Golden tests for UI in Flutter - Mini App: Build & Ship

Build: Simple Counter Screen

A screen with a counter number and a button to increment it. We want to create a golden test to verify the UI looks correct.

Target UI

---------------------
|   Counter Screen   |
|                   |
|       0           |
|                   |
|  [Increment Button]|
---------------------

Display a number starting at 0 in the center

A button labeled 'Increment' below the number

Pressing the button increases the number by 1

Create a golden test that captures the initial UI

Create a golden test that captures the UI after one increment

Starter Code

Flutter

import 'package:flutter/material.dart';

class CounterScreen extends StatefulWidget {
  const CounterScreen({super.key});

  @override
  State<CounterScreen> createState() => _CounterScreenState();
}

class _CounterScreenState extends State<CounterScreen> {
  int _count = 0;

  void _increment() {
    setState(() {
      _count++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter Screen')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('$_count', style: const TextStyle(fontSize: 48)),
            const SizedBox(height: 20),
            ElevatedButton(
              onPressed: _increment,
              child: const Text('Increment'),
            ),
          ],
        ),
      ),
    );
  }
}

// TODO: Add golden tests in test/counter_screen_test.dart

Task 1

Task 2

Solution

Flutter

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:golden_toolkit/golden_toolkit.dart';
import 'package:your_app/counter_screen.dart';

void main() {
  testGoldens('CounterScreen initial state', (tester) async {
    await tester.pumpWidgetBuilder(const CounterScreen());
    await screenMatchesGolden(tester, 'counter_screen_initial');
  });

  testGoldens('CounterScreen after increment', (tester) async {
    await tester.pumpWidgetBuilder(const CounterScreen());
    await tester.tap(find.text('Increment'));
    await tester.pumpAndSettle();
    await screenMatchesGolden(tester, 'counter_screen_after_increment');
  });
}

This solution uses the golden_toolkit package to create golden tests for the CounterScreen.

First, it tests the initial UI with the counter at 0 and saves a golden snapshot named counter_screen_initial.

Second, it taps the 'Increment' button, waits for the UI to update, and saves another golden snapshot named counter_screen_after_increment.

These golden images help ensure the UI does not change unexpectedly in future code changes.

Final Result

Completed Screen

---------------------
|   Counter Screen   |
|                   |
|       1           |
|                   |
|  [Increment Button]|
---------------------

User taps 'Increment' button

Number increases from 0 to 1

UI updates to show new number

Stretch Goal

Add a golden test for dark mode appearance

💡 Hint

Wrap the widget with a Theme widget using ThemeData.dark() before capturing the golden snapshot

Practice

1. What is the main purpose of a golden test in Flutter UI testing?

easy

A. To compare the current UI rendering with a saved image to detect visual changes

B. To test the performance speed of UI animations

C. To check if the UI code compiles without errors

D. To verify the network requests made by the UI

Solution

1. Step 1: Understand the purpose of golden tests

   Golden tests compare the UI's current look to a saved image to find visual differences.

2. Step 2: Eliminate unrelated options

   Performance, compilation, and network checks are not the focus of golden tests.

3. Final Answer:

   To compare the current UI rendering with a saved image to detect visual changes -> Option A

4. Quick Check:

   Golden test purpose = Visual comparison [OK]

Hint: Golden tests check UI images, not code or speed [OK]

Common Mistakes:

• Confusing golden tests with performance tests
• Thinking golden tests check code syntax
• Assuming golden tests verify network calls

2. Which Flutter test function is used to compare a widget's UI to a golden file?

easy

A. runApp(widget).toMatchGolden('golden.png')

B. expect(widget, equals('golden.png'))

C. testWidgets('golden test', () => expect(widget, isTrue))

D. expectLater(widget, matchesGoldenFile('golden.png'))

Solution

1. Step 1: Identify the correct golden test syntax

   The function expectLater with matchesGoldenFile is the standard way to compare UI to a golden image.

2. Step 2: Check other options for correctness

   Options B, C, and D use incorrect or non-existent syntax for golden tests.

3. Final Answer:

   expectLater(widget, matchesGoldenFile('golden.png')) -> Option D

4. Quick Check:

   Golden test syntax = expectLater + matchesGoldenFile [OK]

Hint: Use expectLater with matchesGoldenFile for golden tests [OK]

Common Mistakes:

• Using expect instead of expectLater
• Missing matchesGoldenFile matcher
• Trying to call golden test on runApp directly

3. Given this Flutter golden test snippet:

testWidgets('Button golden test', (tester) async {
  await tester.pumpWidget(MyButton());
  await expectLater(find.byType(MyButton), matchesGoldenFile('button.png'));
});

What will happen if the current button UI differs visually from 'button.png'?

medium

A. The test will throw a syntax error

B. The test will fail indicating a visual mismatch

C. The test will pass regardless of UI differences

D. The test will skip without running

Solution

1. Step 1: Understand golden test behavior on mismatch

   If the rendered UI does not match the saved golden image, the test fails to alert visual changes.

2. Step 2: Verify other options

   Passing despite differences, syntax errors, or skipping are not expected behaviors here.

3. Final Answer:

   The test will fail indicating a visual mismatch -> Option B

4. Quick Check:

   Golden test mismatch = test failure [OK]

Hint: Golden test fails if UI differs from saved image [OK]

Common Mistakes:

• Expecting test to pass despite UI changes
• Confusing runtime errors with test failures
• Thinking golden tests skip on mismatch

4. Identify the error in this golden test code snippet:

testWidgets('Screen golden test', (tester) async {
  await tester.pumpWidget(MyScreen());
  expect(find.byType(MyScreen), matchesGoldenFile('screen.png'));
});

medium

A. Using expect instead of expectLater for matchesGoldenFile

B. Missing await before tester.pumpWidget

C. Incorrect widget type passed to find.byType

D. Golden file name should be a full path

Solution

1. Step 1: Check the matcher usage

   Golden tests require expectLater with matchesGoldenFile to work asynchronously.

2. Step 2: Validate other parts

   Await is correctly used with pumpWidget, widget type is correct, and relative file names are allowed.

3. Final Answer:

   Using expect instead of expectLater for matchesGoldenFile -> Option A

4. Quick Check:

   Golden test needs expectLater, not expect [OK]

Hint: Use expectLater, not expect, with matchesGoldenFile [OK]

Common Mistakes:

• Using expect instead of expectLater
• Forgetting async/await on pumpWidget
• Assuming golden file needs full absolute path

5. You want to write a golden test for a button that changes color based on theme. Which approach ensures your golden test covers both light and dark themes correctly?

hard

A. Skip golden tests for theme changes and rely on manual testing

B. Write one golden test with default theme only, assuming dark theme looks similar

C. Write separate golden tests pumping the button with light and dark theme widgets and compare to separate golden files

D. Use a single golden file and toggle theme at runtime without repumping widget

Solution

1. Step 1: Understand theme impact on UI

   Light and dark themes can change colors significantly, so testing both separately is needed.

2. Step 2: Evaluate testing approaches

   Separate tests with repumping widgets under each theme and comparing to distinct golden files ensures accurate visual checks.

3. Final Answer:

   Write separate golden tests pumping the button with light and dark theme widgets and compare to separate golden files -> Option C

4. Quick Check:

   Test each theme separately with distinct golden files [OK]

Hint: Test light and dark themes separately with different golden files [OK]

Common Mistakes:

• Assuming one golden file covers all themes
• Not repumping widget after theme change
• Skipping golden tests for theme variations