Examples for https://github.com/flutter/flutter/pull/55489

few_items.dart

import 'package:flutter/material.dart';

class BasicAppBarSample extends StatefulWidget {
  @override
  _BasicAppBarSampleState createState() => _BasicAppBarSampleState();
}

class _BasicAppBarSampleState extends State<BasicAppBarSample> {
  Choice _selectedChoice = choices[0];

  void _select(Choice choice) {
    setState(() {
      _selectedChoice = choice;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Basic AppBar'),
          actions: <Widget>[
            PopupMenuButton<Choice>(
              onSelected: _select,
              initialValue: _selectedChoice,
              itemBuilder: (BuildContext context) {
                return choices
                    .skip(2)
                    .map<PopupMenuItem<Choice>>((Choice choice) {
                  return PopupMenuItem<Choice>(
                    value: choice,
                    child: Text(choice.title),
                  );
                }).toList();
              },
            ),
          ],
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: ChoiceCard(choice: _selectedChoice),
        ),
      ),
    );
  }
}

class Choice {
  const Choice({this.title, this.icon});
  final String title;
  final IconData icon;
}

const List<Choice> choices = <Choice>[
  Choice(title: 'Car', icon: Icons.directions_car),
  Choice(title: 'Bicycle', icon: Icons.directions_bike),
  Choice(title: 'Boat', icon: Icons.directions_boat),
  Choice(title: 'Bus', icon: Icons.directions_bus),
  Choice(title: 'Train', icon: Icons.directions_railway),
  Choice(title: 'Walk', icon: Icons.directions_walk),
];

class ChoiceCard extends StatelessWidget {
  const ChoiceCard({Key key, this.choice}) : super(key: key);

  final Choice choice;

  @override
  Widget build(BuildContext context) {
    final TextStyle textStyle = Theme.of(context).textTheme.headline4;
    return Card(
      color: Colors.white,
      child: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            Icon(choice.icon, size: 128.0, color: textStyle.color),
            Text(choice.title, style: textStyle),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(BasicAppBarSample());
}

many_items.dart

import 'package:flutter/material.dart';

class BasicAppBarSample extends StatefulWidget {
  @override
  _BasicAppBarSampleState createState() => _BasicAppBarSampleState();
}

class _BasicAppBarSampleState extends State<BasicAppBarSample> {
  int _selectedValue;

  void _select(int value) {
    setState(() {
      _selectedValue = value;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Basic AppBar'),
          actions: <Widget>[
            PopupMenuButton<int>(
              onSelected: _select,
              initialValue: _selectedValue,
              itemBuilder: (BuildContext context) {
                return List.generate(100, (int value) {
                  return PopupMenuItem<int>(
                    value: value,
                    child: Text('Item $value'),
                  );
                }).toList();
              },
            ),
          ],
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Center(
            child: Text('Item $_selectedValue'),
          ),
        ),
      ),
    );
  }
}

void main() {
  runApp(BasicAppBarSample());
}