Contributing to Flutter
=======================

[![Build Status](https://api.cirrus-ci.com/github/flutter/flutter.svg)](https://cirrus-ci.org/flutter/flutter)

_See also: [Flutter's code of conduct](https://flutter.io/design-principles/#code-of-conduct)_

Welcome
-------

We gladly accept contributions via GitHub pull requests.

Please become familiar with our
[style guide](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo) and
[design philosophy](https://flutter.io/design-principles/). These guidelines are intended to
keep the code consistent and avoid common pitfalls, and being familiar with them will
make everything much easier for you. If you have questions about our processes or are looking
for random tips and tricks, you may be interested in the [engine wiki](https://github.com/flutter/engine/wiki) and [framework wiki](https://github.com/flutter/flutter/wiki).

This document will introduce you to the basic steps for developing for the Flutter framework (Dart).
If you're interested in developing for the Flutter engine (C++, Java, Objective C), please
switch to [the engine repo's `CONTRIBUTING.md` document](https://github.com/flutter/engine/blob/master/CONTRIBUTING.md).

If you have an itch, work on that. If you are just looking for something good to start with, consider
[the issues marked "easy fix"](https://github.com/flutter/flutter/issues?q=is%3Aopen+is%3Aissue+label%3A%22easy+fix%22+sort%3Areactions-%2B1-desc) in our issues list.

Things you will need
--------------------

 * Linux, Mac OS X, or Windows
 * git (used for source version control).
 * An IDE. We recommend [Android Studio with the Flutter plugin](https://flutter.io/using-ide/).
 * An ssh client (used to authenticate with GitHub).
 * Python (used by some of our tools).
 * The Android platform tools (see [Issue #55](https://github.com/flutter/flutter/issues/55)
   about downloading the Android platform tools automatically).
   _If you're also working on the Flutter engine, you can use the
   copy of the Android platform tools in
   `.../engine/src/third_party/android_tools/sdk/platform-tools`._
   - Mac: `brew cask install android-platform-tools`
   - Linux: `sudo apt-get install android-tools-adb`

Getting the code and configuring your environment
-------------------------------------------------

 * Ensure all the dependencies described in the previous section, in particular
   git, ssh, and python are installed. Ensure that `adb`
   (from the Android platform tools) is in your path (e.g.,
   that `which adb` prints sensible output).
 * Fork `https://github.com/flutter/flutter` 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/.
 * `git clone git@github.com:<your_name_here>/flutter.git`
 * `cd flutter`
 * `git remote add upstream git@github.com:flutter/flutter.git` (So that you
   fetch from the master repository, not your clone, when running `git fetch`
   et al.)
 * Add this repository's `bin` directory to your path. That will let you use the
   `flutter` command in this directory more easily.
 * Run `flutter update-packages` This will fetch all the Dart packages that
   Flutter depends on. You can replicate what this script does by running
   `pub get` in each directory that contains a `pubspec.yaml` file.
 * If you plan on using IntelliJ as your IDE, then also run
   `flutter ide-config --overwrite` to create all of the IntelliJ configuration
   files so you can open the main flutter directory as a project and run examples
   from within the IDE.

Running the examples
--------------------

To run an example, switch to that example's directory, and use `flutter run`.
Make sure you have an emulator running, or a device connected over USB and
debugging enabled on that device.

 * `cd examples/hello_world`
 * `flutter run`

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 `widgets/spinning_square.dart` example in the [examples/layers](examples/layers)
directory on a connected Android device, from that directory you would run:
`flutter run -t widgets/spinning_square.dart`

When running code from the examples directory, any changes you make to the
example code, as well as any changes to Dart code in the
[packages/flutter](packages/flutter) directory and subdirectories, will
automatically be picked when you relaunch the app.  You can do the same for your
own code by mimicking the `pubspec.yaml` files in the `examples` subdirectories.

Running the analyzer
--------------------

When editing Flutter code, it's important to check the code with the
analyzer. There are two main ways to run it. In either case you will
want to run `flutter update-packages` first, or you will get bogus
error messages about core classes like Offset from `dart:ui`.

For a one-off, use `flutter analyze --flutter-repo`. This uses the `analysis_options.yaml` file
at the root of the repository for its configuration.

For continuous analysis, use `flutter analyze --flutter-repo --watch`. This uses normal
`analysis_options.yaml` files, and they can differ from package to package.

If you want to see how many members are missing dartdocs, you should use the first option,
providing the additional command `--dartdocs`.

If you omit the `--flutter-repo` option you may end up in a confusing state because that will
assume you want to check a single package and the flutter repository has several packages.


Running the tests
-----------------

To automatically find all files named `_test.dart` inside a package's `test/` subdirectory, and
run them inside the flutter shell as a test, use the `flutter test` command, e.g:

 * `cd examples/stocks`
 * `flutter test`

Individual tests can also be run directly, e.g. `flutter test lib/my_app_test.dart`

Flutter tests use [package:flutter_test](https://github.com/flutter/flutter/tree/master/packages/flutter_test)
which provides flutter-specific extensions on top of [package:test](https://pub.dartlang.org/packages/test).

`flutter test` runs tests inside the flutter shell. To debug tests in Observatory, use the `--start-paused`
option to start the test in a paused state and wait for connection from a debugger. This option lets you
set breakpoints before the test runs.

To run analysis and all the tests for the entire Flutter repository, the same way that Cirrus runs them, run `dart dev/bots/test.dart` and `dart dev/bots/analyze.dart`.

If you've built [your own flutter engine](#working-on-the-engine-and-the-framework-at-the-same-time), you
can pass `--local-engine` to change what flutter shell `flutter test` uses. For example,
if you built an engine in the `out/host_debug_unopt` directory, you can pass
`--local-engine=host_debug_unopt` to run the tests in that engine.

Flutter tests are headless, you won't see any UI. You can use
`print` to generate console output or you can interact with the DartVM
via observatory at [http://localhost:8181/](http://localhost:8181/).

Adding a test
-------------

To add a test to the Flutter package, create a file whose name
ends with `_test.dart` in the `packages/flutter/test` directory. The
test should have a `main` function and use the `flutter_test` package.

Working with flutter tools
--------------------------

The flutter tool itself is built when you run `flutter` for the first time and each time
you run `flutter upgrade`. If you want to alter and re-test the tool's behavior itself,
locally commit your tool changes in git and the tool will be rebuilt from Dart sources
in `packages/flutter_tools` the next time you run `flutter`.

Alternatively, delete the `bin/cache/flutter_tools.snapshot` file. Doing so will
force a rebuild of the tool from your local sources the next time you run `flutter`.

flutter_tools' tests run inside the Dart command line VM rather than in the
flutter shell. To run the tests, ensure that no devices are connected,
then navigate to `flutter_tools` and execute:

```shell
../../bin/cache/dart-sdk/bin/pub run test -j1
```

The pre-built flutter tool runs in release mode with the observatory off by default.
To enable debugging mode and the observatory on the `flutter` tool, uncomment the
`FLUTTER_TOOL_ARGS` line in the `bin/flutter` shell script.

Using git
---------

To start working on a patch:

 * `git fetch upstream`
 * `git checkout upstream/master -b name_of_your_branch`
 * Hack away.
 * `git commit -a -m "<your informative commit message>"`
 * `git push origin name_of_your_branch`

To send us a pull request:

* `git pull-request` (if you are using [Hub](http://github.com/github/hub/)) or
  go to `https://github.com/flutter/flutter` and click the
  "Compare & pull request" button

Please make sure all your checkins have detailed commit messages explaining the patch.

Once you've gotten an LGTM from a project maintainer and once your PR has received
the green light from all our automated testing (running on Cirrus, etc), and once
the tree is green (see the [design principles](https://flutter.io/design-principles/)
document for more details), 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)

You must complete the
[Contributor License Agreement](https://cla.developers.google.com/clas).
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](AUTHORS) file.

We grant commit access to people who have gained our trust and demonstrated
a commitment to Flutter.

Tools for tracking and improving test coverage
----------------------------------------------

We strive for a high degree of test coverage for the Flutter framework. We use
Coveralls to [track our test coverage](https://coveralls.io/github/flutter/flutter?branch=master).
You can download our current coverage data from cloud storage and visualize it
in Atom as follows:

 * Install [Atom](https://atom.io/).
 * Install the [lcov-info](https://atom.io/packages/lcov-info) package for Atom.
 * Open the `packages/flutter` folder in Atom.
 * Open a Dart file in the `lib` directory an type `Ctrl+Alt+C` to bring up the
   coverage data.

If you don't see any coverage data, check that you have an `lcov.info` file in
the `packages/flutter/coverage` directory. It should have been downloaded by the
`flutter update-packages` command you ran previously.

If you want to iterate quickly on improving test coverage, consider using this
workflow:

 * Open a file and observe that some line is untested.
 * Write a test that exercises that line.
 * Run `flutter test --merge-coverage path/to/your/test_test.dart`.
 * After the test passes, observe that the line is now tested.

This workflow merges the coverage data from this test run with the base coverage
data downloaded by `flutter update-packages`.

See [issue 4719](https://github.com/flutter/flutter/issues/4719) for ideas about
how to improve this workflow.

Communication
-------------

If you would like to chat to other people who work on Flutter, consider joining the
https://gitter.im/flutter/contributors chat channel. (We also have a [general chat
channel](https://gitter.im/flutter/flutter) for people who aren't working on Flutter
but who use Flutter.)

Working on the engine and the framework at the same time
--------------------------------------------------------

You can work both with this repository (flutter.git) and the Flutter
[engine repository](https://github.com/flutter/engine) at the same time using
the following steps.

1. Follow the instructions above for creating a working copy of this repository.

2. Follow the [contributing instructions](https://github.com/flutter/engine/blob/master/CONTRIBUTING.md)
   in the engine repository to create a working copy of the engine. The instructions
   also explain how to use a locally-built engine instead of the one bundled with
   your installation of the Flutter framework.

Making a breaking change to the engine
--------------------------------------

If you make a breaking change to the engine, you'll need to land your change in a
few steps:

1. Land your change in the engine repository.

2. Publish a new version of the engine that contains your change. See the
   engine's [release process](https://github.com/flutter/engine/wiki/Release-process)
   for instructions about how to publish a new version of the engine. Publishing
   a new version is important in order to not break folks using prebuilt
   binaries in their workflow (e.g., our customers).

API docs for master branch
--------------------------

To view the API docs for the `master` branch,
visit https://master-docs-flutter-io.firebaseapp.com/.

Those docs should be updated after a successful CI build
of Flutter's `master` branch.

(Looking for the API docs for our releases?
Please visit https://docs.flutter.io.)

Build infrastructure
--------------------

We build and test Flutter on:

- [Cirrus](https://cirrus-ci.com/) ([details](.cirrus.yml))
- Chromebots (a.k.a. "recipes", [details](dev/bots/README.md))
- Devicelab (a.k.a. "cocoon", [details](dev/devicelab/README.md))
