Writing a golden file test for package:flutter

(This page is referenced by comments in the Flutter codebase.)

Tutorial

Write your test as a normal test, using testWidgets and await tester.pumpWidget and so on.

Put a RepaintBoundary widget around the part of the subtree that you want to verify. If you don't, the output will be a 2400x1800 image, since the tests by default use an 800x600 viewport with a device pixel ratio of 3.0.

Add an expectation along the following lines:

  await expectLater(
    find.byType(RepaintBoundary),
    matchesGoldenFile(
      'test_name.subtest.subfile.png',
      version: 0,
    ),
    skip: !Platform.isLinux, // explanation
  );

The first argument is a finder that specifies the widget to screenshot.

The arguments to matchesGoldenFile are the filename for the screen shot and a version number. For the filename, the part up to the first dot should exactly match the test filename (e.g. if your test is widgets/foo_bar_test.dart, use foo_bar). The subtest part should be unique to this testWidgets entry, and the part after that should be unique within the testWidgets entry. This allows each file to have multiple testWidgets tests each with their own namespace for the images, and then allows for disambiguation within each test in case there are multiple screen shots per test. The version number is used to differentiate historical golden files and is incorporated into the file. When we update a golden file, a new file is created rather than updating the old one, and the version number provides distinction between the old and the new.

The skip argument is used if we have slight rendering differences across platforms, as we don't yet have the infrastructure in place to allow the developer to update their golden files on all three supported host platforms. When you find that the golden files differ from platform to platform (by finding that the precommit tests on Windows or Mac fail, typically!), add in the skip line as above, then describe the difference in the comment. We chose Linux as the host platform upon which we run golden file tests. If you don't have a Linux box and need someone to generate the goldens for you, cc @Hixie on your PR.

Once you have written your test, run flutter test --update-goldens test/foo/bar_test.dart in the flutter package directory (where the filename is the relative path to your new test). This will update the images in bin/cache/pkg/goldens/packages/flutter/test/; the directories below that will match the hierarchy of the directories in the test directory of the flutter package. Verify that the images are what you expect; update your test and repeat this step until you are happy.

Run flutter test again, but without the --update-goldens flag, to verify that the goldens match. If they don't, check that you're using the right filenames, and that they're all unique. It's not uncommon to copy-paste the expectation lines and so accidentally have duplicate golden filenames.

Commit the images to the goldens repo. You do this by going into the bin/cache/pkg/goldens/ directory, which is actually a git checkout of our goldens repo, using git add for all your new files, using git commit -a to create a commit (in the commit message, link to the bug you are fixing, if possible, e.g. "Goldens for https://github.com/flutter/flutter/issues/17262"), and then pushing the files to the repo using git push [email protected]:flutter/goldens.git master.

This updates the goldens repo but does not make the images actually available yet. To make them available, you then update the bin/internal/goldens.version file in your Flutter repo to the hash of the commit you just pushed to the goldens repo.

Now run flutter test again, without the --update-goldens flag, to verify that the goldens match the uploaded images.

If they do, you are ready to submit your PR for review. The reviewer should also verify your golden files, so make sure to point to your goldens repo commit in your PR description. If you find the golden tests fail on some platforms, see the notes above about adding a skip line.

Updating a golden file

If renderings change, then rather than replacing the golden file in-situ, create new files with new names (incrementing the version number), and update the tests to point to those. Then, add the old file names to the README file.

This allows multiple people to contribute simultaneously without conflicting with each other.

Once your main PR has landed, please come back and delete the obsolete files listed in the README.

Summary of rules for the golden repo

• Commit messages with updates to the golden files must be of the form Goldens for https://github.com/flutter/flutter/issues/123456

• Don't update files, create new ones with an incremented version number.

• Delete obsolete files once your PR has landed.