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Contributing to OpenLayers 3

Thanks for your interest in contributing to OpenLayers 3.

Contributing Code

Our preferred means of receiving contributions is through pull requests. Make sure that your pull request follows our pull request guidelines below before submitting it.

This page describes what you need to know to contribute code to ol3 as a developer.

Contributor License Agreement

Before accepting a contribution, we ask that you provide us a Contributor License Agreement. If you are making your contribution as part of work for your employer, please follow the guidelines on submitting a Corporate Contributor License Agreement. If you are making your contribution as an individual, you can submit a digital Individual Contributor License Agreement.

Setting up development environment

You will obviously start by forking the ol3 repository.

Travis CI

The Travis CI hook is enabled on the Github repository. This means every pull request is run through a full test suite to ensure it compiles and passes the tests. Failing pull requests will not be merged.

Although not mandatory, it is also recommended to set up Travis CI for your ol3 fork. For that go to your ol3 fork's Service Hooks page and set up the Travis hook. Then every time you push to your fork, the test suite will be run. This means errors can be caught before creating a pull request. For those making small or occasional contributions, this may be enough to check that your contributions are ok; in this case, you do not need to install the build tools on your local environment as described below.

Development dependencies

The minimum requirements are:

  • Git
  • Node.js (0.10.x or higher)
  • Python 2.6 or 2.7 with a couple of extra modules (see below)
  • Java 7 (JRE and JDK)

The executables git, java, jar, and python should be in your PATH.

You can check your configuration by running:

$ ./build.py checkdeps

To install the Node.js dependencies run

$ npm install

To install the extra Python modules, run:

$ sudo pip install -r requirements.txt

or

$ cat requirements.txt | sudo xargs easy_install

depending on your OS and Python installation.

Working with the build tool

As an ol3 developer you will need to use the build.py Python script. This is the script to use to run the linter, the compiler, the tests, etc. Windows users can use build.cmd which is a thin wrapper around build.py.

The build.py script is equivalent to a Makefile. It is actually based on pake, which is a simple implementation of make in Python.

The usage of the script is:

$ ./build.py <target>

where <target> is the name of the build target you want to execute. For example:

$ ./build.py test

The main build targets are serve, lint, build, test, and check. The latter is a meta-target that basically runs lint, build, and test.

The serve target starts a node-based web server, which we will refer to as the dev server. You'll need to start that server for running the examples and the tests in a browser. More information on that further down.

Other targets include apidoc and ci. The latter is the target used on Travis CI. See ol3's Travis configuration file.

Running the check target

The check target is to be run before pushing code to GitHub and opening pull requests. Branches that don't pass check won't pass the integration tests, and have therefore no chance of being merged into master.

To run the check target:

$ ./build.py check

If you want to run the full suite of integration tests, see "Running the integration tests" below.

Running examples

To run the examples you first need to start the dev server:

$ ./build.py serve

Then, just point your browser http://localhost:3000/examples in your browser. For example http://localhost:3000/examples/side-by-side.html.

Run examples against the ol.js standalone build:

The examples can also be run against the ol.js standalone lib, just like the examples hosted on GitHub. Start by executing the host-examples build target:

$ ./build.py host-examples

After running host-examples you can now open the examples index page in the browser, for example: http://localhost/~elemoine/ol3/build/hosted/master/examples/. (This assumes that the hosted directory is a web directory, served by Apache for example.)

Append ?mode=raw to make the example work in full debug mode. In raw mode the OpenLayers and Closure Library scripts are loaded individually by the Closure Library's base.js script (which the example page loads and executes before any other script).

Running tests

To run the tests in a browser start the dev server (./build.py serve) and open http://localhost:3000/test/index.html in the browser.

To run the tests on the console (headless testing with PhantomJS) use the test target:

$ ./build.py test

See also the test-specific README.

Running the integration tests

When you submit a pull request the Travis continuous integration server will run a full suite of tests, including building all versions of the library and checking that all of the examples work. You will receive an email with the results, and the status will be displayed in the pull request.

To run the full suite of integration tests use the ci target:

$ ./build.py ci

Running the full suite of integration tests currently takes 5-10 minutes.

This makes sure that your commit won't break the build. It also runs JSDoc3 to make sure that there are no invalid API doc directives.

Adding examples

Adding functionality often implies adding one or several examples. This section provides explanations related to adding examples.

The examples are located in the examples directory. Adding a new example implies creating two files in this directory, an .html file and a .js file. See examples/simple.html and examples/simple.js for instance.

The .html file needs to include a script tag with loader.js?id=<example_name> as its src. For example, if the two files for the example are myexample.js and myexample.html then the script tag's src should be set to myexample.

You can use simple.js and simple.html as templates for new examples.

Use of the goog namespace in examples

Short story: the ol3 examples should not use the goog namespace, except for goog.require.

Longer story: we want that the ol3 examples work in multiple modes, with the standalone lib (which has implications of the symbols and properties we export), and compiled together with the ol3 library.

Compiling the examples together with the library makes it mandatory to declare dependencies with goog.require statements.

Pull request guidelines

Your pull request must:

  • Follow OpenLayers 3's coding style.

  • Pass the integration tests run automatically by the Travis Continuous Integration system.

  • Address a single issue or add a single item of functionality.

  • Contain a clean history of small, incremental, logically separate commits, with no merge commits.

  • Use clear commit messages.

  • Be possible to merge automatically.

The check build target

It is strongly recommended that you run

$ ./build.py check

before every commit. This will catch many problems quickly, and it is much faster than waiting for the Travis CI integration tests to run.

The check build target runs a number of quick tests on your code. These include:

  • Lint
  • Compile
  • Tests

Follow OpenLayers 3's coding style

OpenLayers 3 follows Google's JavaScript Style Guide. This is checked using the Closure Linter in strict mode. You can run the linter locally on your machine before committing using the lint target to build.py:

$ ./build.py lint

In addition to fixing problems identified by the linter, please also follow the style of the existing OpenLayers 3 code, which includes:

  • Always wrap the body of for, if, and while statements in braces.

  • Class methods should be in alphabetical order.

  • var declarations should not span multiple lines. If you cannot fit all the declarations in a single line, then start a new var declaration on a new line. Within a single line, variables should be declared in alphabetical order.

  • Do not use assignments inside expressions.

  • Avoid the use of goog.array.clone with arrays (use slice instead).

  • Use array.length = 0 instead of goog.array.clear.

  • Use bracket notation instead of goog.object.set and goog.object.get (with two arguments).

  • Use uppercase for @const variables.

Pass the integration tests run automatically by the Travis CI system

The integration tests contain a number of automated checks to ensure that the code follows the OpenLayers 3 style and does not break tests or examples. You can run the integration tests locally using the ci target:

$ ./build.py ci

Address a single issue or add a single item of functionality

Please submit separate pull requests for separate issues. This allows each to be reviewed on its own merits.

Contain a clean history of small, incremental, logically separate commits, with no merge commits

The commit history explains to the reviewer the series of modifications to the code that you have made and breaks the overall contribution into a series of easily-understandable chunks. Any individual commit should not add more than one new class or one new function. Do not submit commits that change thousands of lines or that contain more than one distinct logical change. Trivial commits, e.g. to fix lint errors, should be merged into the commit that introduced the error. See the Atomic Commit Convention on Wikipedia for more detail.

git apply --patch and git rebase can help you create a clean commit history. Reviewboard.org and Pro GIT have explain how to use them.

Use clear commit messages

Commit messages should be short, begin with a verb in the imperative, and contain no trailing punctuation. We follow http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html for the formatting of commit messages.

Git commit message should look like:

Header line: explaining the commit in one line

Body of commit message is a few lines of text, explaining things
in more detail, possibly giving some background about the issue
being fixed, etc etc.

The body of the commit message can be several paragraphs, and
please do proper word-wrap and keep columns shorter than about
74 characters or so. That way "git log" will show things
nicely even when it's indented.

Further paragraphs come after blank lines.

Please keep the header line short, no more than 50 characters.

Be possible to merge automatically

Occasionally other changes to master might mean that your pull request cannot be merged automatically. In this case you may need to rebase your branch on a more recent master, resolve any conflicts, and git push --force to update your branch so that it can be merged automatically.