Concepts

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

• Versions
• ManifestsThere are two types of manifest files: Project manifests and package manifests.
  See in Glossary
• Registry
• Package Management
• Package states
• Package sources

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), a 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
  See in Glossary (manifest.json) These store information that the Package Manager needs to locate and load the right packages, including a list of packages and versions declared as dependencies.
• 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
  See in Glossary (package.json). These 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

Unity maintains a central registry of official packages that are available for distribution. A package registry stores package contents and information (metadata) on each package version. By default, all Projects use the official Unity package registry, but you can add additional registries to store and distribute private packages or stage packages in development.

Package Management

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

• It communicates with the Unity registry and any additional registries you specify.
• It reads your Project manifest and fetches package contents and metadata.
• It installs, upgrades, and uninstalls packages, whether they are dependencies of the Project or one of the installed packages.
• It enables and disables Unity’s 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).
  See in Glossary.
• It displays information about every version of every package.
• It resolves conflicts when the Project and its packages require more than one package version.

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 read-only global cache to store downloaded package metadata and contents. Once installed, Unity treats package Assets just like any other Asset in the Project, except that these Assets 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.
See in Glossary (read-only). You can only change Assets from Local and Embedded package sources.

Package states

States indicate where the package is in the development cycle:

The Package Manager window displays a tag that corresponds to some of these states. 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
See in Glossary.

Package sources

Sources describe where the package came from:

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 Tags.