Style guide for Flutter repo

Summary

Optimize for readability. Write detailed documentation. Make error messages useful. Never use timeouts or timers. Avoid is, print, part of, extension and _.

Introduction

This document contains some high-level philosophy and policy decisions for the Flutter project, and a description of specific style issues for some parts of the codebase.

The style portion describes the preferred style for code written as part of the Flutter project (the framework itself and all our sample code). Flutter application developers are welcome to follow this style as well, but this is by no means required. Flutter will work regardless of what style is used to author applications that use it.

The engine repository uses other style guides for non-Dart code. The language-neutral sections in this document still apply to engine code, however.

Overview

This document describes our approach to designing and programming Flutter, from high-level architectural principles all the way to indentation rules. These are our norms, written down so that we can easily convey our shared understanding with new team members.

The primary goal of these style guidelines is to improve code readability so that everyone, whether reading the code for the first time or maintaining it for years, can quickly determine what the code does. Secondary goals are to design systems that are simple; to increase the likelihood of catching bugs quickly; and to avoid arguments when there are disagreements over subjective matters.

For anything not covered by this document, check the Dart style guide for more advice. That document is focused primarily on Dart-specific conventions, while this document is more about Flutter conventions.

In some cases (for example, line wrapping around if statements) the Dart style guide differs from the Flutter guide. For Flutter project code, the Flutter guide governs. The differences are a result of slightly different priorities. The Flutter guide is designed for making code highly readable even to people who have never seen the code before and are new to Dart, as the Flutter framework code will be read millions of times more than it is written. The Dart guide, on the other hand, is designed to provide a more balanced approach that assumes that the writing of the code will be a bigger proportion of the interactions with the code, and that the reader is more experienced with Dart. (The dart format tool uses the Dart guide, so we do not use it in the flutter/flutter and flutter/engine repositories. However, we do recommend its use in general.)

A word on designing APIs

Designing an API is an art. Like all forms of art, one learns by practicing. The best way to get good at designing APIs is to spend a decade or more designing them, while working closely with people who are using your APIs. Ideally, one would first do this in very controlled situations, with small numbers of developers using one's APIs, before graduating to writing APIs that will be used by hundreds of thousands or even millions of developers.

In the absence of one‘s own experience, one can attempt to rely on the experience of others. The biggest problem with this is that sometimes explaining why an API isn’t optimal is a very difficult and subtle task, and sometimes the reasoning doesn't sound convincing unless you already have a lot of experience designing them.

Because of this, and contrary to almost any other situation in engineering, when you are receiving feedback about API design from an experienced API designer, they will sometimes seem unhappy without quite being able to articulate why. When this happens, seriously consider that your API should be scrapped and a new solution found.

This requires a different and equally important skill when designing APIs: not getting attached to one‘s creations. One should try many wildly different APIs, and then attempt to write code that uses those APIs, to see how they work. Throw away APIs that feel frustrating, that lead to buggy code, or that other people don’t like. If it isn‘t elegant, it’s usually better to try again than to forge ahead.

An API is for life, not just for the one PR you are working on.

Philosophy

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.

Write Test, Find Bug

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. (See also: Running and writing tests, and the section on writing tests below.)

Check the code coverage to make sure every line of your new code is tested. See also: Test coverage for package:flutter.

If something isn't tested, it is very likely to regress or to get “optimized away”. If you want your code to remain in the codebase, you should make sure to test it.

Don't submit code with the promise to “write tests later”. Just take the time to write the tests properly and completely in the first place.

Avoid duplicating state

There should be no objects that represent live state that reflect some state from another source, since they are expensive to maintain. (The Web‘s HTMLCollection object is an example of such an object.) In other words, keep only one source of truth, and **don’t replicate live state**.

Getters feel faster than methods

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. (Looking at the Web again: we would have document.getForms(), not document.forms, since it walks the entire tree).

Similarly, a getter that returns a Future should not kick-off the work represented by the future, since getters appear idempotent and side-effect free. Instead, the work should be started from a method or constructor, and the getter should just return the preexisting Future.

No synchronous slow work

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

Layers

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. Text uses RichText and DefaultTextStyle).

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

Avoid interleaving multiple concepts together

Each API should be self-contained and should not know about other features. Interleaving concepts leads to complexity.

For example:

• Many Widgets take a child. Widgets should be entirely agnostic about the type of that child. Don't use is or similar checks to act differently based on the type of the child.

• Render objects each solve a single problem. Rather than having a render object handle both clipping and opacity, we have one render object for clipping, and one for opacity.

• In general, prefer immutable objects over mutable data. Immutable objects can be passed around safely without any risk that a downstream consumer will change the data. (Sometimes, in Flutter, we pretend that some objects are immutable even when they technically are not: for example, widget child lists are often technically implemented by mutable List instances, but the framework will never modify them and in fact cannot handle the user modifying them.) Immutable data also turns out to make animations much simpler through lerping.

Avoid secret (or global) state

A function should operate only on its arguments and, if it is an instance method, data stored on its object. This makes the code significantly easier to understand.

For example, when reading this code:

// ... imports something that defines foo and bar ...

void main() {
  foo(1);
  bar(2);
}

...the reader should be confident that nothing in the call to foo could affect anything in the call to bar.

This usually means structuring APIs so that they either take all relevant inputs as arguments, or so that they are based on objects that are created with the relevant input, and can then be called to operate on those inputs.

This significantly aids in making code testable and in making code understandable and debuggable. When code operates on secret global state, it's much harder to reason about.

Prefer general APIs, but use dedicated APIs where there is a reason

For example, having dedicated APIs for performance reasons is fine. If one specific operation, say clipping a rounded rectangle, is expensive using the general API but could be implemented more efficiently using a dedicated API, then that is where we would create a dedicated API.

Avoid the lowest common denominator

It is common for SDKs that target multiple platforms (or meta-platforms that themselves run on multiple platforms, like the Web) to provide APIs that work on all their target platforms. Unfortunately, this usually means that features that are unique to one platform or another are unavailable.

For Flutter, we want to avoid this by explicitly aiming to be the best way to develop for each platform individually. Our ability to be used cross- platform is secondary to our ability to be used on each platform. For example, TextInputAction has values that only make sense on some platforms. Similarly, our platform channel mechanism is designed to allow separate extensions to be created on each platform.

Avoid APIs that encourage bad practices

For example, don't provide APIs that walk entire trees, or that encourage O(N^2) algorithms, or that encourage sequential long-lived operations where the operations could be run concurrently.

In particular:

• 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 an operation is expensive, that expense should be represented in the API (e.g. by returning a Future or a Stream). Avoid providing APIs that hide the expense of tasks.

Avoid exposing API cliffs

Convenience APIs that wrap some aspect of a service from one environment for exposure in another environment (for example, exposing an Android API in Dart), should expose/wrap the complete API, 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).

Avoid exposing API oceans

APIs that wrap underlying services but prevent the underlying API from being directly accessed (e.g. how dart:ui exposes Skia) should carefully expose only the best parts of the underlying API. This may require refactoring features so that they are more usable. It may mean avoiding exposing convenience features that abstract over expensive operations unless there's a distinct performance gain from doing so. A smaller API surface is easier to understand.

For example, this is why dart:ui doesn't expose Path.fromSVG(): we checked, and it is just as fast to do that work directly in Dart, so there is no benefit to exposing it. That way, we avoid the costs (bigger API surfaces are more expensive to maintain, document, and test, and put a compatibility burden on the underlying API).

Avoid heuristics and magic

Predictable APIs that the developer feels gives them control are generally preferred over APIs that mostly do the right thing but don't give the developer any way to adjust the results.

Predictability is reassuring.

Solve real problems by literally solving a real problem

Where possible, especially for new features, you should partner with a real customer who wants that feature and is willing to help you test it. Only by actually using a feature in the real world can we truly be confident that a feature is ready for prime time.

Listen to their feedback, too. If your first customer is saying that your feature doesn‘t actually solve their use case completely, don’t dismiss their concerns as esoteric. Often, what seems like the problem when you start a project turns out to be a trivial concern compared to the real issues faced by real developers.

Get early feedback when designing new APIs

If you're designing a new API or a new feature, consider writing a design doc. Then, get feedback from the relevant people, e.g. send it to flutter-dev or post it on the relevant chat channel.

Start designing APIs from the closest point to the developer

When we create a new feature that requires a change to the entire stack, it‘s tempting to design the lowest-level API first, since that’s the closest to the “interesting” code (the “business end” of the feature, where we actually do the work). However, that then forces the higher level APIs to be designed against the lower-level API, which may or may not be a good fit, and eventually the top-level API, which developers will primarily be using, may be forced to be a tortured and twisted mess (either in implementation or in terms of the exposed API). It may even be that the final API doesn't fit how people think about the problem or solve their actual issues, but instead merely exposes the lowest-level feature almost verbatim.

Instead, always design the top-level API first. Consider what the most ergonomic API would be at the level that most developers will be interacting with it. Then, once that API is cleanly designed and usability-tested, build the lower levels so that the higher level can be layered atop.

Concretely, this means designing the API at the material or widgets layer first, then the API in the rendering, scheduler, or services layer, then the relevant binding, then the dart:ui API or the message channel protocol, then the internal engine API or the plugin API. (The details may vary from case to case.)

Only log actionable messages to the console

If the logs contain messages that the user can safely ignore, then they will do so, and eventually their logs will be so chatty and verbose that they will miss the critical messages. Therefore, only log actual errors and actionable warnings (warnings that can always be dealt with and fixed).

Never log “informational” messages by default. It is possible that it may be useful to have messages on certain topics while debugging those topics. To deal with that, have debug flags you can enable that enable extra logging for particular topics. For example, setting debugPrintLayouts to true enables logging of layouts.

This also applies to our unopt builds. It‘s annoying for other people on the team to have to wade through messages that aren’t directly relevant to their work. Rely on feature flags, not verbosity levels, when deciding to output messages. The one exception to this is reporting useful milestones; for example, the flutter tool in verbose mode (-v) reports meaningful steps that it is executing because those are almost always useful.

Error messages should be useful

Every time you find the need to report an error (e.g. throwing an exception in the framework, handling some bad state in the engine, reporting a syntax error in the Dart compiler, etc), consider how you can make this the most useful and helpful error message ever.

Put yourself in the shoes of whoever sees that error message. Why did they see it? What can we do to help them? They are at a crossroads, having seen your error message: they can either get frustrated and hate Flutter, or they can feel thankful that the error helped them resolve an actual issue. Every error message is an opportunity to make someone love our product.

Template values should set developers up for success

Template defaults should focus on providing the best developer experience. Templates should help developers understand the code, be easy to run now and support in the future. Help developers by picking dependencies that are broadly used and/or broadly supported and by leaving comments that are helpful.

See flutter create's templates for an example.

Policies

This section defines some policies that we have decided to honor. In the absence of a very specific policy in this section, the general philosophies in the section above are controlling.

Plugin compatibility

We guarantee that a plugin published with a version equal to or greater than 1.0.0 will require no more recent a version of Flutter than the latest stable release at the time that the plugin was released. (Plugins may support older versions too, but that is not guaranteed.)

Workarounds

We are willing to implement temporary (one week or less) workarounds (e.g. //ignore hacks) if it helps a high profile developer or prolific contributor with a painful transition. Please contact @Hixie (ian@hixie.ch) if you need to make use of this option.

Avoid abandonware

Code that is no longer maintained should be deleted or archived in some way that clearly indicates that it is no longer maintained.

For example, we delete rather than commenting out code. Commented-out code will bitrot too fast to be useful, and will confuse people maintaining the code.

Similarly, all our repositories should have an owner that does regular triage of incoming issues and PRs, and fixes known issues. Repositories where nobody is doing triage at least monthly, preferably more often, should be deleted, hidden, or otherwise archived.

Widget libraries follow the latest OEM behavior

For our material and cupertino libraries, we generally implement the latest behavior unless doing so would be a seriously disruptive breaking change. For example, we use the latest stylings for iOS switch controls, but when Material Design introduced a whole new type of button, we created a new widget for that rather than updating the existing buttons to have the new style.

Code that is not copyrighted “The Flutter Authors”

All code in all Flutter repositories must be contributed by developers who have signed the Google CLA, and must be licensed using our normal BSD license with a copyright referencing “The Flutter Authors”, except if it is “third party code”.

“Third party code” that is not part of a Dart package must be in a subdirectory of a third_party directory at the root of the relevant repository, and the subdirectory in question must contain a LICENSE file that details the license covering that code and a README describing the provenance of that code.

“Third party code” that is part of a Dart package and is not Dart code must be in a subdirectory of a third_party directory at the root of the package, and the subdirectory in question must contain a LICENSE file that details the license covering that code and a README describing the provenance of that code. The license must then also be duplicated into the package's LICENSE file using the syntax described in the LicenseRegistry API docs.

“Third party code” that is part of a Dart package and is Dart code must be in a subdirectory of the package‘s lib/src/third_party directory, and the subdirectory in question must contain a LICENSE file that details the license covering that code and a README describing the provenance of that code. The license must then also be duplicated into the package’s LICENSE file using the syntax described in the LicenseRegistry API docs.

All licenses included in this manner must have been reviewed and determined to be legally acceptable licenses.

All such “third party code” must either be a fork for which we take full responsibility, or there must be an automated rolling mechanism that keeps the code up to date when the upstream source changes.

In general it is strongly recommended that we avoid any such code unless strictly necessary. In particular, we aim for all code in the flutter/flutter repository to be single-licensed, which is why it does not contain any “third party code” at all.

Documentation (dartdocs, javadocs, etc)

We use “dartdoc” for our Dart documentation, and similar technologies for the documentation of our APIs in other languages, such as ObjectiveC and Java. All public members in Flutter libraries should have a documentation.

In general, follow the Dart documentation guide except where that would contradict this page.

Answer your own questions straight away

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. That way, the documentation will consist of answers to real questions, where people would look to find them. Do this right away; it's fine if your otherwise-unrelated PR has a bunch of documentation fixes in it to answer questions you had while you were working on your PR.

We try to avoid reliance on “oral tradition”. It should be possible for anyone to begin contributing without having had to learn all the secrets from existing team members. To that end, all processes should be documented (typically on the wiki), code should be self-explanatory or commented, and conventions should be written down, e.g. in our style guide.

There is one exception: it‘s better to not document something in our API docs than to document it poorly. This is 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 rules below (especially the next one), so as to make it reappear on the list.

Avoid useless documentation

If someone could have written the same documentation without knowing anything about the class other than its name, then it's useless.

Avoid checking in such documentation, because it is no better than no documentation but will prevent us from noticing that the identifier is not actually documented.

Example (from CircleAvatar):

// BAD:

/// The background color.
final Color backgroundColor;

/// Half the diameter of the circle.
final double radius;


// GOOD:

/// The color with which to fill the circle.
///
/// Changing the background color will cause the avatar to animate to the new color.
final Color backgroundColor;

/// The size of the avatar.
///
/// Changing the radius will cause the avatar to animate to the new size.
final double radius;

Writing prompts for good documentation

If you are having trouble coming up with useful documentation, here are some prompts that might help you write more detailed prose:

• If someone is looking at this documentation, it means that they have a question which they couldn't answer by guesswork or by looking at the code. What could that question be? Try to answer all questions you can come up with.

• If you were telling someone about this property, what might they want to know that they couldn‘t guess? For example, are there edge cases that aren’t intuitive?

• Consider the type of the property or arguments. Are there cases that are outside the normal range that should be discussed? e.g. negative numbers, non-integer values, transparent colors, empty arrays, infinities, NaN, null? Discuss any that are non-trivial.

• Does this member interact with any others? For example, can it only be non-null if another is null? Will this member only have any effect if another has a particular range of values? Will this member affect whether another member has any effect, or what effect another member has?

• Does this member have a similar name or purpose to another, such that we should point to that one, and from that one to this one? Use the See also: pattern.

• Are there timing considerations? Any potential race conditions?

• Are there lifecycle considerations? For example, who owns the object that this property is set to? Who should dispose() it, if that's relevant?

• What is the contract for this property/method? Can it be called at any time? Are there limits on what values are valid? If it's a final property set from a constructor, does the constructor have any limits on what the property can be set to? If this is a constructor, are any of the arguments not nullable?

• If there are Future values involved, what are the guarantees around those? Consider whether they can complete with an error, whether they can never complete at all, what happens if the underlying operation is canceled, and so forth.

Introduce terms as if every piece of documentation is the first the reader has ever seen

It's easy to assume that the reader has some basic knowledge of Dart or Flutter when writing API documentation.

Unfortunately, the reality is that everyone starts knowing nothing, and we do not control where they will begin their journey.

For this reason, avoid using terms without first defining them, unless you are linking to more fundamental documentation that defines that term without reference to the API you are documenting.

For example, a fancy widget in the Material library can refer to the StatefulWidget documentation and assume that the reader either knows about the StatefulWidget class, or can learn about it by following the link and then later returning to the documentation for the fancy widget. However, the documentation for the StatefulWidget class should avoid assuming that the reader knows what a State class is, and should avoid deferring to it for its definition, because State could is likely to defer back to StatefulWidget and the reader would be stuck in a loop unable to grasp the basic principles. This is the documentation equivalent of a bootstrapping problem.

Another way to express this is that API documentation should follow a similar layering philosophy as code. The goal of documentation is not just to act as a refresher for experts, but to act as a tutorial for new developers.

Avoid empty prose

It's easy to use more words than necessary. Avoid doing so where possible, even if the result is somewhat terse.

// BAD:

/// Note: It is important to be aware of the fact that in the
/// absence of an explicit value, this property defaults to 2.

// GOOD:

/// Defaults to 2.

In particular, avoid saying “Note:”, or starting a sentence with “Note that”. It adds nothing.

Leave breadcrumbs in the comments

This is especially important for documentation at the level of classes.

If a class is constructed using a builder of some sort, or can be obtained via some mechanism other than merely calling the constructor, then include this information in the documentation for the class.

If a class is typically used by passing it to a particular API, then include that information in the class documentation also.

If a method is the main mechanism used to obtain a particular object, or is the main way to consume a particular object, then mention that in the method's description.

Typedefs should mention at least one place where the signature is used.

These rules result in a chain of breadcrumbs that a reader can follow to get from any class or method that they might think is relevant to their task all the way up to the class or method they actually need.

Example:

// GOOD:

/// An object representing a sequence of recorded graphical operations.
///
/// To create a [Picture], use a [PictureRecorder].
///
/// A [Picture] can be placed in a [Scene] using a [SceneBuilder], via
/// the [SceneBuilder.addPicture] method. A [Picture] can also be
/// drawn into a [Canvas], using the [Canvas.drawPicture] method.
abstract class Picture ...

You can also use “See also” links, as in:

/// See also:
///
/// * [FooBar], which is another way to peel oranges.
/// * [Baz], which quuxes the wibble.

Each line should end with a period. Prefer “which...” rather than parentheticals on such lines. There should be a blank line between “See also:” and the first item in the bulleted list.

Refactor the code when the documentation would be incomprehensible

If writing the documentation proves to be difficult because the API is convoluted, then rewrite the API rather than trying to document it.

Canonical terminology

The documentation should use consistent terminology:

• method - a member of a class that is a non-anonymous closure
• function - a callable non-anonymous closure that isn't a member of a class
• parameter - a variable defined in a closure signature and possibly used in the closure body.
• argument - the value passed to a closure when calling it.

Prefer the term “call” to the term “invoke” when talking about jumping to a closure.

Prefer the term “member variable” to the term “instance variable” when talking about variables associated with a specific object.

Typedef dartdocs should usually start with the phrase “Signature for...”.

Use correct grammar

Avoid starting a sentence with a lowercase letter.

// BAD

/// [foo] must not be null.

// GOOD

/// The [foo] argument must not be null.

Similarly, end all sentences with a period.

Use the passive voice; recommend, do not require; never say things are simple

Never use “you” or “we”. Avoid the imperative voice. Avoid value judgements.

Rather than telling someone to do something, use “Consider”, as in “To obtain the foo, consider using [bar].”.

In general, you don‘t know who is reading the documentation or why. Someone could have inherited a terrible codebase and be reading our documentation to find out how to fix it; by saying “you should not do X” or “avoid Y” or “if you want Z”, you will put the reader in a defensive state of mind when they find code that contradicts the documentation (after all, they inherited this codebase, who are we to say that they’re doing it wrong, it's not their fault).

For similar reasons, never use the word “simply”, or say that the reader need “just” do something, or otherwise imply that the task is easy. By definition, if they are looking at the documentation, they are not finding it easy.

Provide sample code

Sample code helps developers learn your API quickly. Writing sample code also helps you think through how your API is going to be used by app developers.

Sample code should go in a documentation comment that typically begins with /// {@tool dartpad}, and ends with /// {@end-tool}, with the example source and corresponding tests placed in a file under the API examples directory. This will then be checked by automated tools, and formatted for display on the API documentation web site api.flutter.dev. For details on how to write sample code, see the API example documentation.

Provide full application samples.

Our UX research has shown that developers prefer to see examples that are in the context of an entire app. So, whenever it makes sense, provide an example that can be presented as part of an entire application instead of just a snippet that uses the {@tool snippet} or ```dart ... ``` indicators.

An application sample can be created using the {@tool dartpad} ... {@end-tool} or {@tool sample} ... {@end-tool} dartdoc indicators. See here for more details about writing these kinds of examples.

Dartpad examples (those using the dartdoc {@tool dartpad} indicator) will be presented on the API documentation website as an in-page executable and editable example. This allows developers to interact with the example right there on the page, and is the preferred form of example. Here is one such example.

For examples that don't make sense in a web page (for example, code that interacts with a particular platform feature), application examples (using the dartdoc {@tool sample} indicator) are preferred, and will be presented on the API documentation website along with information about how to instantiate the example as an application that can be run.

Supported IDEs viewing the Flutter source code using the Flutter plugin also offer the option of creating a new project with either kind of example.

Provide illustrations, diagrams or screenshots

For any widget that draws pixels on the screen, showing how it looks like in its API doc helps developers decide if the widget is useful and learn how to customize it. All illustrations should be easily reproducible, e.g. by running a Flutter app or a script.

Examples:

• A diagram for the AppBar widget

• A screenshot for the Card widget

When creating diagrams, make sure to provide alternative text as described in the HTML specification.

Link to Widget of the Week videos

Link to a widget's Flutter Widget of the Week video if it has one:

/// {@youtube 560 315 https://www.youtube.com/watch?v=<id>}

The first two arguments are the video's width and height. These should be 560 and 315 respectively.

Clearly mark deprecated APIs

We have conventions around deprecation. See the Tree Hygiene page for more details.

Use /// for public-quality private documentation

In general, private code can and should also be documented. If that documentation is of good enough quality that we could include it verbatim when making the class public (i.e. it satisfies all the style guidelines above), then you can use /// for those docs, even though they're private.

Documentation of private APIs that is not of sufficient quality should only use //. That way, if we ever make the corresponding class public, those documentation comments will be flagged as missing, and we will know to examine them more carefully.

Feel free to be conservative in what you consider “sufficient quality”. It‘s ok to use // even if you have multiple paragraphs of documentation; that’s a sign that we should carefully rereview the documentation when making the code public.

Dartdoc templates and macros

Dartdoc supports creating templates that can be reused in other parts of the code. They are defined like so:

/// {@template <id>}
/// ...
/// {@endtemplate}

and used via:

/// {@macro <id>}

The <id> should be a unique identifier that is of the form flutter.library.Class.member[.optionalDescription].

For example:

// GOOD:
/// {@template flutter.rendering.Layer.findAnnotations.aboutAnnotations}
/// Annotations are great!
/// {@endtemplate

// BAD:
/// {@template the_stuff!}
/// This is some great stuff!
/// {@endtemplate}

The optionalDescription component of the identifier is only necessary if there is more than one template defined in one Dartdoc block. If a symbol is not part of a library, or not part of a class, then just omit those parts from the ID.

Dartdoc-specific requirements

The first paragraph of any dartdoc section must be a short self-contained sentence that explains the purpose and meaning of the item being documented. Subsequent paragraphs then must elaborate. Avoid having the first paragraph have multiple sentences. (This is because the first paragraph gets extracted and used in tables of contents, etc, and so has to be able to stand alone and not take up a lot of room.)

When referencing a parameter, use backticks. However, when referencing a parameter that also corresponds to a property, use square brackets instead. (This contradicts the Dart style guide, which says to use square brackets for both. We do this because of dartdoc issue 1486. Currently, there's no way to unambiguously reference a parameter. We want to avoid cases where a parameter that happens to be named the same as a property despite having no relationship to that property gets linked to the property.)

// GOOD

  /// Creates a foobar, which allows a baz to quux the bar.
  ///
  /// The [bar] argument must not be null.
  ///
  /// The `baz` argument must be greater than zero.
  Foo({ this.bar, int baz }) : assert(bar != null), assert(baz > 0);

Avoid using terms like “above” or “below” to reference one dartdoc section from another. Dartdoc sections are often shown alone on a Web page, the full context of the class is not present.

Coding patterns and catching bugs early

Use asserts liberally to detect contract violations and verify invariants

assert() allows us to be diligent about correctness without paying a performance penalty in release mode, because Dart only evaluates asserts in debug mode.

It should be used to verify contracts and invariants are being met as we expect. Asserts do not enforce contracts, since they do not run at all in release builds. They should be used in cases where it should be impossible for the condition to be false without there being a bug somewhere in the code.

The following example is from box.dart:

abstract class RenderBox extends RenderObject {
  // ...

  double getDistanceToBaseline(TextBaseline baseline, {bool onlyReal: false}) {
    // simple asserts:
    assert(!needsLayout);
    assert(!_debugDoingBaseline);
    // more complicated asserts:
    assert(() {
      final RenderObject parent = this.parent;
      if (owner.debugDoingLayout) {
        return (RenderObject.debugActiveLayout == parent) && parent.debugDoingThisLayout;
      }
      if (owner.debugDoingPaint) {
        return ((RenderObject.debugActivePaint == parent) && parent.debugDoingThisPaint)
            || ((RenderObject.debugActivePaint == this)   && debugDoingThisPaint);
      }
      assert(parent == this.parent);
      return false;
    });
    // ...
    return 0.0;
  }

  // ...
}

Prefer specialized functions, methods and constructors

Use the most relevant constructor or method, when there are multiple options.

Example:

// BAD:
const EdgeInsets.TRBL(0.0, 8.0, 0.0, 8.0);

// GOOD:
const EdgeInsets.symmetric(horizontal: 8.0);

Minimize the visibility scope of constants

Prefer using a local const or a static const in a relevant class than using a global constant.

As a general rule, when you have a lot of constants, wrap them in a class. For examples of this, see lib/src/material/colors.dart.

Avoid using if chains or ?: or == with enum values

Use switch with no default case if you are examining an enum, since the analyzer will warn you if you missed any of the values when you use switch. The default case should be avoided so that the analyzer will complain if a value is missing. Unused values can be grouped together with a single break or return as appropriate.

Avoid using if chains, ? ... : ..., or, in general, any expressions involving enums.

Avoid using var and dynamic

All variables and arguments are typed; avoid dynamic or Object in any case where you could figure out the actual type. Always specialize generic types where possible. Explicitly type all list and map literals. Give types to all parameters, even in closures and even if you don't use the parameter.

This achieves two purposes: it verifies that the type that the compiler would infer matches the type you expect, and it makes the code self-documenting in the case where the type is not obvious (e.g. when calling anything other than a constructor).

Always avoid var and dynamic. If the type is unknown, prefer using Object (or Object?) and casting, as using dynamic disables all static checking.

Avoid using library and part of.

Prefer that each library be self-contained. Only name a library if you are documenting it (see the documentation section).

We avoid using part of because that feature makes it very hard to reason about how private a private really is, and tends to encourage “spaghetti” code (where distant components refer to each other) rather than “lasagna” code (where each section of the code is cleanly layered and separable).

Avoid using extension.

Extension methods are confusing to document and discover. To an end developer, they appear no different than the built-in API of the class, and discovering the documentation and implementation of an extension is more challenging than for class members.

Prefer instead adding methods directly to relevant classes. If that is not possible, create a method that clearly identifies what object(s) it works with and is part of.

(A rare exception can be made for extensions that provide temporary workarounds when deprecating features. In those cases, however, the extensions and all their members must be deprecated in the PR that adds them, and they must be removed in accordance with our deprecation policy.)

Avoid using FutureOr<T>

The FutureOr type is a Dart-internal type used to explain certain aspects of the Future API. In public APIs, avoid the temptation to create APIs that are both synchronous and asynchronous by returning this type, as it usually only results in the API being more confusing and less type safe.

In certain extreme cases where the API absolutely needs to be asynchronous but a synchronous “escape hatch” is needed for performance, consider using SynchronousFuture (but be aware that this still suffers from many of the same risks of making the API only subtle and complicated). This is used, for example, when loading images in the Flutter framework.

You may use FutureOr to accept a callback that may or may not return a Future.

Avoid using Expando

Generally speaking, Expando objects are a sign of an architectural problem. Carefully consider whether your usage is actually necessary. When your classes have clear documented ownership rules, there is usually a better solution.

Expando objects tend to invite code that is hard to understand because one cannot simply follow references to find all the dependencies.

Avoid using @visibleForTesting

The @visibleForTesting annotation marks a public API so that developers that have not disabled the invalid_use_of_visible_for_testing_member analyzer error get a warning when they use this API outside of a test directory.

This means that the API has to be treated as being public (nothing prevents a developer from using the API even in non-test code), meaning it must be designed to be a public API, it must be documented, it must be tested, etc. At which point, there‘s really no reason not to just make it a public API. If anything, the use of @visibleForTesting becomes merely a crutch to convince ourselves that it’s ok that we're making something public that we should really not have made public.

So rather than rely on @visibleForTesting, consider designing your APIs so that they are directly testable using the public API, without exposing any sensitive internals.

(One exception is combining @visibleForTesting with @protected. The @protected annotation marks a member as one that is intended for subclasses, so it is already a public API and considered as such. The @visibleForTesting annotation in that case merely enables the member to be called directly in tests without having to create a fake subclass and without having to add //ignore pragmas.)

Never check if a port is available before using it, never add timeouts, and other race conditions.

If you look for an available port, then try to open it, it's extremely likely that several times a week some other code will open that port between your check and when you open the port, and that will cause a failure.

Instead, have the code that opens the port pick an available port and return it, rather than being given a (supposedly) available port.

If you have a timeout, then it's very likely that several times a week some other code will happen to run while your timeout is running, and your “really conservative” timeout will trigger even though it would have worked fine if the timeout was one second longer, and that will cause a failure.

Instead, have the code that would time out just display a message saying that things are unexpectedly taking a long time, so that someone interactively using the tool can see that something is fishy, but an automated system won't be affected.

Race conditions like this are the primary cause of flaky tests, which waste everyone's time.

Similarly, avoid delays or sleeps that are intended to coincide with how long something takes. You may think that waiting two seconds is fine because it normally takes 10ms, but several times a week your 10ms task will actually take 2045ms and your test will fail because waiting two seconds wasn't long enough.

Instead, wait for a triggering event.

Avoid mysterious and magical numbers that lack a clear derivation

Numbers in tests and elsewhere should be clearly understandable. When the provenance of a number is not obvious, consider either leaving the expression or adding a clear comment (bonus points for leaving a diagram).

// BAD
expect(rect.left, 4.24264068712);

// GOOD
expect(rect.left, 3.0 * math.sqrt(2));

Have good hygiene when using temporary directories

Give the directory a unique name that starts with flutter_ and ends with a period (followed by the autogenerated random string).

For consistency, name the Directory object that points to the temporary directory tempDir, and create it with createTempSync unless you need to do it asynchronously (e.g. to show progress while it's being created).

Always clean up the directory when it is no longer needed. In tests, use the tryToDelete convenience function to delete the directory. (We use tryToDelete because on Windows it's common to get “access denied” errors when deleting temporary directories. We have no idea why; if you can figure it out then that could simplify a lot of code!)

Perform dirty checks in setters

Dirty checks are processes to determine whether a changed values have been synchronized with the rest of the app.

When defining mutable properties that mark a class dirty when set, use the following pattern:

/// Documentation here (don't wait for a later commit).
TheType get theProperty => _theProperty;
TheType _theProperty;
void set theProperty(TheType value) {
  assert(value != null);
  if (_theProperty == value) {
    return;
  }
  _theProperty = value;
  markNeedsWhatever(); // the method to mark the object dirty
}

The argument is called ‘value’ for ease of copy-and-paste reuse of this pattern. If for some reason you don't want to use ‘value’, use ‘newProperty’ (where ‘Property’ is the property name).

Start the method with any asserts you need to validate the value.

Don‘t do anything else in setters, other than marking the object as dirty and updating internal state. Getters and setters should not have significant side-effects. For example, setting a property whose value is a callback should not result in that callback being invoked. Setting a property whose value is an object of some sort should not result in any of that object’s methods being called.

Common boilerplates for operator == and hashCode

We have many classes that override operator == and hashCode (“value classes”). To keep the code consistent, we use the following style for these methods:

  @override
  bool operator ==(Object other) {
    if (other.runtimeType != runtimeType) {
      return false;
    }
    return other is Foo
        && other.bar == bar
        && other.baz == baz
        && other.quux == quux;
  }

  @override
  int get hashCode => Object.hash(bar, baz, quux);

For objects with a lot of properties, consider adding the following at the top of the operator ==:

    if (identical(other, this)) {
      return true;
    }

(We don‘t yet use this exact style everywhere, so feel free to update code you come across that isn’t yet using it.)

In general, consider carefully whether overriding operator == is a good idea. It can be expensive, especially if the properties it compares are themselves comparable with a custom operator ==. If you do override equality, you should use @immutable on the class hierarchy in question.

Override toString

Use Diagnosticable (rather than directly overriding toString) on all but the most trivial classes. That allows us to inspect the object from devtools and IDEs.

For trivial classes, override toString as follows, to aid in debugging:

  @override
  String toString() => '${objectRuntimeType(this, 'NameOfObject')}($bar, $baz, $quux)';

...but even then, consider using Diagnosticable instead. Avoid using $runtimeType, since it adds a non-trivial cost even in release and profile mode. The objectRuntimeType method handles this for you, falling back to a supplied constant string when asserts are disabled.

Be explicit about dispose() and the object lifecycle

Even though Dart is garbage collected, having a defined object lifecycle and explicit ownership model (describing in the API documentation who is allowed to mutate the object, for instance) is important to avoid subtle bugs and confusing designs.

If your class has a clear “end of life”, for example, provide a dispose() method to clean up references such as listeners that would otherwise prevent some objects from getting garbage collected. For example, consider a widget that has a subscription on a global broadcast stream (that might have other listeners). That subscription will keep the widget from getting garbage collected until the stream itself goes away (which, for a global stream, might never happen).

In general, pretending that Dart does not have garbage collection is likely to lead to less confusing and buggy code, because it forces you to think about the implications of object ownership and lifecycles.

Test APIs belong in the test frameworks

Mechanisms that exist for test purposes do not belong in the core libraries, they belong in test harnesses. This keeps the cost of the main library down in production and avoids the risk that people might abuse test APIs.

Immutable classes should not have hidden state

Immutable classes (those with const constructors) should not have hidden state. For example, they should not use private statics or Expandos. If they are stateful, then they should not be const.

Avoid sync*/async*

Using generator functions (sync*/async*) can be a powerful improvement when callers will actually lazily evaluate the iterable and each iteration is expensive or there are a very large number of iterations.

It should not be used in place of building and returning a List, particularly for trivial methods that only yield a small number of members or when callers will evaluate the whole collection anyway. It should also be avoided in very large functions.

It incurs runtime overhead in maintaining and using an iterator, and space overhead for the compiler to actually desugar the generator into something that uses an iterator class.

Writing tests

Make each test entirely self-contained

Embrace code duplication in tests. It makes it easier to make new tests by copying and pasting them and tweaking a few things.

Avoid using setUp, tearDown, and similar features, as well as test-global variables or other state shared between tests. They make writing tests easier but make maintaining them, debugging them, and refactoring code much harder. (These are commonly used in Flutter‘s codebase today, but that is almost always a mistake. When you are editing a file that uses those features, aim to reduce the number of tests using them while you’re there.)

Specifically, we are trying to avoid shared state, which could persist across tests, and non-local side-effects, which would prevent being able to move a test to another file without breaking the test. (It‘s fine to factor out code into functions that are called by tests, so long as the functions don’t have side-effects that might change how other tests run.)

Prefer more test files, avoid long test files

Avoid adding tests to files that already have more than one or two hundred lines of code. It's easier to understand a test file when it has only a few related tests, rather than when it has an entire test suite. (It also makes developing the tests faster because you can run the test file faster.)

Avoid using pumpAndSettle

As per the API docs for pumpAndSettle, prefer using explicit pump calls rather than pumpAndSettle.

Using pumpAndSettle, especially without checking its return value, makes it very easy for bugs to sneak in where we trigger animations across multiple frames instead of immediately. It is almost always the case that a call to pumpAndSettle is more strictly correctly written as two pump calls, one to trigger the animations and one (with a duration) to jump to the point after the animations.

Naming

Begin global constant names with prefix “k”

Examples:

const double kParagraphSpacing = 1.5;
const String kSaveButtonTitle = 'Save';

However, where possible avoid global constants. Rather than kDefaultButtonColor, consider Button.defaultColor. If necessary, consider creating an abstract final class to hold relevant constants.

Avoid abbreviations

Unless the abbreviation is more recognizable than the expansion (e.g. XML, HTTP, JSON), expand abbrevations when selecting a name for an identifier. In general, avoid one-character names unless one character is idiomatic (for example, prefer index over i, but prefer x over horizontalPosition).

Avoid anonymous parameter names

Provide full type information and names even for parameters that are otherwise unused. This makes it easier for people reading the code to tell what is actually going on (e.g. what is being ignored). For example:

  onTapDown: (TapDownDetails details) { print('hello!'); }, // GOOD
  onTapUp: (_) { print('good bye'); }, // BAD

Naming rules for typedefs and function variables

When naming callbacks, use FooCallback for the typedef, onFoo for the callback argument or property, and handleFoo for the method that is called. If Foo is a verb, prefer the present tense to the past tense (e.g. onTap instead of onTapped).

If you have a callback with arguments but you want to ignore the arguments, give the type and names of the arguments anyway. That way, if someone copies and pastes your code, they will not have to look up what the arguments are.

Never call a method onFoo. If a property is called onFoo it must be a function type. (For all values of “Foo”.)

Prefer using typedefs to declare callbacks. Typedefs benefit from having documentation on the type itself and make it easier to read and find common callsites for the signature.

Spell words in identifiers and comments correctly

Our primary source of truth for spelling is the Material Design Specification. Our secondary source of truth is dictionaries.

Avoid “cute” spellings. For example, ‘colors’, not ‘colorz’.

Prefer US English spellings. For example, ‘colorize’, not ‘colourise’, and ‘canceled’, not ‘cancelled’.

Prefer compound words over “cute” spellings to avoid conflicts with reserved words. For example, ‘classIdentifier’, not ‘klass’.

Capitalize identifiers consistent with their spelling

In general, we use Dart's recommendations for naming identifiers. Please consider the following additional guidelines:

If a word is correctly spelled (according to our sources of truth as described in the previous section) as a single word, then it should not have any inner capitalization or spaces.

For examples, prefer toolbar, scrollbar, but appBar (‘app bar’ in documentation), tabBar (‘tab bar’ in documentation).

Similarly, prefer offstage rather than offStage.

Avoid using class names with iOS when possible. The capitalization of iOS is supposed to be exactly that, but that doesn't work well with camelCase and even less with UpperCamelCase; use alternatives like “Cupertino” or “UIKit” instead when possible. If you really really must use “iOS” in an identifier, capitalize it to IOS. Whether or not the two-letter exception applies to “iOS” is debatable, but IOS is consistent with Dart APIs, and the alternatives (IOs, Ios) are even more jarring. (Previous versions of this guide incorrectly indicated that Ios was the correct capitalization when necessary; this form should not be used in new code.)

Avoid double negatives in APIs

Name your boolean variables in positive ways, such as “enabled” or “visible”, even if the default value is true.

This is because, when you have a property or argument named “disabled” or “hidden”, it leads to code such as input.disabled = false or widget.hidden = false when you're trying to enable or show the widget, which is very confusing.

Prefer naming the argument to a setter value

Unless this would cause other problems, use value for the name of a setter's argument. This makes it easier to copy/paste the setter later.

Qualify variables and methods used only for debugging

If you have variables or methods (or even classes!) that are only used in debug mode, prefix their names with debug or _debug (or, for classes, _Debug).

Do not use debugging variables or methods (or classes) in production code.

Avoid naming undocumented libraries

In other words, do not use the library keyword, unless it is a documented top-level library intended to be imported by users.

Avoid “new/old” modifiers in code

The definition of “New” changes as code grows and time passes. If the code needed a replacement version the odds of needing another replacement in the future is higher. Instead find a name that represents the idea being used or replaced.

Comments

Avoid checking in comments that ask questions

Find the answers to the questions, or describe the confusion, including references to where you found answers.

If commenting on a workaround due to a bug, also leave a link to the issue and a TODO to clean it up when the bug is fixed.

Example:

// BAD:

// What should this be?

// This is a workaround.


// GOOD:

// According to this specification, this should be 2.0, but according to that
// specification, it should be 3.0. We split the difference and went with
// 2.5, because we didn't know what else to do.

// TODO(username): Converting color to RGB because class Color doesn't support
//                 hex yet. See http://link/to/a/bug/123

TODOs should include the string TODO in all caps, followed by the GitHub username of the person with the best context about the problem referenced by the TODO in parentheses. A TODO is not a commitment that the person referenced will fix the problem, it is intended to be the person with enough context to explain the problem. Thus, when you create a TODO, it is almost always your username that is given.

Including an issue link in a TODO description is required.

(See also #37519, which tracks a proposal to change the syntax of TODOs to not include usernames.)

Comment all // ignores

Sometimes, it is necessary to write code that the analyzer is unhappy with.

If you find yourself in this situation, consider how you got there. Is the analyzer actually correct but you don't want to admit it? Think about how you could refactor your code so that the analyzer is happy. If such a refactor would make the code better, do it. (It might be a lot of work... embrace the yak shave.)

If you are really really sure that you have no choice but to silence the analyzer, use // ignore: . The ignore directive should be on the same line as the analyzer warning.

If the ignore is temporary (e.g. a workaround for a bug in the compiler or analyzer, or a workaround for some known problem in Flutter that you cannot fix), then add a link to the relevant bug, as follows:

  foo(); // ignore: lint_code, https://link.to.bug/goes/here

If the ignore directive is permanent, e.g. because one of our lints has some unavoidable false positives and in this case violating the lint is definitely better than all other options, then add a comment explaining why:

  foo(); // ignore: lint_code, sadly there is no choice but to do
  // this because we need to twiddle the quux and the bar is zorgle.

Comment all test skips

On very rare occasions it may be necessary to skip a test. To do that, use the skip argument. Any time you use the skip argument, file an issue describing why it is skipped and include a link to that issue in the code.

Comment empty closures to setState

Generally the closure passed to setState should include all the code that changes the state. Sometimes this is not possible because the state changed elsewhere and the setState is called in response. In those cases, include a comment in the setState closure that explains what the state is that changed.

  setState(() {
    // The animation ticked. We use the animation's value in the build method.
  });

Formatting

These guidelines have no technical effect, but they are still important purely for consistency and readability reasons.

We do not yet use dartfmt (except in flutter/packages). Flutter code tends to use patterns that the standard Dart formatter does not handle well. We are working with Dart team to make dartfmt aware of these patterns.

In defense of the extra work that hand-formatting entails

Flutter code might eventually be read by hundreds of thousands of people each day. Code that is easier to read and understand saves these people time. Saving each person even a second each day translates into hours or even days of saved time each day. The extra time spent by people contributing to Flutter directly translates into real savings for our developers, which translates to real benefits to our end users as our developers learn the framework faster.

Constructors come first in a class

The default (unnamed) constructor should come first, then the named constructors. They should come before anything else (including, e.g., constants or static methods).

This helps readers determine whether the class has a default implied constructor or not at a glance. If it was possible for a constructor to be anywhere in the class, then the reader would have to examine every line of the class to determine whether or not there was an implicit constructor or not.

Order other class members in a way that makes sense

The methods, properties, and other members of a class should be in an order that will help readers understand how the class works.

If there's a clear lifecycle, then the order in which methods get invoked would be useful, for example an initState method coming before dispose. This helps readers because the code is in chronological order, so they can see variables get initialized before they are used, for instance. Fields should come before the methods that manipulate them, if they are specific to a particular group of methods.

For example, RenderObject groups all the layout fields and layout methods together, then all the paint fields and paint methods, because layout happens before paint.

If no particular order is obvious, then the following order is suggested, with blank lines between each one:

1. Constructors, with the default constructor first.
2. Constants of the same type as the class.
3. Static methods that return the same type as the class.
4. Final fields that are set from the constructor.
5. Other static methods.
6. Static properties and constants.
7. Members for mutable properties, without new lines separating the members of a property, each property in the order:
   • getter
   • private field
   • setter
8. Read-only properties (other than hashCode).
9. Operators (other than ==).
10. Methods (other than toString and build).
11. The build method, for Widget and State classes.
12. operator ==, hashCode, toString, and diagnostics-related methods, in that order.

Be consistent in the order of members. If a constructor lists multiple fields, then those fields should be declared in the same order, and any code that operates on all of them should operate on them in the same order (unless the order matters).

Constructor syntax

If you call super() in your initializer list, put a space between the constructor arguments' closing parenthesis and the colon. If there‘s other things in the initializer list, align the super() call with the other arguments. Don’t call super if you have no arguments to pass up to the superclass.

// one-line constructor example
class ConstantTween<T> extends Tween<T> {
  ConstantTween(T value) : super(begin: value, end: value);

  // ...
}

// fully expanded constructor example
class ConstantTween<T> extends Tween<T> {
  ConstantTween(
    T value,
  ) : super(
        begin: value,
        end: value,
      );

  // ...
}

Prefer a maximum line length of 80 characters

Aim for a maximum line length of roughly 80 characters, but prefer going over if breaking the line would make it less readable, or if it would make the line less consistent with other nearby lines. Prefer avoiding line breaks after assignment operators.

// BAD (breaks after assignment operator and still goes over 80 chars)
final int a = 1;
final int b = 2;
final int c =
    a.very.very.very.very.very.long.expression.that.returns.three.eventually().but.is.very.long();
final int d = 4;
final int e = 5;

// BETTER (consistent lines, not much longer than the earlier example)
final int a = 1;
final int b = 2;
final int c = a.very.very.very.very.very.long.expression.that.returns.three.eventually().but.is.very.long();
final int d = 4;
final int e = 5;

// BAD (breaks after assignment operator)
final List<FooBarBaz> _members =
  <FooBarBaz>[const Quux(), const Qaax(), const Qeex()];

// BETTER (only slightly goes over 80 chars)
final List<FooBarBaz> _members = <FooBarBaz>[const Quux(), const Qaax(), const Qeex()];

// BETTER STILL (fits in 80 chars)
final List<FooBarBaz> _members = <FooBarBaz>[
  const Quux(),
  const Qaax(),
  const Qeex(),
];

Indent multi-line argument and parameter lists by 2 characters

When breaking an argument list into multiple lines, indent the arguments two characters from the previous line.

Example:

Foo f = Foo(
  bar: 1.0,
  quux: 2.0,
);

When breaking a parameter list into multiple lines, do the same.

If you have a newline after some opening punctuation, match it on the closing punctuation.

And vice versa.

Example:

// BAD:
  foo(
    bar, baz);
  foo(
    bar,
    baz);
  foo(bar,
    baz
  );

// GOOD:
  foo(bar, baz);
  foo(
    bar,
    baz,
  );

Use a trailing comma for arguments, parameters, and list items, but only if they each have their own line.

Example:

List<int> myList = [
  1,
  2,
];
myList = <int>[3, 4];

foo1(
  bar,
  baz,
);
foo2(bar, baz);

If one of the items is a multi-line callback, collection literal, or switch expression, it can be added without a trailing comma.

// GOOD:
foo(
  bar,
  baz,
  switch (value) {
    true  => ScrollDirection.forward,
    false => ScrollDirection.reverse,
    null  => ScrollDirection.idle,
  },
);

// also GOOD:
foo(bar, baz, switch (value) {
  true  => ScrollDirection.forward,
  false => ScrollDirection.reverse,
  null  => ScrollDirection.idle,
});

// The same applies to collection literals and callbacks:
foo(<String>[
  'list item 1',
  'list item 2',
  'list item 3',
]);

Future.delayed(Durations.short1, () {
  if (mounted && _shouldOpenDrawer) {
    _drawerController.forward();
  }
});

Whether to put things all on one line or whether to have one line per item is an aesthetic choice. We prefer whatever ends up being most readable. Typically this means that when everything would fit on one line, put it all on one line, otherwise, split it one item to a line.

However, there are exceptions. For example, if there are six back-to-back lists and all but one of them need multiple lines, then one would not want to have the single case that does fit on one line use a different style than the others.

  // BAD (because the second list is unnecessarily and confusingly different than the others):
  List<FooBarBaz> myLongList1 = <FooBarBaz>[
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
  ];
  List<Quux> myLongList2 = <Quux>[ Quux(1), Quux(2) ];
  List<FooBarBaz> myLongList3 = <FooBarBaz>[
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
  ];

  // GOOD (code is easy to scan):
  List<FooBarBaz> myLongList1 = <FooBarBaz>[
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
  ];
  List<Quux> myLongList2 = <Quux>[
    Quux(1),
    Quux(2),
  ];
  List<FooBarBaz> myLongList3 = <FooBarBaz>[
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
    FooBarBaz(one: firstArgument, two: secondArgument, three: thirdArgument),
  ];

Consider using => for short functions and methods

But only use => when everything, including the function declaration, fits on a single line.

Example:

// BAD:
String capitalize(String s) =>
  '${s[0].toUpperCase()}${s.substring(1)}';

// GOOD:
String capitalize(String s) => '${s[0].toUpperCase()}${s.substring(1)}';

String capitalize(String s) {
  return '${s[0].toUpperCase()}${s.substring(1)}';
}

Use => for getters and callbacks that just return literals or switch expressions

// GOOD:
List<Color> get favorites => <Color>[
  const Color(0xFF80FFFF),
  const Color(0xFF00FFF0),
  const Color(0xFF4000FF),
  _mysteryColor(),
];

// GOOD:
bool get isForwardOrCompleted => switch (status) {
  AnimationStatus.forward || AnimationStatus.completed => true,
  AnimationStatus.reverse || AnimationStatus.dismissed => false,
};

It's important to use discretion, since there are cases where a function body is easier to visually parse:

// OKAY, but the code is more dense than it could be:
String? get validated => switch(input[_inputIndex]?.trim()) {
  final String value when value.isNotEmpty => value,
  _ => null,
}

// BETTER (more verbose, but also more readable):
String? get validated {
  final String? value = input[_inputIndex]?.trim();

  if (value != null && value.isNotEmpty) {
    return value;
  }
  return null;
}

If your code is passing an inline closure containing only a return statement, you can instead use the => form.
When doing this, the closing ], }, or ) bracket will have the same indentation as the line where the callback starts.

For example:

    // GOOD, but slightly more verbose than necessary since it doesn't use =>
    @override
    Widget build(BuildContext context) {
      return PopupMenuButton<String>(
        onSelected: (String value) { print('Selected: $value'); },
        itemBuilder: (BuildContext context) {
          return <PopupMenuItem<String>>[
            PopupMenuItem<String>(
              value: 'Friends',
              child: MenuItemWithIcon(Icons.people, 'Friends', '5 new'),
            ),
            PopupMenuItem<String>(
              value: 'Events',
              child: MenuItemWithIcon(Icons.event, 'Events', '12 upcoming'),
            ),
          ];
        }
      );
    }

    // GOOD, does use =>, slightly briefer
    @override
    Widget build(BuildContext context) {
      return PopupMenuButton<String>(
        onSelected: (String value) { print('Selected: $value'); },
        itemBuilder: (BuildContext context) => <PopupMenuItem<String>>[
          PopupMenuItem<String>(
            value: 'Friends',
            child: MenuItemWithIcon(Icons.people, 'Friends', '5 new'),
          ),
          PopupMenuItem<String>(
            value: 'Events',
            child: MenuItemWithIcon(Icons.event, 'Events', '12 upcoming'),
          ),
        ]
      );
    }

The important part is that the closing punctuation lines up with the start of the line that has the opening punctuation, so that you can easily determine what's going on by just scanning the indentation on the left edge.

Prefer single line for short collection-if and collection-for

If the code fits in a single line don't split it.

For example:

// BAD
final List<String> args = <String>[
  'test',
  if (useFlutterTestFormatter) '-rjson'
  else '-rcompact',
  '-j1',
  if (!hasColor)
    '--no-color',
  for (final String opt in others)
    opt,
];

// GOOD
final List<String> args = <String>[
  'test',
  if (useFlutterTestFormatter) '-rjson' else '-rcompact',
  '-j1',
  if (!hasColor) '--no-color',
  for (final String opt in others) opt,
];

Otherwise indent with 2 spaces

// GOOD
final List<String> args = <String>[
  'test',
  if (useFlutterTestFormatter)
    '-rjson.very.very.very.very.very.very.very.very.long'
  else
    '-rcompact.very.very.very.very.very.very.very.very.long',
  '-j1',
  if (!hasColor)
    '--no-color.very.very.very.very.very.very.very.very.long',
  for (final String opt in others)
    methodVeryVeryVeryVeryVeryVeryVeryVeryVeryLong(opt),
];

Put spread inside collection-if or collection-for on the same line

Spreads inside collection-if or collection-for are used to insert several elements. It's easier to read to have spread on the line of if, else, or for.

// BAD
final List<String> args = <String>[
  'test',
  if (condA)
    ...<String>[
      'b',
      'c',
    ]
  else
    '-rcompact',
  for (final String opt in others)
    ...<String>[
      m1(opt),
      m2(opt),
    ],
];

// GOOD
final List<String> args = <String>[
  'test',
  if (condA) ...<String>[
    'b',
    'c',
  ] else
    '-rcompact',
  for (final String opt in others) ...<String>[
    m1(opt),
    m2(opt),
  ],
];

Use braces for long functions and methods

Use a block (with braces) when a body would wrap onto more than one line (as opposed to using =>; the cases where you can use => are discussed in the previous two guidelines).

Align expressions

Where possible, subexpressions on different lines should be aligned, to make the structure of the expression easier. When doing this with a return statement chaining || or && operators, consider putting the operators on the left hand side instead of the right hand side.

// BAD:
if (foo.foo.foo + bar.bar.bar * baz - foo.foo.foo * 2 +
    bar.bar.bar * 2 * baz > foo.foo.foo) {
  // ...
}

// GOOD (notice how it makes it obvious that this code can be simplified):
if (foo.foo.foo     + bar.bar.bar     * baz -
    foo.foo.foo * 2 + bar.bar.bar * 2 * baz   > foo.foo.foo) {
  // ...
}
// After simplification, it fits on one line anyway:
if (bar.bar.bar * 3 * baz > foo.foo.foo * 2) {
  // ...
}

// BAD:
return foo.x == x &&
    foo.y == y &&
    foo.z == z;

// GOOD:
return foo.x == x &&
       foo.y == y &&
       foo.z == z;

// ALSO GOOD:
return foo.x == x
    && foo.y == y
    && foo.z == z;

Prefer += over ++

We generally slightly prefer += over ++.

In some languages/compilers postfix ++ is an antipattern because of performance reasons, and so it's easier to just avoid it in general.

Because of the former, some people will use the prefix ++, but this leads to statements that lead with punctuation, which is aesthetically displeasing.

In general, mutating variables as part of larger expressions leads to confusion about the order of operations, and entwines the increment with another calculation.

Using ++ does not make it obvious that the underlying variable is actually being mutated, whereas += more clearly does (it's an assignment with an = sign).

Finally, += is more convenient when changing the increment to a number other than 1.

Use double literals for double constants

To make it clearer when something is a double or an integer, even if the number is a round number, include a decimal point in double literals. For example, if a function foo takes a double, write foo(1.0) rather than foo(1) because the latter makes it look like the function takes an integer.

Conventions

Expectations around potential crashes in the engine

The engine should never crash in an uncontrolled fashion.

In unopt mode, the engine C++ code should have asserts that check for contract violations.

In opt debug mode, the dart:ui code should have asserts that check for contract violations. These asserts should have messages that are detailed and useful, if they are not self-explanatory.

In opt release mode, the exact behavior can be arbitrary so long as it is defined and non-vulnerable for every input. For example, a contract violation could be checked in Dart, with an exception thrown for invalid data; but equally valid would be for the C++ code to return early when faced with invalid data. The idea is to optimize for speed in the case where the data is valid.

For practical purposes we don't currently check for out-of-memory errors. Ideally we would.

Features we expect every widget to implement

Now that the Flutter framework is mature, we expect every new widget to implement all of the following:

• full accessibility, so that on both Android and iOS the widget works with the native accessibility tools.
• full localization with default translations for all our default languages.
• full support for both right-to-left and left-to-right layouts, driven by the ambient Directionality.
• full support for text scaling up to at least 3.0x.
• documentation for every member; see the section above for writing prompts to write documentation.
• good performance even when used with large amounts of user data.
• a complete lifecycle contract with no resource leaks (documented, if it differs from usual widgets).
• tests for all the above as well as all the unique functionality of the widget itself.

It's the job of the programmer to provide these before submitting a PR.

It's the job of the reviewer to check that all these are present when reviewing a PR.

Use of streams in Flutter framework code

In general we avoid the use of Stream classes in Flutter framework code (and dart:ui). Streams in general are fine and we encourage people to use them. However, they have some disadvantages and we prefer to keep them out of the framework for this reason. For example:

• Streams have a heavy API. For example, they can be synchronous or asynchronous, broadcast or single-client, and they can be paused and resumed. It is non-trivial to determine the right semantics for a particular stream when it will be used in all the ways framework code could be used, and it is non-trivial to fully implement the semantics correctly.

• Streams don't have a “current value” accessor, which makes them difficult to use in build methods.

• The APIs for manipulating streams are non-trivial (e.g. transformers).

We generally prefer Listenable subclasses (e.g. ValueNotifier or ChangeNotifier).

In the specific case of exposing a value from dart:ui via a callback, we expect the bindings in the framework to register a single listener and then provide a mechanism to fan the notification to multiple listeners. Sometimes this is a rather involved process (e.g. the SchedulerBinding exists almost entirely for the purpose of doing this for onBeginFrame/onDrawFrame, and the GesturesBinding exists exclusively for the purpose of doing this for pointer events). Sometimes it's simpler (e.g. propagating changes to life cycle events).

Packages

Structure

As per normal Dart conventions, a package should have a single import that reexports all of its API.

For example, rendering.dart exports all of lib/src/rendering/*.dart

If a package uses, as part of its exposed API, types that it imports from a lower layer, it should reexport those types.

For example, material.dart reexports everything from widgets.dart. Similarly, the latter reexports many types from rendering.dart, such as BoxConstraints, that it uses in its API. On the other hand, it does not reexport, say, RenderProxyBox, since that is not part of the widgets API.

Flutter packages should not have “private” APIs other than those that are prefixed with underscores. Every file in a Flutter package should be exported. (“Private” files can still be imported so they are still actually public APIs; by not exporting them explicitly we are tricking ourselves into thinking of them as private APIs which may lead to poor design.)

When developing new features in Flutter packages, one should follow the philosophy:

Only expose the APIs that are necessities to the features.

Since the private classes in dart language are file-bound, this may often result in large file sizes. In Flutter, this is considered to be more preferable than creating multiple smaller files but exposing intermediate classes that are not needed to use the features.

Import conventions

Under lib/src, for in-folder import, use relative import. For cross-folder import, import the entire package with absolute import.

When importing the rendering.dart library into higher level libraries, if you are creating new RenderObject subclasses, import the entire library. If you are only referencing specific RenderObject subclasses, then import the rendering.dart library with a show keyword explicitly listing the types you are importing. This latter approach is generally good for documenting why exactly you are importing particularly libraries and can be used more generally when importing large libraries for very narrow purposes.

By convention, dart:ui is imported using import 'dart:ui' show ...; for common APIs (this isn't usually necessary because a lower level will have done it for you), and as import 'dart:ui' as ui show ...; for low-level APIs, in both cases listing all the identifiers being imported. See basic_types.dart in the painting package for details of which identifiers we import which way. Other packages are usually imported undecorated unless they have a convention of their own (e.g. path is imported as path).

The dart:math library is always imported as math.

Deciding where to put code

As a general rule, if a feature is entirely self-contained (not requiring low-level integration into the Flutter framework) and is not something with universal appeal, we would encourage that that feature be provided as a package.

We try to be very conservative with what we put in the core framework, because there‘s a high cost to having anything there. We have to commit to supporting it for years to come, we have to document it, test it, create samples, we have to consider everyone’s varied desires which they may have as they use the feature, we have to fix bugs. If there are design problems, we may not find out for a long time but then once we do we then have to figure out how to fix them without breaking downstream code, or we have to migrate all our existing widgets to the new architecture, etc.

Basically, code is expensive. So before we take it, if possible, we like to see if we can prove the code's value. By creating a package, we can see if people use the feature, how they like it, whether it would be useful for the framework, etc, without having to take on the costs.

We have two main kinds of packages that are maintained by the Flutter team, both of which live in flutter/packages:

1. Regular packages, which are pure Dart. Packages can also be written and maintained by people outside the Flutter team.

2. Plugin packages, which provide access to platform features and therefore include native code (such as Java or Objective-C) as well as Dart.

You can also consider making an independent package. Packages are published to pub.

Often once we have made a package we find that that is actually sufficient to solve the problem that the code sets out to solve, and there ends up being no need to bring it into the framework at all.