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 and lifecycle
• Package sources
• Global cache
• Dependency and resolution

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
  See in Glossary (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 dependencies
  See in Glossary.
• 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) 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:

• It communicates with the Unity package registry server 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 imports Asset Store packages that you previously downloaded or purchased.
• It enables and disables Unity’s built-in packages.
• 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 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.
See in Glossary. You can only permanently change content from Local and Embedded package sources.

Package states and lifecycle

After developing a package to the point where it is ready for Unity users to test it and provide feedback, a package enters the Preview state. Packages in this state usually appear in the Unity Editor with the preview label and often use preview as part of their version.

Unity’s release management only grants a package the verified status after it passes several testing stages and validation procedures, which also include checks for appropriate documentation, changelog, and license files. Packages in this state may appear in the Unity Editor with the 2019.4 verified label and never use preview as part of their version.

If the package developer introduces changing or breaking changes to a package (either by introducing a change that breaks an API or affects its usage), that package returns to the Preview state and the Package Manager does not suggest the new preview package as a possible update for the verified version.

These states indicate where the package is in the development cycle:

The Package Manager window displays a tagA 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 that corresponds to some of these states.

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.

Note: The Package Manager stores packages that you download from the Asset StoreA growing library of free and commercial assets created by Unity and members of the community. Offers a wide variety of assets, from textures, models and animations to whole project examples, tutorials and Editor extensions. More info
See in Glossary in a cache which is separate from the global cache. See Asset Store packages for more information.