Libraries are a great way to create modular code that can be easily shared. In the Dart ecosystem, libraries are created and distributed as packages. Dart has two kinds of packages: application packages, which may include local libraries, and library packages.

This document explains how to create a library package and points you to further resources. For information on using libraries, see Install Shared Packages or the language tour’s Libraries and visibility section.

What makes a library package

The following diagram shows the layout of the simplest library package:

The minimal requirements for a library are:

pubspec file

The pubspec.yaml file for a library is the same as for an application package—there is no special designation to indicate that the package is a library.

lib directory

As you might expect, the library code lives under the lib directory and is public to other packages. You can create any hierarchy under lib, as needed. By convention, implementation code is placed under lib/src. Code under lib/src is considered private; other packages should never need to import src/.... To make APIs under lib/src public, you can export lib/src files from a file that’s directly under lib.

Organizing a library package

Library packages are easiest to maintain, extend, and test when you create small, individual libraries, referred to as mini libraries. In most cases, each class should be in its own mini library, unless you have a situation where two classes are tightly coupled.

Create a “main” library file directly under lib, lib/<package-name>.dart, that exports all of the public APIs. This allows the user to get all of a library’s functionality by importing a single file.

The lib directory might also include other importable, non-src, libraries. For example, perhaps your main library works across platforms, but you create separate libraries that rely on dart:io or dart:html. Some packages have separate libraries that are meant to be imported with a prefix, when the main library is not.

Let’s look at the organization of a real-world library package: shelf. The shelf package provides an easy way to create web servers using Dart, and is laid out in a structure that is commonly used for Dart library packages:

Directly under lib, the main library file, shelf.dart, exports several files from lib/src:

export 'src/cascade.dart';
export 'src/handler.dart';
export 'src/handlers/logger.dart';
export 'src/hijack_exception.dart';
export 'src/middleware.dart';
export 'src/pipeline.dart';
export 'src/request.dart';
export 'src/response.dart';
export 'src/server.dart';
export 'src/server_handler.dart';

The shelf package also contains a mini library: shelf_io. This adapter handles HttpRequest objects from dart:io.

Importing library files

When importing a library file, you can use the the package: directive to specify the URI of that file.

import 'package:utilities/utilities.dart';

You can import a library using a relative path when both files are inside of lib, or when both files are outside of lib. However, you must use package: when importing a file that reaches inside, or outside, of lib. When in doubt, use the package: directive; it works in all cases.

The following graphic shows how to import lib/foo/a.dart from both lib and web.

Providing additional files

A well designed library package is easy to test. We recommend that you write tests using the test package, placing the test code in the test directory at the top of the package.

If you create any command-line tools intended for public consumption, place those in the bin directory, which is public. Enable running a tool from the command line, using pub global activate. Listing the tool in the executables section of the pubspec allows a user to run it directly without calling pub global run.

It’s helpful if you include an example of how to use your library. This goes into the example directory at the top of the package.

Any tools or executables that you create during development that aren’t for public use go into the tool directory.

Other files that are required if you publish your library to the Pub site, such as a README and a CHANGELOG, are described in Publishing a Package. Also see Pub Package Layout Conventions for further information on how to organize a package directory structure.

Documenting a library

You can generate API docs for your library using the dartdoc tool. Dartdoc parses the source looking for documentation comments, which use the /// syntax:

/// The event handler responsible for updating the badge in the UI.
void updateBadge() {
  ...
}

For an example of generated docs, see the shelf documentation.

Distributing an open source library

If your library is open source, we recommend sharing it on the Pub site. To publish or update the library, use pub publish, which uploads your package and creates or updates its page. For example, see the page for the shelf package. See Publishing a Package for details on how to prepare your package for publishing.

The pub site not only hosts your package, but also generates and hosts your package’s API reference docs. A link to the latest generated docs is in the package’s About box; for example, see the shelf package’s API docs. Links to previous versions’ docs are in the Versions tab of the package’s page.

To ensure that your package’s API docs look good on the pub site, follow these steps:

• Before publishing your package, run the dartdoc tool to make sure that your docs generate successfully and look as expected.
• After publishing your package, check the Versions tab to make sure that the docs generated successfully.
• If the docs didn’t generate at all, click failed in the Versions tab to see the dartdoc output.

Resources

Use the following resources to learn more about library packages:

• Libraries and visibility in the language tour covers using library files.
• The pub documentation is useful, particularly Pub Package Layout Conventions.
• What Not to Commit covers what should not be checked into a source code repository.
• The newer library packages under the dart-lang organization tend to show best practices. Consider studying these examples: dart_style, path, shelf, source_gen, and test.