This directory contains the source code for the Web Engine.
If you are setting up a workspace for the first time, start by following the instructions at Setting up the Engine development environment. In addition, it is useful to add the following to your PATH environment variable:
ENGINE_ROOT/src/flutter/lib/web_ui/dev, so you can run the felt command from anywhere.FLUTTER_ROOT/bin, so you can run dart and flutter commands from anywhere.feltfelt (stands for “Flutter Engine Local Tester”) is a command-line tool that aims to make development in the Flutter web engine more productive and pleasant.
To tell felt to do anything you call felt SUBCOMMAND, where SUBCOMMAND is one of the available subcommands, which can be listed by running felt help. To get help for a specific subcommand, run felt help SUBCOMMAND.
The most useful subcommands are:
felt build - builds a local Flutter Web engine ready to be used by the Flutter framework. To use the local engine build, pass --local-engine=host_debug_unopt to the flutter command, or to dev/bots/test.dart when running a web shard, such as web_tests.felt test - runs web engine tests. By default, this runs all tests using Chromium. Passing one or more paths to specific tests would run just the specified tests. Run felt help test for more options.build and test take the --watch option, which automatically reruns the subcommand when a source file changes. This is handy when you are iterating quickly.
Builds the web engine, the runs a Flutter app using it:
felt build cd path/to/some/app flutter --local-engine=host_debug_unopt run -d chrome
Runs all tests in Chromium:
felt test
Runs a specific test:
felt test test/engine/util_test.dart
Runs multiple specific tests:
felt test test/engine/util_test.dart test/alarm_clock_test.dart
Enable watch mode so that the test re-runs every time a source file changes:
felt test --watch test/engine/util_test.dart
Runs tests in Firefox (requires a Linux computer):
felt test --browser=firefox
Chromium and Firefox support debugging tests using the browser's developer tools. To run tests in debug mode add --debug to the test command, e.g.:
felt test --debug --browser=firefox test/alarm_clock_test.dart
Concurrency of various build steps can be configured via environment variables:
FELT_DART2JS_CONCURRENCY specifies the number of concurrent dart2js processes used to compile tests. Default value is 8.FELT_TEST_CONCURRENCY specifies the number of tests run concurrently. Default value is 10.If you are a Google employee, you can use an internal instance of Goma (go/ma) to parallelize your ninja builds. Because Goma compiles code on remote servers, this option is particularly effective for building on low-powered laptops.
Chromium, Firefox, and Safari for iOS are version-locked using the browser_lock.yaml configuration file. Safari for macOS is supplied by the computer's operating system. Tests can be run in Edge locally, but Edge is not enabled on LUCI. Chromium is used as a proxy for Chrome, Edge, and other Chromium-based browsers.
Changing parameters in the browser lock is effective immediately when running tests locally. To make changes effective on LUCI follow instructions in [Rolling Browsers][#rolling-browsers].
When running tests on LUCI using Chromium, LUCI uses the version of Chromium fetched from CIPD.
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.
Chromium is an independent project that gets rolled into Flutter manually, and as needed. Flutter consumes a pre-built Chromium version from chromium.org. When a new version of Chromium (check here) is needed, follow these steps to roll the new version:
depot_tools installed (if you are regularly hacking on the engine code, you probably do).cipd auth-login and follow instructions (this step requires sufficient privileges; contact #hackers-infra-🌡 on Flutter's Discord server).dev/browser_lock.yaml and update the following values under chrome:Windows, Mac and Linux to the branch_base_positions given in this table. (Pick from linux, mac and win as os, and the stable channel.)version to a string composed of the Major Version of the browser, and the number of times that major version has been uploaded to CIPD. For example, start with '99' for version 99.0.4844.51 of Chromium, and update to '99.1', '99.2' and so on if you need to upload newer bundles of the same major version. (This is required because tags can't be repeated in CIPD).dart dev/browser_roller.dart and make sure it completes successfully. The script uploads the specified versions of Chromium (and Chromedriver) to the right locations in CIPD: Chrome, Chromedriver.If you have questions, contact the Flutter Web team on Flutter Discord on the #hackers-web-🌍 channel.
The script has the following command-line options:
--dry-run - The script will stop before uploading artifacts to CIPD. The location of the data will be reported at the end of the script, if the script finishes successfullyThe output of the script will be visible in /tmp/browser-roll-RANDOM_STRING--verbose - Greatly increase the amount of information printed to stdout by the script.Try the following!
dart ./dev/browser_roller.dart --dry-run --verbose
In general, the manual process goes like this:
browser_lock.yaml file.Resources:
CanvasKit is versioned separately from Skia and rolled manually. Flutter consumes a pre-built CanvasKit provided by the Skia team, currently hosted on unpkg.com. When a new version of CanvasKit is available (check https://www.npmjs.com/package/canvaskit-wasm or consult the Skia team directly), follow these steps to roll to the new version:
depot_tools installed (if you are regularly hacking on the engine code, you probably do).cipd auth-login and follow instructions (this step requires sufficient privileges; file a github infra ticket queue issue: https://github.com/flutter/flutter/wiki/Infra-Ticket-Queue to get access)dev/canvaskit_lock.yaml and update the value of canvaskit_version to the new version.dart dev/canvaskit_roller.dart and make sure it completes successfully. The script uploads the new version of CanvasKit to the flutter/web/canvaskit_bundle CIPD package, and writes the CIPD package instance ID to the DEPS file.gclient sync and do a clean build to test that the new version is picked up.If you have questions, contact the Flutter Web team on Flutter Discord on the #hackers-web-🌍 channel.
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.
canvaskit_lock.yaml locks the version of CanvasKit for tests and production use.
Sometimes .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 under lib/web_ui:
rm .dart_tool/felt.snapshot*
felt tool itselfIf 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 run felt from sources, disable the snapshot using the FELT_USE_SNAPSHOT environment variable:
FELT_USE_SNAPSHOT=false felt <command>
To build CanvasKit locally, you must first set up your gclient config to activate the Emscripten SDK, which is the toolchain used to build CanvasKit. To do this, replace the contents of your .gclient file at the root of the project (i.e. in the parent directory of the src directory) with:
solutions = [
{
"managed": False,
"name": "src/flutter",
"url": "git@github.com:<your_username_here>/engine.git",
"custom_deps": {},
"deps_file": "DEPS",
"safesync_url": "",
"custom_vars": {
"download_emsdk": True,
},
},
]
Now run gclient sync and it should pull in the Emscripten SDK and activate it.
To build CanvasKit with felt, run:
felt build --build-canvaskit
This will build CanvasKit in out/wasm_debug. If you now run
felt test
it will detect that you have built CanvasKit and use that instead of the one from CIPD to run the tests against.
The version of the Emscripten SDK should be kept up to date with the version used in the Skia build. That version can be found in third_party/skia/bin/activate-emsdk. It will probably also be necessary to roll the dependency on third_party/emsdk in DEPS to the same version as in third_party/skia/DEPS.
Once you know the version for the Emscripten SDK, change the line in tools/activate_emsdk.py which defines EMSDK_VERSION to match Skia.