Flutter PopupMenuButton

Flutter series overall directory

Attributes	Explanation
itemBuilder	item children
initialValue	The initial value
onSelected	Select a callback
onCanceled	Click on the space, the callback is not selected
tooltip	prompt
elevation	Z-axis shadow
child	Child control, can not and icon are set
icon	IconButton child controls, and the child are not set

In addition to AppBar use and at the same time, you can also click on the Button displayed.

import 'package:flutter/material.dart';

class PopupMenuButtonDemo extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _PopupMenuButtonDemo();
}

class _PopupMenuButtonDemo extends State<PopupMenuButtonDemo> {


  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(
        actions: <Widget>[_popMenu()],
      ),
      body: Center(
        child: RaisedButton(
          child: Text(_selectedValue == null ? '未选择' : _selectedValue),
          onPressed: () {
            _showMenu(context);
          },
        ),
      ),
    );
  }

  String _selectedValue;

  PopupMenuButton _popMenu() {
    return PopupMenuButton<String>(
      itemBuilder: (context) => _getPopupMenu(context),
      onSelected: (String value) {
        print('onSelected');
        _selectValueChange(value);
      },
      onCanceled: () {
        print('onCanceled');
      },
//      child: RaisedButton(onPressed: (){},child: Text('选择'),),
      icon: Icon(Icons.shopping_basket),
    );
  }

  _selectValueChange(String value) {
    setState(() {
      _selectedValue = value;
    });
  }

  _showMenu(BuildContext context) {
    final RenderBox button = context.findRenderObject();
    final RenderBox overlay = Overlay.of(context).context.findRenderObject();
    final RelativeRect position = RelativeRect.fromRect(
      Rect.fromPoints(
        button.localToGlobal(Offset(0, 0), ancestor: overlay),
        button.localToGlobal(button.size.bottomRight(Offset.zero),
            ancestor: overlay),
      ),
      Offset.zero & overlay.size,
    );
    var pop = _popMenu();
    showMenu<String>(
      context: context,
      items: pop.itemBuilder(context),
      position: position,
    ).then<void>((String newValue) {
      if (!mounted) return null;
      if (newValue == null) {
        if (pop.onCanceled != null) pop.onCanceled();
        return null;
      }
      if (pop.onSelected != null) pop.onSelected(newValue);
    });
  }

  _getPopupMenu(BuildContext context) {
    return <PopupMenuEntry<String>>[
      PopupMenuItem<String>(
        value: '语文',
        child: Text('语文'),
      ),
      PopupMenuItem<String>(
        value: '数学',
        child: Text('数学'),
      ),
      PopupMenuItem<String>(
        value: '英语',
        child: Text('英语'),
      ),
      PopupMenuItem<String>(
        value: '生物',
        child: Text('生物'),
      ),
      PopupMenuItem<String>(
        value: '化学',
        child: Text('化学'),
      ),
    ];
  }
}

effect: