Sample code for meant to demonstrating writing simple widget test for your applications. Code used as part of an upcoming Flutter testing talk by @mkiisoft and myself.

app_bar_flutter_coffee_example.dart

import 'package:flutter/material.dart';

const kBreakpoint = 860.0;

extension Width on BuildContext {
  double get width => MediaQuery.of(this).size.width;
}

void main() {
  runApp(
    MaterialApp(
      home: ExamplePage(),
    ),
  );
}

class ExamplePage extends StatelessWidget {
  final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey();
  final List<String> _items = [
    "Item One",
    "Item Two",
  ];

  Widget _buildNavItem(String item) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: RaisedButton(
        color: Colors.redAccent,
        onPressed: () {},
        child: Text(
          item,
          style: TextStyle(color: Colors.white),
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: scaffoldKey,
      appBar: ExampleAppBar(
        scaffoldKey: scaffoldKey,
        menuItems: _items.map((item) => _buildNavItem(item)).toList(),
      ),
      body: Container(
        child: Center(
          child: Text("Hello"),
        ),
      ),
      drawer: context.width <= kBreakpoint
          ? Drawer(
              child: Column(
                children: _items
                    .map(
                      (item) => ListTile(
                        title: Text(item),
                      ),
                    )
                    .toList(),
              ),
            )
          : null,
    );
  }
}

class ExampleAppBar extends StatefulWidget implements PreferredSizeWidget {
  const ExampleAppBar({
    Key key,
    @required this.menuItems,
    @required this.scaffoldKey,
  })  : assert(menuItems != null),
        assert(menuItems.length > 1),
        super(key: key);

  final List<Widget> menuItems;
  final GlobalKey<ScaffoldState> scaffoldKey;

  @override
  ExampleAppBarState createState() => ExampleAppBarState();

  @override
  Size get preferredSize => Size.fromHeight(80.0);
}

class ExampleAppBarState extends State<ExampleAppBar> {
  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, dimens) {
        return Container(
          color: Colors.redAccent,
          padding: const EdgeInsets.all(12),
          child: Row(
            children: [
              if (context.width > kBreakpoint) ...widget.menuItems,
              if (context.width <= kBreakpoint)
                IconButton(
                  icon: Icon(Icons.subject),
                  color: Colors.white,
                  onPressed: () => widget.scaffoldKey.currentState.openDrawer(),
                ),
              Spacer(),
              AnimatedSwitcher(
                duration: const Duration(milliseconds: 300),
                child: InkWell(
                  onTap: () {},
                  child: Icon(
                    Icons.account_circle,
                    color: Colors.white,
                    size: 36.0,
                  ),
                ),
              )
            ],
          ),
        );
      },
    );
  }
}

app_bar_flutter_coffee_example_test.dart

import 'package:flutter/material.dart';
import 'package:flutter_sandbox/responsive_appbar/appbar_example.dart';
import 'package:flutter_test/flutter_test.dart';

Widget materialWidget(Widget child) {
  return MaterialApp(
    home: child,
  );
}

void main() {
  group(
    "AppBar",
    () {
      testWidgets(
        "AppBar item shows on large devices",
        (WidgetTester tester) async {
          final widget = MediaQuery(
            data: MediaQueryData(size: Size(900.0, 1200.0)),
            child: ExamplePage(),
          );

          // First build
          await tester.pumpWidget(materialWidget(widget));

          // Create finders
          final Finder firstItemFinder = find.text("Item One");
          final Finder secondItemFinder = find.text("Item Two");

          // Expectations
          expect(firstItemFinder, findsOneWidget);
          expect(secondItemFinder, findsOneWidget);
        },
      );

      testWidgets(
        "AppBar item does not show on small devices",
        (WidgetTester tester) async {
          final widget = MediaQuery(
            data: MediaQueryData(size: Size(600.0, 600.0)),
            child: ExamplePage(),
          );

          await tester.pumpWidget(materialWidget(widget));
          final Finder firstItemFinder = find.text("Item One");
          final Finder secondItemFinder = find.text("Item Two");
          expect(firstItemFinder, findsNothing);
          expect(secondItemFinder, findsNothing);
        },
      );
    },
  );

  group(
    "Page drawer",
    () {
      testWidgets(
        "Drawer icon is not visible on large screens",
        (WidgetTester tester) async {
          final widget = MediaQuery(
            data: MediaQueryData(size: Size(900.0, 1200.0)),
            child: ExamplePage(),
          );

          await tester.pumpWidget(materialWidget(widget));
          final Finder iconFinder = find.byIcon(Icons.subject);
          expect(iconFinder, findsNothing);
        },
      );

      testWidgets(
        "Drawer  icon is visible on large screens",
        (WidgetTester tester) async {
          final widget = MediaQuery(
            data: MediaQueryData(size: Size(600.0, 600.0)),
            child: ExamplePage(),
          );

          await tester.pumpWidget(materialWidget(widget));
          final Finder iconFinder = find.byIcon(Icons.subject);
          expect(iconFinder, findsOneWidget);
        },
      );

      testWidgets(
        "Drawer items is visible on large screens",
        (WidgetTester tester) async {
          final widget = MediaQuery(
            data: MediaQueryData(size: Size(600.0, 600.0)),
            child: ExamplePage(),
          );

          await tester.pumpWidget(materialWidget(widget));
          final Finder iconFinder = find.byIcon(Icons.subject);
          await tester.tap(iconFinder);
          await tester.pumpAndSettle();
          final Finder firstItemFinder = find.text("Item One");
          final Finder secondItemFinder = find.text("Item Two");
          expect(firstItemFinder, findsOneWidget);
          expect(secondItemFinder, findsOneWidget);
        },
      );
    },
  );
}