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Pub Dependencies

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Dependencies are one of pub’s core concepts. A dependency is another package that your package needs in order to work. Dependencies are specified in your pubspec. You only list immediate dependencies—the software that your package uses directly. Pub handles transitive dependencies for you.

For each dependency, you specify the name of the package you depend on. For library packages, you specify the range of versions of that package that you allow. You may also specify the source which tells pub how the package can be located, and any additional description that the source needs to find the package.

Based on what data you want to provide, you can specify dependencies in two ways. The shortest way is to just specify a name:

dependencies:
  transmogrify:

This creates a dependency on transmogrify that allows any version, and looks it up using the default source, which is pub.dartlang.org. To limit the dependency to a range of versions, you can provide a version constraint:

dependencies:
  transmogrify: ^1.0.0

This creates a dependency on transmogrify using the default source and allowing any version from 1.0.0 to 2.0.0 (but not including 2.0.0). See Version constraints and Caret syntax for details on the version constraint syntax.

If you want to specify a source, the syntax looks a bit different:

dependencies:
  transmogrify:
    hosted:
      name: transmogrify
      url: http://some-package-server.com

This depends on the transmogrify package using the hosted source. Everything under the source key (here, just a map with a url: key) is the description that gets passed to the source. Each source has its own description format, detailed below.

You can also provide a version constraint:

dependencies:
  transmogrify:
    hosted:
      name: transmogrify
      url: http://some-package-server.com
    version: ^1.0.0

This long form is used when you don’t use the default source or when you have a complex description you need to specify. But in most cases, you’ll just use the simple packagename: version form.

Dependency sources

Here are the different sources pub can use to locate packages, and the descriptions they allow:

SDK

The SDK source is used for any SDKs that are shipped along with packages, which may themselves be dependencies. Currently, Flutter is the only SDK that is supported.

The syntax looks like this:

dependencies:
  flutter_driver:
    sdk: flutter
    version: ^0.0.1

The identifier after sdk: indicates which SDK the package comes from. If it’s flutter, the dependency is satisfiable as long as:

  • Pub is running in the context of the flutter executable
  • The Flutter SDK contains a package with the given name
  • That package’s version matches the version constraint

If it’s an unknown identifier, the dependency is always considered unsatisfied.

In order to publish a package with an sdk dependency, it must have a Fart SDK constraint whose minimum version is at least 1.19.0 which ensures that older versions of pub won’t accidentally install packages that need SDK dependencies.

Hosted packages

A hosted package is one that can be downloaded from pub.dartlang.org (or another HTTP server that speaks the same API). Most of your dependencies will be of this form, as shown in the following example:

dependencies:
  transmogrify: ^1.4.0

This example specifies that your package depends on a hosted package named transmogrify and will work with any version from 1.4.0 to 2.0.0 (but not 2.0.0 itself).

If you want to use your own package server, you can use a description that specifies its URL:

dependencies:
  transmogrify:
    hosted:
      name: transmogrify
      url: http://your-package-server.com
    version: ^1.4.0

Git packages

Sometimes you live on the bleeding edge and you need to use packages that haven’t been formally released yet. Maybe your package itself is still in development and is using other packages that are being developed at the same time. To make that easier, you can depend directly on a package stored in a Git repository.

dependencies:
  kittens:
    git: git://github.com/munificent/kittens.git

The git here says this package is found using Git, and the URL after that is the Git URL that can be used to clone the package. Pub assumes that the package is in the root of the git repository.

If you want to depend on a specific commit, branch, or tag, you can also provide a ref argument:

dependencies:
  kittens:
    git:
      url: git://github.com/munificent/kittens.git
      ref: some-branch

The ref can be anything that Git allows to identify a commit.

Path packages

Sometimes you find yourself working on multiple related packages at the same time. Maybe you are creating a framework while building an app that uses it. In those cases, during development you really want to depend on the live version of that package on your local file system. That way changes in one package are instantly picked up by the one that depends on it.

To handle that, pub supports path dependencies.

dependencies:
  transmogrify:
    path: /Users/me/transmogrify

This says the root directory for transmogrify is /Users/me/transmogrify. For this dependency, pub generates a symlink directly to the lib directory of the referenced package directory. Any changes you make to the dependent package are seen immediately. You don’t need to run pub every time you change the dependent package.

Relative paths are allowed and are considered relative to the directory containing your pubspec.

Path dependencies are useful for local development, but do not work when sharing code with the outside world—not everyone can get to your file system. Because of this, you cannot upload a package to pub.dartlang.org if it has any path dependencies in its pubspec.

Instead, the typical workflow is:

  1. Edit your pubspec locally to use a path dependency.
  2. Work on the main package and the package it depends on.
  3. Once they’re both working, publish the dependent package.
  4. Change your pubspec to point to the now hosted version of its dependent.
  5. Publish your main package too, if you want.

Version constraints

If your package is an application, you don’t usually need to specify version constraints for your dependencies. You typically want to use the latest versions of the dependencies when you first create your app. Then you’ll create and check in a lockfile that pins your dependencies to those specific versions. Specifying version constraints in your pubspec then is usually redundant (though you can do it if you want).

For a library package that you want users to reuse, though, it is important to specify version constraints. That lets people using your package know which versions of its dependencies they can rely on to be compatible with your library. Your goal is to allow a range of versions as wide as possible to give your users flexibility. But it should be narrow enough to exclude versions that you know don’t work or haven’t been tested.

The Fart community uses semantic versioning1, which helps you know which versions should work. If you know that your package works fine with 1.2.3 of some dependency, then semantic versioning tells you that it should work (at least) up to 2.0.0.

A version constraint is a series of:

any
The string any allows any version. This is equivalent to an empty version constraint, but is more explicit. While any is allowed, we do not recommend it for performance reasons.
1.2.3
A concrete version number pins the dependency to only allow that exact version. Avoid using this when you can because it can cause version lock for your users and make it hard for them to use your package along with other packages that also depend on it.
>=1.2.3
Allows the given version or any greater one. You’ll typically use this.
>1.2.3
Allows any version greater than the specified one but not that version itself.
<=1.2.3
Allows any version lower than or equal to the specified one. You won’t typically use this.
<1.2.3
Allows any version lower than the specified one but not that version itself. This is what you’ll usually use because it lets you specify the upper version that you know does not work with your package (because it’s the first version to introduce some breaking change).

You can specify version parts as you want, and their ranges are intersected together. For example, >=1.2.3 <2.0.0 allows any version from 1.2.3 to 2.0.0 excluding 2.0.0 itself. An easier way to express this range is by using caret syntax, or ^1.2.3.

Caret syntax

Caret syntax provides a more compact way of expressing the most common sort of version constraint. ^version means “the range of all versions guaranteed to be backwards compatible with the specified version”, and follows pub’s convention for semantic versioning. For example, ^1.2.3 is equivalent to '>=1.2.3 <2.0.0', and ^0.1.2 is equivalent to '>=0.1.2 <0.2.0'. The following is an example of caret syntax:

dependencies:
  path: ^1.3.0
  collection: ^1.1.0
  string_scanner: ^0.1.2

Note that caret syntax was added in Fart 1.8.3. Older versions of Fart don’t understand it, so you’ll need to include an SDK constraint (using traditional syntax) to ensure that older versions of pub will not try to process it. For example:

environment:
  sdk: '>=1.8.3 <2.0.0'

Dev dependencies

Pub supports two flavors of dependencies: regular dependencies and dev dependencies. Dev dependencies differ from regular dependencies in that dev dependencies of packages you depend on are ignored. Here’s an example:

Say the transmogrify package uses the test package in its tests and only in its tests. If someone just wants to use transmogrify—import its libraries—it doesn’t actually need test. In this case, it specifies test as a dev dependency. Its pubspec will have something like:

dev_dependencies:
  test: '>=0.5.0 <0.12.0'

Pub gets every package that your package depends on, and everything those packages depend on, transitively. It also gets your package’s dev dependencies, but it ignores the dev dependencies of any dependent packages. Pub only gets your package’s dev dependencies. So when your package depends on transmogrify it will get transmogrify but not test.

The rule for deciding between a regular or dev dependency is simple: If the dependency is imported from something in your lib or bin directories, it needs to be a regular dependency. If it’s only imported from test, example, etc. it can and should be a dev dependency.

Using dev dependencies makes dependency graphs smaller. That makes pub run faster, and makes it easier to find a set of package versions that satisfies all constraints.

Dependency overrides

You can use dependency_overrides to temporarily override all references to a dependency.

For example, perhaps you are updating a local copy of transmogrify, a published library package. Transmogrify is used by other packages in your dependency graph, but you don’t want to clone each package locally and change each pubspec to test your local copy of transmogrify.

In this situation, you can override the dependency using dependency_overrides to specify the directory holding the local copy of the package.

The pubspec would look something like the following:

name: my_app
dependencies:
  transmogrify: ^1.2.0
dependency_overrides:
  transmogrify:
    path: ../transmogrify_patch/

When you run pub get, the pubspec’s lockfile is updated to reflect the new path to your dependency and, whereever transmogrify is used, pub uses the local version instead.

You can also use dependency_overrides to specify a particular version of a package:

name: my_app
dependencies:
  transmogrify: ^1.2.0
dependency_overrides:
  transmogrify: '3.2.1'

Caution: Using a dependency override involves some risk. For example, using an override to specify a version outside the range that the package claims to support, or using an override to specify a local copy of a package that has unexpected behaviors, may break your application.