí)用簡(jiǎn)單菜單彈出框 PopupMenuButton功能,希望對(duì)大家有所幫助，如果大家有任何疑問(wèn)請(qǐng)給我留言，小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)武林網(wǎng)網(wǎng)站的支持！
如果你覺(jué)得本文對(duì)你有幫助，歡迎轉(zhuǎn)載，煩請(qǐng)注明出處，謝謝！