Introduction

Flutter is written based on some core principles that were mostly intuited from past experiences with other platforms such as the Web and Android, some of which are summarised below.

This document is most useful if you want to contribute to Flutter, as then we hope you will also follow these philosophies. Please also read our style guide for more specific guidelines regarding writing Dart code for Flutter.

Code review and checking in code

Every PR must be code-reviewed before check-in, including things like rolling a dependency. Getting a review means that a regular Flutter contributor (someone with commit access) has written a comment saying “LGTM” on your PR, and you have addressed all their feedback. (“LGTM” means “Looks Good To Me”.)

Code review serves many critical purposes. There’s the obvious purpose: catching errors. Even the most experienced engineers frequently make errors that are caught by code review. But there are also many other benefits of code reviews:

  • It spreads knowledge among the team. Since every line of code will have been read by two people, it’s more likely that once you move on, someone else will understand the code.

  • It keeps you honest. Knowing that someone will be reading your code, you are less tempted to cut corners and more motivated to write code you are proud of.

  • It exposes you to different modes of thinking. Your code reviewer has probably not thought about the problem in the same way you have, and so may have a fresh perspective and may find you a better way to solve the problem.

If you’re working on a big patch, don’t hesitate to get reviews early, before you’re ready to check code in. Also, don’t hesitate to ask for multiple people to review your code.

Reviewers should carefully read the code and make sure they understand it. A reviewer should check the code for both high level concerns, such as whether the code’s structure makes sense, as well as readability and adherence to the style guide. Reviewers should not give an LGTM unless the patch has tests that verify all the affected code, or unless a test would make no sense. If you review a patch, you are sharing the responsibility for the patch with its author. You should only give an LGTM if you would feel confident answering questions about the code.

A reviewer may in some circumstances consider the code satisfactory without having fully reviewed or understood it. If a reviewer has not fully reviewed the code, they admit to this by saying “RSLGTM” rather than just “LGTM”. If you feel your code needs a real review, please find someone to actually review it. (“RSLGTM” means “Rubber Stamp Looks Good To Me”.) If you mark a patch as RSLGTM, you are still sharing the responsibility for the patch with its author. Reviewing a patch as RSLGTM should be a rare event.

If you really need to check something in in a hurry, for example because everything is broken and you can fix it, then pick someone on the team who you want to have review the code, and then mark the PR as “TBR” with their name. (“TBR” means “To Be Reviewed”.) This is only to be used in emergencies. (Nobody being around to review your 50,000 line patch at midnight on December 31st is not an emergency!) If someone marks a patch as TBR and gives your name as the reviewer, you should review the patch as soon as possible. If a reviewer finds problems with a patch marked TBR, the issues should be fixed as soon as possible.

Wait for Travis to give the green light before merging a PR. Travis runs a bunch of precommit checks (see the tests for the framework, the engine, and the website). You can run these locally as well. For the engine repository, Travis does not actually build the engine, so you should make sure to do that locally first too before checking anything in.

The only time it is ok to check code in without Travis having given the go-ahead is if the tests are already failing, and your fix is intended to improve matters, and you’re going to watch the results once you’ve checked it in (in which case the bots are essentially doing the same job as Travis would have on your PR).

Make sure all the trees and dashboards are green before checking in: the infra waterfall, our travis dashboard, our test dashboard (Google-only, sorry), and our benchmarks dashboard (Google-only, sorry).

If the trees or dashboards are showing any regressions, only fixes that improve the situation are allowed to go in.

Handling breaking changes

Any time you make a breaking change that you expect will impact anyone developing Flutter applications, you should e-mail flutter-dev@googlegroups.com. The e-mail should include the following:

  1. A subject line that clearly summarises the change and sounds like it matters (so that people can spot these e-mails among the noise).

  2. A summary of each change.

  3. Clear mechanical steps for porting the code from the old form to the new form, if possible. If not possible, clear steps for figuring out how to port the code.

  4. A brief justification for the change.

  5. A sincere offer to help port code, which includes the preferred venue for contacting the person who made the change.

Lazy programming

Write what you need and no more, but when you write it, do it right.

Avoid implementing features you don’t need. You can’t design a feature without knowing what the constraints are. Implementing features “for completeness” results in unused code that is expensive to maintain, learn about, document, test, etc.

When you do implement a feature, implement it the right way. Avoid workarounds. Workarounds merely kick the problem further down the road, but at a higher cost: someone will have to relearn the problem, figure out the workaround and how to dismantle it (and all the places that now use it), and implement the feature. It’s much better to take longer to fix a problem properly, than to be the one who fixes everything quickly but in a way that will require cleaning up later.

You may hear team members say “embrace the yak shave!”. This is an encouragement to take on the larger effort necessary to perform a proper fix for a problem rather than just applying a band-aid.

Tests

Write Tests, Find Bugs

When you fix a bug, first write a test that fails, then fix the bug and verify the test passes.

When you implement a new feature, write tests for it.

Documentation

When working on Flutter, if you find yourself asking a question about our systems, please place whatever answer you subsequently discover into the documentation in the same place where you first looked for the answer.

It’s better to not document something than to document it poorly (because if you don’t document it, it still appears on our list of things to document). Feel free to remove documentation that violates our style guide, so as to make it reappear on the list.

API design

  • There should be no objects that represent live state that reflects some other state, since they are expensive to maintain. e.g. no HTMLCollection.

  • Property getters should be efficient (e.g. just returning a cached value, or an O(1) table lookup). If an operation is inefficient it should be a method instead. e.g. document.getForms(), not document.forms.

  • There should be no APIs that require synchronously completing an expensive operation (e.g. computing a full app layout outside of the layout phase).

  • We use a layered framework design, where each layer addresses a narrowly scoped problem and is then used by the next layer to solve a bigger problem. This is true both at a high level (widgets relies on rendering relies on painting) and at the level of individual classes and methods (e.g. in the rendering library, having one class for clipping and one class for opacity rather than one class that does both at the same time).

    • Convenience APIs belong at the layer above the one they are simplifying.

    • Having dedicated APIs for performance reasons is fine. If one specific operation, say clipping a rounded rectangle, is expensive using the generic API but could be implemented more efficiently using a dedicated API, then a dedicated API is fine.

  • APIs that encourage bad practices should not exist. e.g., no document.write(), innerHTML, insertAdjacentHTML(), etc.

    • String manipulation to generate data or code that will subsequently be interpreted or parsed is a bad practice as it leads to code injection vulnerabilities.
  • If we wrap some aspect of a service from one environment for exposure in another environment (for example, exposing an Android API in Dart), we should expose/wrap all of it, so that there’s no cognitive cliff when interacting with that service (where you are fine using the exposed API up to a point, but beyond that have to learn all about the underlying service).

Bugs

Only assign a bug to yourself when you are actively working on it. If you’re not working on it, leave it unassigned. Don’t assign bugs to people unless you know they are going to work on it. If you find yourself with bugs assigned that you are not going to be working on in the very near future, unassign the bug so that other people feel empowered to work on them.

You may hear team members refer to “licking the cookie”. Assigning a bug to yourself, or otherwise indicating that you will work on it, tells others on the team to not fix it. If you then don’t work on it straight away, you are acting like someone who has taken a cookie, licked it to unappetising to other people, and then not eaten it right away. By extension, “unlicking the cookie” means indicating to the rest of the team that you are not actually going to work on the bug right away after all, e.g. by unassigning the bug from yourself.

File bugs for anything that you come across that needs doing. When you implement something but know it’s not complete, file bugs for what you haven’t done. That way, we can keep track of what still needs doing.

Regressions

If a check-in has caused a regression on the trunk, roll back the check-in (even if it isn’t yours) unless doing so would take longer than fixing the bug. When the trunk is broken, it slows down everyone else on the project.

If things are broken, the priority of everyone on the team should be helping the team fix the problem. Someone should own the issue, and they can delegate responsibilities to others on the team. Once the problem is resolved, write a post-mortem. Postmortems are about documenting what went wrong and how to avoid the problem (and the entire class of problems like it) from recurring in the future. Postmortems are emphatically not about assigning blame.

There is no shame in making mistakes.

Questions

It’s always ok to ask questions. Our systems are large, nobody will be an expert in all the systems.

Conflict resolution

When multiple contributors disagree on the direction for a particular patch or the general direction of the project, the conflict should be resolved by communication. The people who disagree should get together, try to understand each other’s points of view, and work to find a design that addresses everyone’s concerns.

This is usually sufficient to resolve issues. If you cannot come to an agreement, ask for the advice of a more senior member of the team.

Be wary of agreement by attrition, where one person argues a point repeatedly until other participants give up in the interests of moving on. This is not conflict resolution, as it does not address everyone’s concerns. Be wary of agreement by compromise, where two good competing solutions are merged into one mediocre solution. A conflict is addressed when the participants agree that the final solution is better than all the conflicting proposals. Sometimes the solution is more work than either of the proposals. Please see the comments above where we introduce the phrase “embrace the yak shave”.

Code of conduct

This section is the last section in this document because it should be the most obvious. However, it is also the most important.

We expect Flutter’s contributors to act professionally and respectfully, and we expect our social spaces to be safe and dignified environments.

Specifically:

  • Respect people, their identities, their culture, and their work.
  • Be kind. Be courteous. Be welcoming.
  • Listen. Consider and acknowledge people’s points before responding.

Should you experience anything that makes you feel unwelcome in Flutter’s community, please contact someone on the team, for instance Ian or Adam. We will not tolerate harrasment from anyone in Flutter’s community, even outside of Flutter’s public communication channels.