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 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.
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.
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 build --watch
If you don't want to add
felt to your path, you can still invoke it using a relative path like
You can speed up your builds by using more CPU cores. Pass
-j to specify the desired level of parallelism, like so:
felt build [-w] -j 100
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.
To run all tests on Chrome. This will run both integration tests and the unit tests:
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.
felt test --integration-tests-only
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
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.
If 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 achived through the environment variable
FELT_USE_SNAPSHOT=false felt <command>
FELT_USE_SNAPSHOT=0 felt <command>
FELT_USE_SNAPSHOT is omitted or has any value other than “false” or “0”, the snapshot mode will be enabled.