Simple 3-dot popup menu in the app bar in Flutter

main.dart

// This gist is part of the https://github.com/dartsidedev/gists repository.
// There, you'll find further information about this gist.
// https://github.com/dartsidedev/gists#3cd82881966c0e1314504e4e2e54e373

import 'package:flutter/material.dart';

void main() => runApp(PopupDemo());

class PopupDemo extends StatefulWidget {
  @override
  _PopupDemoState createState() => _PopupDemoState();
}

/// Example enum.
enum Option {
  first,
  second,
  third,
  fourth,
  fifth,
}

/// Convert the [option] to "human-readable" string.
String str(Option option) {
  switch (option) {
    case Option.first:
      return 'First!';
    case Option.second:
      return 'Second';
    case Option.third:
      return 'Third';
    case Option.fourth:
      return 'Fourth';
    case Option.fifth:
      return 'Fifth';
  }
}

/// A simple stateful widget to demo how the popup menu button in the app bar
/// works.
class _PopupDemoState extends State<PopupDemo> {
  /// The selected option in the state.
  ///
  /// Initial value is null to make the example more interesting :)
  Option? _option;

  /// Handle the option selection.
  ///
  /// In this example, we simply set the selected option to the state
  /// to keep things simple.
  ///
  /// We could also perform any asynchronous calls, push something to the
  /// navigator stack, or add some more logic.
  void _onOptionSelected(Option option) => setState(() => _option = option);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Popup Demo',
      // Debug banner is especially annoying for this example,
      // so we turn it off.
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: Text('Popup Demo'),
          actions: [
            // In this example, we use the Option enum, but the generic parameter
            // could also be strings, integers, or any other type
            // (custom class, etc...).
            //
            // The PopupMenuButton automatically uses the three-dots icon.
            // On Android, the three dots are vertical,
            // on iOS, the dots are horizontal.
            // All this is managed by Flutter automatically.
            // You can test this either with two simulators/emulators,
            // physical devices, or just simply change the platform in the Flutter
            // inspector.
            PopupMenuButton<Option>(
              // Called when the user taps on a popup menu item.
              onSelected: _onOptionSelected,
              // Called when the button is pressed to create the items to show in
              // the popup menu.
              itemBuilder: (context) {
                // The popup menu item builder receives the build context
                // and must return a list of PopupMenuEntries.
                // We could build the items manually (probably most common in
                // real apps), or use map().toList().
                //
                // The items are lazily constructed when the button is pressed.
                return [
                  for (final option in Option.values)
                    PopupMenuItem(
                      child: Text(str(option)),
                      value: option,
                    ),
                ];
              },
            ),
          ],
        ),
        // In this example, we just display the last selected option.
        body: Center(
          child: Text(
            // The initial selected option is null, so we handle that case, too.
            _option != null ? str(_option!) : 'None selected',
            style: Theme.of(context).textTheme.headline4,
          ),
        ),
      ),
    );
  }
}