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Contributing to the Flutter engine

Build Status

Things you will need

  • Linux or Mac OS X. (Windows is not yet supported.)
  • git (used for source version control).
  • An IDE. We recommend Atom.
  • An ssh client (used to authenticate with GitHub).
  • Chromium's depot_tools (make sure it's in your path). We use the gclient tool from depot_tools.
  • Python (used by many of our tools, including 'gclient').
  • curl (used by gclient sync).

You do not need Dart installed, as a Dart tool chain is automatically downloaded as part of the "getting the code" step. Similarly for the Android SDK, it's downloaded by the build step below where you run download_android_tools.py.

Getting the code and configuring your environment

  • Ensure all the dependencies described in the previous section, in particular git, ssh, depot_tools, python, and curl, are installed.
  • Fork https://github.com/flutter/engine into your own GitHub account. If you already have a fork, and are now installing a development environment on a new machine, make sure you've updated your fork so that you don't use stale configuration options from long ago.
  • If you haven't configured your machine with an SSH key that's known to github then follow the directions here: https://help.github.com/articles/generating-ssh-keys/.
  • Create an empty directory for your copy of the repository. For best results, call it engine: some of the tools assume this name when working across repositories. (They can be configured to use other names too, so this isn't a strict requirement.)
  • Create a .gclient file in the engine directory with the following contents, replacing <your_name_here> with your GitHub account name:
solutions = [
  {
    "managed": False,
    "name": "src/flutter",
    "url": "[email protected]:<your_name_here>/engine.git",
    "custom_deps": {},
    "deps_file": "DEPS",
    "safesync_url": "",
  },
]
target_os = ["android"]
  • cd engine (Change to the directory in which you put the .gclient file.)
  • gclient sync This will fetch all the source code that Flutter depends on. Avoid interrupting this script, it can leave your repository in an inconsistent state that is tedious to clean up.
  • cd src/flutter (Change to the flutter directory of the src directory that gclient sync created in your engine directory.)
  • git remote add upstream [email protected]:flutter/engine.git (So that you fetch from the master flutter/engine repository, not your clone, when running git fetch et al.)
  • cd .. (Return to the src directory that gclient sync created in your engine directory.)
  • Run ./tools/android/download_android_tools.py to add Android dependencies to your tree. You will need to run this command again if you ever run git clean -xdf, since that deletes these dependencies. (git clean -df is fine since it skips these .gitignored files.)
  • Add .../engine/src/third_party/android_tools/sdk/platform-tools to your path so that you can run the adb tool more easily. This is also required by the flutter tool, which is used to run Flutter apps.
  • Make sure you are still in the src directory that the gclient sync step created earlier.
  • If you're on Linux, run sudo ./build/install-build-deps-android.sh
  • If you're on Linux, run sudo ./build/install-build-deps.sh
  • If you're on Mac, install Oracle's Java JDK, version 1.7 or later.
  • If you're on Mac, install ant: brew install ant

Building and running the code

General

Most developers will use the flutter tool in the main Flutter repository for interacting with their built flutter/engine. To do so, the flutter tool accepts two global parameters local-engine-src-path and local-engine, a typical invocation would be: --local-engine-src-path /path/to/engine/src --local-engine=android_debug_unopt.

Additionally if you've modified dart sources in flutter/engine, you'll need to add a dependency_overrides section to point to your modified package:sky_engine and package:sky_services to the pubspec.yaml for the flutter app you're using the custom engine with. A typical example would be:

dependency_overrides:
  sky_engine:
    path: /path/to/flutter/engine/out/host_debug/gen/dart-pkg/sky_engine
  sky_services:
    path: /path/to/flutter/engine/out/host_debug/gen/dart-pkg/sky_services

Android (cross-compiling from Mac or Linux)

Run the following steps, from the src directory created in the steps above:

  • gclient sync to update your dependencies.
  • ./flutter/tools/gn --android --unoptimized to prepare your build files.
  • ninja -C out/android_debug_unopt to actually build the Android binary.

This builds a debug-enabled ("unoptimized") binary configured to run Dart in checked mode ("debug"). There are other versions, discussed on the wiki.

To run an example with your locally built binary, you'll also need to clone the main Flutter repository. See the instructions for contributing to the main Flutter repository for detailed instructions. For your convenience, the engine and flutter directories should be in the same parent directory.

Once you've got everything set up, you can run an example using your locally built engine by switching to that example's directory, running pub get to make sure its dependencies have been downloaded, and using flutter run with an explicit --local-engine-src-path pointing at the engine/src directory. Make sure you have a device connected over USB and debugging enabled on that device:

  • cd /path/to/flutter/examples/hello_world
  • pub get
  • ../../bin/flutter run --local-engine-src-path /path/to/engine/src --local-engine=android_debug_unopt

If you put the engine and flutter directories side-by-side, you can skip the tedious --local-engine-src-path option and the flutter tool will automatically determine the path.

You can also specify a particular Dart file to run if you want to run an example that doesn't have a lib/main.dart file using the -t command-line option. For example, to run the tabs.dart example in the examples/widgets directory on a connected Android device, from that directory you would run:

  • flutter run --local-engine=android_debug_unopt -t tabs.dart

If you're going to be debugging crashes in the engine, make sure you add android:debuggable="true" to the <application> element in the android/AndroidManifest.xml file for the Flutter app you are using to test the engine.

iOS (cross-compiling from Mac)

  • Make sure you have Xcode 7.3.0+ installed.
  • gclient sync to update dependencies.
  • ./flutter/tools/gn --ios --unoptimized to prepare build files.
  • ninja -C out/ios_debug_unopt to build iOS artifacts.

Once the artifacts are built, you can start using them in your application by following these steps:

  • cd /path/to/flutter/examples/hello_world
  • pub get
  • ../../bin/flutter run --local-engine-src-path /path/to/engine/src --local-engine=ios_debug_unopt
    • Depending on the configuration you built, modify the local-engine flag.
  • If you are debugging crashes in the engine, you can connect the LLDB debugger from Xcode by opening ios/Runner.xcodeproj and starting the application by clicking the Run button (CMD + R).

Desktop (Mac and Linux), for tests

  • gclient sync to update your dependencies.
  • ./flutter/tools/gn to prepare your build files.
  • ninja -C out/host_debug_unopt to build a desktop unoptimized binary.

To run the tests, you'll also need to clone the main Flutter repository. See the instructions for contributing to the main Flutter repository for detailed instructions.

Building all the builds that matter on Linux and Android

The following script will update all the builds that matter if you're developing on Linux and testing on Android and created the .gclient file in ~/dev/engine:

set -ex

cd ~/dev/engine/src/flutter
git fetch upstream
git rebase upstream/master
gclient sync
cd ..

flutter/tools/gn --unoptimized --runtime-mode=debug
flutter/tools/gn --android --unoptimized --runtime-mode=debug
flutter/tools/gn --android --unoptimized --runtime-mode=profile
flutter/tools/gn --android --unoptimized --runtime-mode=release
flutter/tools/gn --android --runtime-mode=debug
flutter/tools/gn --android --runtime-mode=profile
flutter/tools/gn --android --runtime-mode=release

cd out
ls | xargs -n 1 ninja -C

flutter update-packages --upgrade

Contributing code

We gladly accept contributions via GitHub pull requests.

To start working on a patch:

  • Make sure you are in the engine/src/flutter directory.
  • git fetch upstream
  • git checkout upstream/master -b name_of_your_branch
  • Hack away. Please peruse our style guides and design principles before working on anything non-trivial. These guidelines are intended to keep the code consistent and avoid common pitfalls.
  • git commit -a -m "<your brief but informative commit message>"
  • git push origin name_of_your_branch

To send us a pull request:

  • git pull-request (if you are using Hub) or go to https://github.com/flutter/engine and click the "Compare & pull request" button

Once you've gotten an LGTM from a project maintainer, submit your changes to the master branch using one of the following methods:

  • Wait for one of the project maintainers to submit it for you
  • Click the green "Merge pull request" button on the GitHub UI of your pull request (requires commit access)
  • git push upstream name_of_your_branch:master (requires commit access)

Then, make sure it doesn't make our tree catch fire by watching the waterfall. The waterfall runs slightly different tests than Travis, so it's possible for the tree to go red even if Travis did not. If that happens, please immediately revert your change. Do not check anything in while the tree is red unless you are trying to resolve the problem.

Please make sure all your checkins have detailed commit messages explaining the patch. If you made multiple commits for a single pull request, either make sure each one has a detailed message explaining that specific commit, or squash your commits into one single checkin with a detailed message before sending the pull request.

You must complete the Contributor License Agreement. You can do this online, and it only takes a minute. If you've never submitted code before, you must add your (or your organization's) name and contact info to the AUTHORS file.