GoldenFileComparator class Null safety

Compares image pixels against a golden image file.

Instances of this comparator will be used as the backend for matchesGoldenFile.

Instances of this comparator will be invoked by the test framework in the TestWidgetsFlutterBinding.runAsync zone and are thus not subject to the fake async constraints that are normally imposed on widget tests (i.e. the need or the ability to call WidgetTester.pump to advance the microtask queue).

What is Golden File Testing?

The term golden file refers to a master image that is considered the true rendering of a given widget, state, application, or other visual representation you have chosen to capture.

By keeping a master reference of visual aspects of your application, you can prevent unintended changes as you develop by testing against them.

Here, a minor code change has altered the appearance of a widget. A golden file test has compared the image generated at the time of the test to the golden master file that was generated earlier. The test has identified the change, preventing unintended modifications.

SampleImage
Golden Master ImageA golden master image
DifferenceThe pixel difference
Test image after modificationTest image

Including Fonts

Custom fonts may render differently across different platforms, or between different versions of Flutter. For example, a golden file generated on Windows with fonts will likely differ from the one produced by another operating system. Even on the same platform, if the generated golden is tested with a different Flutter version, the test may fail and require an updated image.

By default, the Flutter framework uses a font called 'Ahem' which shows squares instead of characters, however, it is possible to render images using custom fonts. For example, this is how to load the 'Roboto' font for a golden test:

How to load a custom font for golden images.
testWidgets('Creating a golden image with a custom font', (tester) async {
  // Assuming the 'Roboto.ttf' file is declared in the pubspec.yaml file
  final font = rootBundle.load('path/to/font-file/Roboto.ttf');

  final fontLoader = FontLoader('Roboto')..addFont(font);
  await fontLoader.load();

  await tester.pumpWidget(const SomeWidget());

  await expectLater(
    find.byType(SomeWidget),
    matchesGoldenFile('someWidget.png'),
  );
});

The example above loads the desired font only for that specific test. To load a font for all golden file tests, the FontLoader.load() call could be moved in the flutter_test_config.dart. In this way, the font will always be loaded before a test:

Loading a custom font from the flutter_test_config.dart file.
Future<void> testExecutable(FutureOr<void> Function() testMain) async {
  setUpAll(() async {
    final fontLoader = FontLoader('SomeFont')..addFont(someFont);
    await fontLoader.load();
  });

  await testMain();
});

See also:

Implementers

Constructors

GoldenFileComparator()

Properties

hashCode int
The hash code for this object.
read-only, inherited
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited

Methods

compare(Uint8List imageBytes, Uri golden) Future<bool>
Compares the pixels of decoded png imageBytes against the golden file identified by golden.
getTestUri(Uri key, int? version) Uri
Returns a new golden file Uri to incorporate any version number with the key.
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed.
inherited
toString() String
A string representation of this object.
inherited
update(Uri golden, Uint8List imageBytes) Future<void>
Updates the golden file identified by golden with imageBytes.

Operators

operator ==(Object other) bool
The equality operator.
inherited

Static Methods

compareLists(List<int> test, List<int> master) Future<ComparisonResult>
Returns a ComparisonResult to describe the pixel differential of the test and master image bytes provided.