What Is a Package?

An installation package (also known as a package) is a file package (a directory that appears in the Finder as a single file) created using the PackageMaker application (/Developer/Applications/Utilities). Packages contain a product or product component—the package’s payload—to be installed on a computer, and install configuration information that determines where and how the product is installed.

Packages have the extension .pkg or .mpkg. When a user double-clicks a package in a Finder window, the Installer application opens the package and walks the user through the installation process.

A package can specify details about four aspects of the package itself and its payload:

• Product information:

  • Title

  • Description

  • Welcome file

  • Read Me file

  • License file

  • Conclusion file

• Package properties:

  • Package identifier

  • Package version number

  • Resource fork processing

• Installation properties:

  • System requirements

  • Volume requirements

  • Authentication requirement

  • Allowance for choosing an installation volume other than the boot volume

  • Installation destination on the installation volume

  • Relocation consent (the ability user may have to change the installation destination)

  • Revert consent

  • Directory-permissions overwrite

  • Postinstallation process action

• Install operations:

  • Preflight

  • Preinstall/Preupgrade

  • Postinstall/Postupgrade

  • Postflight

Component Packages

A complex product, such as the Levon product introduced in Overview of Software Delivery, is made up of distinct components. Except for single-component products, component packages are used in conjunction with metapackages or distribution packages (described later in this chapter) to create an install experience. Specifically, component packages:

• Are the basis for the mechanism that allows you to provide users a way to specify which components to install (for example, a user may not want to install a product’s tutorial files)

• Let you identify required components (which must be installed) and specify the locations of components to be installed at specific locations on the installation volume

• Allow you to specify system and volume requirements for the component using executable files (see Specifying System and Volume Requirements in Pre-Tiger Systems)

Component packages have the extension .pkg. Each component package contains a single product component and specifies product information, package properties, installation properties, and install operations.

Component packages can be installed on their own or as part of the install of a multicomponent product. After the payload of a component package is installed, Installer places a receipt in the /Library/Receipts directory of the installation volume. An installation receipt is a token that Installer uses to determine whether a package has already been installed on a system. As long as the receipt is present, subsequent installs of packages using the same package filename on the same volume are processed as upgrades.

Metapackages

A metapackage is an installation package that contains other installation packages. The enclosed packages can be component packages or metapackages (but not distribution packages).

Metapackages allow you to define a simple install experience for a multicomponent product. When users open a metapackage with the Installer application, they can choose to install only the components they need. Each enclosed package becomes an install choice. For example, if a product includes a tutorial-files component that the user performing the install doesn’t need, they can choose not to install that component.

Metapackages have the extension .mpkg. Table 4-1 shows what aspects of a metapackage and the packages (component packages and other metapackages) it contains are used in the installation process.

Table 4-1 indicates that the containing metapackage specifies package properties, installation properties, and specify install operations. However, the only aspect of the metapackage used in the installation process is its product information. Conversely, for the packages the metapackage contains, all aspects except product information are used in the installation process.

Distribution Packages

A distribution package is a metapackage that specifies both product and installation information for a product. Distribution packages provide more sophisticated facilities to tailor the installation process. The major features distribution packages provide are:

• Definition of the entire install experience in one place instead of having it spread out through several component packages

• Definition of system and volume requirements using a requirements editor instead of executables

• Install choices can contain more than one component package

• Users can choose an installation destination for each install choice instead of for the entire install

• Installer loads a distribution package for a multicomponent product with many component packages (and presents the user with the install experience it specifies) faster than a metapackage containing the same product

The central part of a distribution package is the distribution script. This is a JavaScript-based script file that contains all the information that defines an install experience. When you create a distribution package using PackageMaker, the package’s distribution script is created for you.

Distribution packages differ from metapackages in these areas:

• They can be installed only on computers running Mac OS X v10.4 (Tiger) and later.

• They must contain only component packages, not metapackages or distribution packages.

• Installer ignores installation properties specified in the contained component packages (installation properties are specified by the distribution script).

Table 4-2 shows what aspects of a distribution package and the component packages it contains are used in the installation process.

Table 4-2 indicates that a distribution package specifies only product information and installation properties, and both aspects are used in the installation process. The contained packages may specify all package aspects, but only their package properties and install operations contribute to the installation process.

Component Package Installation Process

This is how Installer performs the Requirements Check, Preinstall, and Postinstall phases of a component package’s installation process:

• Requirements Check

  InstallationCheck

  VolumeCheck on each available volume

• Preinstall

  preflight

  preinstall or preupgrade

• Postinstall

  postinstall or postupgrade

  postflight

Metapackage Installation Process

This is how the Installer application performs the Requirements Check, Preinstall, and Postinstall phases of a metapackage’s installation process:

• Requirements Check

  InstallationCheck for each package but not the top metapackage

  VolumeCheck on each available volume for each package but not the top metapackage

• Preinstall

  preflight for top metapackage

  preflight for each package

  preinstall or preupgrade for top metapackage

  preinstall or preupgrade for each package

• Postinstall

  postinstall or postupgrade for each package

  postinstall or postupgrade for top metapackage

  postflight for top metapackage

  postflight for each package

Distribution Package Installation Process

This is how the Installer application performs the Requirements Check, Preinstall, and Postinstall phases of a distribution package’s installation process:

• Requirements Check

  Installation Check script

  Volume Check script on each available volume

• Preinstall

  preflight for each package

  preinstall or preupgrade for each package

• Postinstall

  postinstall or postupgrade for each package

  postflight for each package