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3. Dart 2 中文文档
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5. 如何使用软件包

# How to use packages

The Dart ecosystem uses packages to manage shared software such as libraries and tools. To get Dart packages, you use the pub package manager. You can find publicly available packages on the Pub site, or you can load packages from the local file system or elsewhere, such as Git repositories. Wherever your packages come from, pub manages version dependencies, helping you get package versions that work with each other and with your SDK version.

Most Dart-savvy IDEs offer support for using pub that includes creating, downloading, updating, and publishing packages. Or you can use pub on the command line.

At a minimum, a Dart package is a directory containing a pubspec file. The pubspec contains some metadata about the package. Additionally, a package can contain dependencies (listed in the pubspec), Dart libraries, apps, resources, tests, images, and examples.

To use a package, do the following:

• Create a pubspec (a file named pubspec.yaml that lists package dependencies and includes other metadata, such as a name for your package).
• Use pub to get your package’s dependencies.
• If your Dart code depends on a library in the package, import the library.

## Creating a pubspec

The pubspec is a file named pubspec.yaml that’s in the top directory of your application. The simplest possible pubspec lists only the package name:

Here is an example of a pubspec that declares dependencies on two packages (js and intl) that are hosted on the Pub site:

For details on creating a pubspec, see the pubspec documentation and the documentation for the packages that you want to use.

## Getting packages

Once you have a pubspec, you can run pub get from the top directory of your application:

This process is called getting the dependencies.

The pub get command determines which packages your app depends on, and puts them in a central system cache. If your app depends on a published package, pub downloads that package from the Pub site. For a Git dependency, pub clones the Git repository. Transitive dependencies are included, too. For example, if the js package depends on the test package, pub grabs both the js package and the test package.

Pub creates a .packages file (under your app’s top directory) that maps each package name that your app depends on to the corresponding package in the system cache.

## Importing libraries from packages

To import libraries found in packages, use the package: prefix:

The Dart runtime takes everything after package: and looks it up within the .packages file for your app.

You can also use this style to import libraries from within your own package. Consider the following pubspec file, which declares a dependency on the (fictional) transmogrify package:

Let’s say that your package is laid out as follows:

The parser_test file could import parser.dart like this:

But that’s a fragile relative path. If parser_test.dart ever moves up or down a directory, that path breaks. Instead, you can do as follows:

This way, the import can always get to parser.dart regardless of where the importing file is.

The first time you get a new dependency for your package, pub downloads the latest version of it that’s compatible with your other dependencies. It then locks your package to always use that version by creating a lockfile. This is a file named pubspec.lock that pub creates and stores next to your pubspec. It lists the specific versions of each dependency (immediate and transitive) that your package uses.

If your package is an application package, you should check this file into source control. That way, everyone working on your app uses the same versions of all of the packages. Checking in the lockfile also ensures that your deployed app uses the same versions of code.

That command tells pub to regenerate the lockfile, using the newest available versions of your package’s dependencies. If you want to upgrade only one dependency, you can specify the package to upgrade:

That command upgrades transmogrify to the latest version but leaves everything else the same.

### Pub commands

The pub tool provides the following commands:

For an overview of all the pub commands, see the pub tool documentation.

### Troubleshooting

Troubleshooting pub gives solutions to problems that you might encounter when using pub.