Dart style guide

The Fuchsia project follows the guidelines in Effective Dart, but with some additions.

All code must be formatted using dart format before being checked in.

Additional Style Rules

DON'T follow the Flutter repository style guide.

The Flutter project‘s style guide is meant to be used for code in the Flutter framework itself. It is not intended as style guidance for projects using Flutter. All code in the Fuchsia repository should follow the standard Dart style. Although we are not following the style, the Flutter’s guide on documentation and development is still useful.

DO use trailing commas on all tree structures longer than one line.

Without trailing commas, code that builds widget trees or similar types of code tends to be hard to read. Adding the trailing commas allows dart format to do its job correctly.

Without trailing commas:

  child: Center(
      child: Container(
          width: 64.0, height: 64.0, child: FuchsiaSpinner())),

With trailing commas:

  child: Center(
      child: Container(
        width: 64.0,
        height: 64.0,
        child: FuchsiaSpinner(),

DO order members using the Dart Analyzer.

In Visual Studio Code, this is the Dart: Organize Members command available in the Command Palette. (Control+Shift+P or View -> Command Palette)

This formatter doesn’t appear to be available outside of the supported IDEs.

PREFER to keep lines below 80 characters unless it would be more readable.

This is a slight amendment from the general Dart rule. Unlike that rule, it is fine to have lines above 80 characters in the Fuchsia repository, as long as it improves readability, and dart format won't automatically truncate the line.

Additional Usage Rules

Repositories and Files

DO prefix library names in /lib and /public/lib with lib.


Dart_library("lib.settings") {
  package_name = "lib.settings"

PREFER minimizing the number of public members exposed in a package.

This can be done by only making things public when needed, and keeping all implementation detail libraries in the /src directory. Assume anything public in the lib directory will be re-used.

CONSIDER exporting publicly visible classes in a single .dart file.

For multiple classes that are used together but are in different files, it’s more convenient for users of your library to import a single file rather many at once. If the user wants narrower imports they can always restrict visibility using the show keyword.

This also helps minimize the publicly visible surface.


/// In src/apple.dart
class Apple {}

/// In src/orange.dart
class Orange {}

/// In src/veggies.dart
class Potato {}
class Tomato {}

/// In botanical_fruits.dart
export 'src/apple.dart';
export 'src/orange.dart';
// Can also be: export 'src/veggies.dart' hide Potato;
export 'src/veggies.dart' show Tomato;

/// In squeezer.dart
import 'package:plants/botanical_fruits.dart' show Orange;

DO import all files within a package using relative paths.

Mixing and matching relative and absolute paths within a single package causes Dart to act as if there were two separate imports of identical files, which will introduce errors in typechecking. Either format works as long as it is consistent. Within the Fuchsia repository, relative paths are used.

This does not apply to external libraries, as only the absolute path can be used.


import 'access_point.dart';


import 'package:wifi/access_point.dart';

DO use namespacing when you import FIDL packages.

This adds clarity and readability. FIDL namespaces (library statements) are not respected in Dart (e.g. fuchsia.io.Node becomes Node). Because of tight namespaces, people tend to use more generic names in FIDL (Error, File, Node, etc.), which result in more collisions/ambiguity in Dart.


import 'package:fidl_fuchsia_file/fidl.dart' as file_fidl;

file_fidl.File.get(...) ...


import 'package:fidl_fuchsia_file/fidl.dart';

File.get(...) ...

DO use namespacing when there is ambiguity, e.g. in class names.

There are often functions or classes that can collide, e.g. File or Image. If you don't namespace, there will be a compile error.


import 'dart:ui' as ui;

import 'package:flutter/material.dart';

ui.Image(...) ...


import 'dart:ui';

import 'package:flutter/material.dart';

Image(...) ... // Which Image is this?

PREFER to use show if you only have a few imports from that package. Otherwise, use as.

Using show can avoid collisions without requiring you to prepend namespaces to types, leading to cleaner code.


import 'package:fancy_style_guide/style.dart' as style;
import 'package:flutter/material.dart';
import 'package:math/simple_functions.dart' show Addition, Subtraction;


import 'package:flutter/material.dart show Container, Row, Column, Padding,
  Expanded, ...;

Coding practices

DON'T use new or use const redundantly.

Dart 2 makes the new optional for constructors, with an aim at removing them in time. The const keyword is also optional where it can be inferred by the compiler.

const can be inferred in:

  • A const collection literal.
  • A const constructor call.
  • A metadata annotation.
  • The initializer for a const variable declaration.
  • A switch case expression—the part right after case before the :, not the body of the case.

This guidance will eventually be part of Effective Dart due to the changes for Dart 2.


final foo = Foo();
const foo = Foo();
const foo = const <Widget>[A(), B()];
const Foo(): bar = Bar();


final foo = new Foo();
const foo = const Foo();
foo = const [const A(), const B()];
const Foo(): bar = const Bar();

DON'T do useful work in assert statements.

Code inside assert statements are not executed in production code. Asserts should only check conditions and be side-effect free.

PREFER to use const over final over var.

This minimizes the mutability for each member or local variable.

PREFER return Widget instead of a specific type of Flutter widget.

As your project evolves, you may change the widget type that is returned in your function. For example, you might wrap your widget with a Center. Returning Widget simplifies the refactoring, as the method signature wouldn't have to change.


Widget returnContainerWidget() {
  return Container();


Container returnContainerWidget() {
  return Container();

Additional Design Rules

PREFER storing state in Models instead of state.

When storing state that Flutter widgets need to access, prefer to use ScopedModel and ScopedModelDescendant instead of StatefulWidget. A StatefulWidget should contain only internal widget state that can be lost without any consequences.

Examples of stuff to store in a ScopedModel:

  • User selections
  • App state
  • Anything that needs to be shared by widgets

Examples of stuff to store in a StatefulWidget's State:

  • Animation state that doesn't need to be controlled

AVOID mixing named and positional parameters.

Instead, @required should be used in place of required positional parameters.

PREFER named parameters.

In most situations, named parameters are less error prone and easier to read than positional parameters, optional or not. They give users to pass in the parameters in whatever order they please, and make Flutter trees especially clearer.

In the Fuchsia repository, positional parameters should be reserved for simple operational functions with only a few parameters.


int add(int a, int b);
int addNumbers(int a, [int b, int c, int d]);
Foo fromJson(String json);
void load(String file);

Widget buildButton({
  @required Widget child,
  VoidCallback onTap,
  double width,
  bool isDisabled = false,


int add({int a, int b});
Foo fromJson({@required String json});

Widget buildButton(
  Widget child,
  VoidCallback onTap, [
  double width,
  bool isDisabled = false,

DO add logging statements