Version: 2020.3
Unity's Package Manager
Package states and lifecycle

Concepts

This section explains many of the concepts surrounding the Unity Package Manager functionality:

Versions

Multiple versions of each package are available, marking changes to that package along its life cycle. Every time a developer updates the package, they give it a new version number. A change in package version tells you whether it contains a breaking change (major), new backward-compatible functionality (minor), or bug fixes only (patch), following Semantic Versioning.

To view the list of versions available for a specific package, see Finding a specific version.

Manifests

There are two types of manifest files:

  • Project manifestsEach Unity project has a project manifest, which acts as an entry point for the Package Manager. This file must be available in the <project>/Packages directory. The Package Manager uses it to configure many things, including a list of dependencies for that project, as well as any package repository to query for packages. More info
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    (manifest.json) store information that the Package Manager needs to locate and load the right packages, including a list of packages and versions declared as dependenciesIn the context of the Package Manager, a dependency is a specific package version (expressed in the form package_name@package_version) that a project or another package requires in order to work. Projects and packages use the dependencies attribute in their manifests to define the set of packages they require. For projects, these are considered direct dependencies; for packages, these are indirect, or transitive, dependencies. More info
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    .
  • Package manifestsEach package has a manifest, which provides information about the package to the Package Manager. The manifest contains information such as the name of the package, its version, a description for users, dependencies on other packages (if any), and other details. More info
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    (package.json) store information about a specific package, and a list of packages and versions that the package requires.

Both files use JSON (JavaScript Object Notation) syntax.

Registry

In the domain of Unity’s Package Manager, a package registry is a server that stores package contents and information (metadata) on each package version. Unity maintains a central registry of official packages that are available for distribution. By default, all projects use the official Unity package registry, but you can add additional registries to store and distribute private packages or stage custom packages while you are developing them.

Package Management

The Unity Package Manager is a tool that manages the entire package system. Its primary tasks include the following:

The Unity Package Manager installs samples, tools, and assets on a per-project basis, rather than installing them across all projects for a specific machine or device. It uses a global cache to store downloaded package metadata and contents. Once installed in a project, Unity treats package assets just like any other asset in the project, except that these assets are stored inside the package folder and are immutableYou cannot change the contents of an immutable (read-only) package. This is the opposite of mutable. Most packages are immutable, including packages downloaded from the package registry or by Git URL.
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. You can only permanently change content from Local and Embedded package sources.

Package sources

Sources describe where the package came from:

Source Description
Registry The Unity Package Manager downloads most packages from a package registry server into a global cache on your computer as you request them. These packages are immutable, so you can use them in your project, but you cannot modify them or change their package manifests.
Built-in These packages allow you to enable or disable Unity features (for example, Terrain Physics, Animation, etc.). They are immutable. For more information, see Built-in packagesBuilt-in packages allow users to toggle Unity features on or off through the Package Manager. Enabling or disabling a package reduces the run-time build size. For example, most projects don’t use the legacy Particle System. By removing the abstracted package of this feature, the related code and resources are not part of the final built product. Typically, these packages contain only the package manifest and are bundled with Unity (rather than available on the package registry).
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Embedded Any package stored inside your project folder is embedded. This source corresponds with the Custom state because you typically put all the scripts, libraries, samples, and other assets your new package needs in a folder under your project folder when you begin development on a custom package.
Local You can install a package from any folder on your computer (for example, if you have cloned a development repository locally).
Tarball (local) You can install a package from a tarball file on your computer. The Package Manager extracts the package from the tarball and stores it in the cache. However, these packages are immutable, unlike installations from a local folder.
Git The Package Manager installs Git-based packages directly from a Git repository instead of from the package registry server.

To edit the package manifest for a package, see Inspecting packages.

The Package Manager window displays a tag that corresponds to some of these sources. For more information, see TagsA reference word which you can assign to one or more GameObjects to help you identify GameObjects for scripting purposes. For example, you might define and “Edible” Tag for any item the player can eat in your game. More info
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Unity's Package Manager
Package states and lifecycle