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",
    "url": "git@github.com:<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 (Change to the directory that gclient sync created in your engine directory.)
  • git remote add upstream git@github.com:flutter/engine.git (So that you fetch from the master repository, not your clone, when running git fetch et al.)
  • 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/dart-sdk/dart-sdk/bin/ to your path so that you can run the pub tool more easily.
  • 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

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.
  • ./sky/tools/gn --android to prepare your build files.
  • ninja -C out/android_Debug to build an Android Debug binary.

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.

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 start with an explicit --engine-src-path pointing at the 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 start --engine-src-path /path/to/engine/src

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 start --engine-src-path /path/to/engine/src -t tabs.dart

Desktop (Mac and Linux), for tests

  • gclient sync to update your dependencies.
  • ./sky/tools/gn to prepare your build files.
  • ninja -C out/Debug to build a desktop Debug 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.

Contributing code

We gladly accept contributions via GitHub pull requests.

To start working on a patch:

  • 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

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.