Install Shared Packages

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Borrow and share code.

What’s the point?

  • Following a few conventions, such as having a valid pubspec.yaml file, makes your app a package.
  • Use Stagehand to generate starting files for your app.
  • Use pub get to download packages.
  • pub.dartlang.org is the primary public repository for Dart packages.

Now that you’re able to create and run a Dart application and have a basic understanding of DOM programming, you are ready to leverage code written by other programmers. Many interesting and useful packages of reusable Dart code are available at the pub.dartlang.org repository.

This tutorial shows you how to use pub—a package manager that comes with Dart—to install one of the packages in the repository, the vector_math package. You can follow these same steps to install any package hosted at pub.dartlang.org; just change the package name when you get to that step. This tutorial also describes some of the resources you can expect to find in a well-built package.

This tutorial uses the vector_math package. You can get this package, and many others, from pub.dartlang.org.

About the pubspec.yaml file

To use an external package, your application must itself be a package. Any application with a valid pubspec.yaml file in its top-level directory is a package and can therefore use external packages.

You can use the Stagehand tool to generate packages with valid pubspec.yaml files and directory structures. Stagehand works either at the command line or (behind the scenes) in an IDE, such as WebStorm.

Install Stagehand using pub global activate:

$ pub global activate stagehand

Now run the stagehand command to see what kinds of template files it can generate:

$ stagehand

You’ll see a list of generators, including various web and server apps. One of the web app generators is named web-simple.

In a new directory named vector_victor, use Stagehand to generate a bare-bones web app:

$ mkdir vector_victor
$ cd vector_victor
$ stagehand web-simple

The pubspec.yaml file contains the package specification written in YAML. (Visit Pubspec Format for in-depth coverage.) The contents of your pubspec.yaml file should look something like this:

name: 'vector_victor'
version: 0.0.1
description: An absolute bare-bones web app.
...
dependencies:
  browser: '>=0.10.0 <0.11.0'

The package name is required. Because all web apps depend on the browser package, browser is listed under dependencies.

Name the package dependencies

To use an external library package, you need to add the package to your application’s list of dependencies in the pubspec.yaml file. Each item in the dependencies list specifies the name and version of a package that your application uses.

Let’s make the vector_victor application have a dependency on the vector_math package, which is available at pub.dartlang.org.

  1. Get the current installation details for the package:

    • Go to vector_math's pub.dartlang.org entry.
    • Click the Installing tab.
    • Copy the vector_math line from the sample dependencies entry. The entry should look something like this:
      dependencies:
        vector_math: "^1.4.3"
      
  2. Edit pubspec.yaml.

  3. In the dependencies section, add the string you copied from pub.dartlang.org. Be careful to keep the indentation the same; YAML is picky! For example:

    dependencies:
      browser: '>=0.10.0 <0.11.0'
      vector_math: "^1.4.3"
    

See Pub Versioning Philosophy for details of what version numbers mean, and how you can format them.

pub.dartlang.org is the primary public repository for Dart packages. pub automatically checks that website when resolving package dependencies. To use one of the packages from that site, you can specify it by its simple name, as we have done here.

Install the package dependencies

If you’re using an IDE or Dart-savvy editor to edit pubspec.yaml, it might automatically install the packages your app depends on.

If not, do it yourself by running pub get:

$ pub get
Resolving dependencies... (1.4s)
+ browser 0.10.0+2
+ vector_math 1.4.3
Downloading vector_math 1.4.3...
Changed 2 dependencies!
Precompiling executables...
Loading source assets...
$

The pub get command installs the packages in your app’s dependencies list. Each package can contain libraries and other assets. Pub works recursively; if an included package has dependencies, those packages are installed as well. Pub caches the files for each package your app depends on, pointing to them from a file named .packages.

Pub creates a file called pubspec.lock that identifies the specific versions of the packages that were installed. This helps to provide a stable development environment. Later you can modify the version constraints and use pub upgrade to update to new versions as needed.

What did you get (and not get)?

Besides the Dart libraries, the vector_math package has other resources that might be useful to you that do not get installed into your application directory. Let’s take a step back for a moment to look at what you got and where it came from.

To see the contents of the vector_math package, visit the Dart vector math repository at github. Although many files and directories are in the repository, only one, lib, was installed when you ran pub get.


Dart libraries directory
Dart libraries: The lib directory contains one or more Dart libraries, which can be imported into your Dart programs.

Housekeeping files
Housekeeping files: When using a package written by someone else, the README file is a good place to start. It should contain important information about the package, such as its intent, contents, samples, and instructions. The LICENSE file provides copyright and rules-of-use information. These files can be found at the package repository. They are not installed when you install a package.

Document, scripts, tests, and other resources
Other resources: Along with Dart libraries, a package might also contain other resources such as example code, tests, scripts, and documentation. If a package contains these resources, they should be in the directories as specified in the pub conventions.

Import libraries from a package

Now that you’ve installed the package, you can import its libraries and use them in your Dart file.

As with the SDK libraries, use the import directive to use code from an installed library. The Dart SDK libraries are built in and are identified with the special dart: prefix. For external libraries installed by pub, use the package: prefix.

  1. Get the import details for the package's main library:
    1. Go to vector_math's pub.dartlang.org entry.
    2. Click the Installing tab.
    3. Copy the import line. It should look something like this:
      import 'package:vector_math/vector_math.dart';
      
  2. Edit your main Dart file (web/main.dart).
  3. Import the library from the package. By convention, package imports appear after dart:* imports:
    import 'dart:html';
    
    import 'package:vector_math/vector_math.dart';
    

Other resources

  • Dart developers share packages at pub.dartlang.org. Look there for packages that might be useful to you, or share your own Dart packages.
  • See the pub documentation for more information on using and sharing packages.