felt?felt stands for “Flutter Engine Local Tester”. It's a cli tool that aims to make development in the Flutter web engine more productive and pleasant.
felt do?felt supports multiple commands as follows:
felt check-licenses: Checks that all Dart and JS source code files contain the correct license headers.felt test: Runs all or some tests depending on the passed arguments. It supports a watch mode for convenience.felt build: Builds the engine locally so it can be used by Flutter apps. It also supports a watch mode for more convenience.You could also run felt help or felt help <command> to get more information about the available commands and arguments.
felt?Once you have your local copy of the engine setup, it's recommended that you add /path/to/engine/src/flutter/lib/web_ui/dev to your PATH. Then you would be able to use the felt tool from anywhere:
felt check-licenses
or:
felt build --watch
If you don't want to add felt to your path, you can still invoke it using a relative path like ./web_ui/dev/felt <command>
If you are a Google employee, you can use an internal instance of Goma to parallelize your builds. Because Goma compiles code on remote servers, this option is effective even on low-powered laptops.
By default, when compiling Dart code to JavaScript, we use 8 dart2js workers.
To run all tests on Chrome. This will run both integration tests and the unit tests:
felt test
To run a specific test:
felt test test/engine/util_test.dart
To enable watch mode so that the test re-runs on every change:
felt test --watch test/engine/util_test.dart
To run unit tests only:
felt test --unit-tests-only
To run integration tests only. For now these tests are only available on Chrome Desktop browsers. These tests will fetch the flutter repository for using flutter drive and flutter pub get commands. The repository will be synced to the youngest commit older than the engine commit.
felt test --integration-tests-only
To skip cloning the flutter repository use the following flag. This flag can save internet bandwidth. However use with caution. Note the tests results will not be consistent with CIs when this flag is set. flutter command should be set in the PATH for this flag to be useful. This flag can also be used to test local Flutter changes.
felt test --integration-tests-only --use-system-flutter
To run tests on Firefox (this will work only on a Linux device):
felt test --browser=firefox
For Chrome and Firefox, the tests run on a version locked on the browser_lock.yaml. In order to use another version, add the version argument:
felt test --browser=firefox --firefox-version=70.0.1
To run tests on Safari use the following command. It works on macOS devices and it uses the Safari installed on the OS. Currently there is no option for using another Safari version.
felt test --browser=safari
One can also use Safari running in iOS Simulator for running unit tests. There are few prerequisite steps:
Please make sure that you installed Xcode.
The default version used in the tests are in browser_lock.yaml file. Install the ios version to use for simulators: Xcode > Preferences > Components
run xcrun simctl list devices. If the simulator you want is not installed use step 4.
Use felt to create a simulator:
felt create_simulator --type='iOS' --version='13.1' --device='iPhone.11.Pro'
To run tests on ios-safari use the one of the following commands:
felt test --browser=ios-safari felt test --browser=ios-safari --version='13.1' --device='iPhone.11.Pro' felt test --browser=ios-safari test/alarm_clock_test.dart
To run tests on Windows Edge use the following command. It works on Windows devices and it uses the Edge installed on the OS.
felt_windows.bat test --browser=edge
To run a single test use the following command. Note that it does not work on Windows.
felt test test/golden_tests/engine/canvas_golden_test.dart
To debug a test on Chrome:
felt test --debug test/golden_tests/engine/canvas_golden_test.dart
browser_lock.yaml contains the version of browsers we use to test Flutter for web. Versions are not automatically updated whenever a new release is available. Instead, we update this file manually once in a while.
goldens_lock.yaml refers to a revision in the https://github.com/flutter/goldens repo. Screenshot tests are compared with the golden files at that revision. When making engine changes that affect screenshots, first submit a PR to flutter/goldens updating the screenshots. Then update this file pointing to the new revision.
felt toolIf you are making changes in the felt tool itself, you need to be aware of Dart snapshots. We create a Dart snapshot of the felt tool to make the startup faster.
To make sure you are running the felt tool with your changes included, you would need to stop using the snapshot. This can be achieved through the environment variable FELT_USE_SNAPSHOT:
FELT_USE_SNAPSHOT=false felt <command>
or
FELT_USE_SNAPSHOT=0 felt <command>
Note: if FELT_USE_SNAPSHOT is omitted or has any value other than “false” or “0”, the snapshot mode will be enabled.
Since the engine code and infra recipes do not live in the same repository there are few steps to follow in order to upgrade a browser's version. For now these instructins are most relevant to Chrome.
browser_lock.yaml file.Note that for LUCI builders, for Chrome both unit and integration tests are using the same browser. (For Firefox issue
Some useful links:
Some times .dart_tool cache invalidation fails, and you‘ll end up with a cached version of felt that is not compatible with the Dart SDK that you’re using.
In that case, any invocation to felt will fail with:
Can't load Kernel binary: Invalid kernel binary format version.
The solution is to delete the cached felt.snapshot files within engine/src/flutter/lib/web_ui:
rm .dart_tool/felt.snapshot*