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Bundle structures can vary depending on the type of the bundle and the target platform. The following sections describe the bundle structures used most commonly in both macOS and iOS.

Note: Although bundles are one way of packaging executable code, they are not the only way that is supported. UNIX shell scripts and command-line tools do not use the bundle structure, neither do static and dynamic shared libraries.

Application Bundles

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Application bundles are one of the most common types of bundle created by developers. The application bundle stores everything that the application requires for successful operation. Although the specific structure of an application bundle depends on the platform for which you are developing, the way you use the bundle is the same on both platforms. This chapter describes the structure of application bundles in both iOS and macOS.

What Files Go Into an Application Bundle?

Table 2-1 summarizes the types of files you are likely to find inside an application bundle. The exact location of these files varies from platform to platform and some resources may not be supported at all. For examples and more detailed information, see the platform-specific bundle sections in this chapter.

Although most of the resources in an application bundle are optional, this may not always be the case. For example, iOS applications typically require additional image resources for the application’s icon and default screen. And although not explicitly required, most Mac apps include a custom icon instead of the default one provided by the system.

Anatomy of an iOS Application Bundle

The project templates provided by Xcode do most of the work necessary for setting up the bundle for your iPhone or iPad application. However, understanding the bundle structure can help you decide where you should place your own custom files. The bundle structure of iOS applications is geared more toward the needs of a mobile device. It uses a relatively flat structure with few extraneous directories in an effort to save disk space and simplify access to the files.

The iOS Application Bundle Structure

A typical iOS application bundle contains the application executable and any resources used by the application (for instance, the application icon, other images, and localized content) in the top-level bundle directory. Listing 2-1 shows the structure of a simple iPhone application called MyApp. The only files that are required to be in subdirectories are those that need to be localized; however, you could create additional subdirectories in your own applications to organize resources and other relevant files.

Listing 2-1 Bundle structure of an iOS application

Table 2-2 describes the contents of the application shown in Listing 2-1. Although the application itself is for demonstration purposes only, many of the files it contains represent specific files that iOS looks for when scanning an application bundle. Your own bundles would include some or all of these files depending on the features you support.

Note: An iOS app bundle cannot include a custom folder named “Resources.”

An iOS application should be internationalized and have a language.lproj folder for each language it supports. In addition to providing localized versions of your application’s custom resources, you can also localize your launch images by placing files with the same name in your language-specific project directories. Even if you provide localized versions, however, you should always include a default version of these files at the top-level of your application bundle. The default version is used in situations where a specific localization is not available. For more information about localized resources, see Localized Resources in Bundles.

The Information Property List File

Every iOS application must have an information property list (Info.plist) file containing the application’s configuration information. When you create a new iOS application project, Xcode creates this file automatically and sets the value of some of the key properties for you. Table 2-3 lists some additional keys that you should set explicitly. (Xcode obscures actual key names by default, so the string displayed by Xcode is also listed in parenthesis where one is used. You can see the real key names for all keys by Control-clicking the Information Property List key in the editor and choosing Show Raw Keys/Values from the contextual menu that appears.)

In addition to the keys in the preceding table, Table 2-4 lists some keys that are commonly used by iOS applications. Although these keys are not required, most provide a way to adjust the configuration of your application at launch time. Providing these keys can help ensure that your application is presented appropriately by the system.

Application Icon and Launch Images

Application icons and launch images are standard graphics that must be present in every application. Every application must specify an icon to be displayed on the device’s Home screen and in the App Store. And an application may specify several different icons for use in different situations. For example, applications can provide a small version of the application icon to use when displaying search results. Launch images provide visual feedback to the user that your application launched.

The image files used to represent icons and launch images must all reside in the root level of your bundle. How you identify these images to the system can vary, but the recommended way to specify your application icons is to use the CFBundleIconFiles key. For detailed information about how to specify the icons and launch images in your application, see the discussion of these items in Advanced App Tricks in App Programming Guide for iOS.

Note: In addition to the icons and launch image at the top level of your bundle, you can also include localized versions of launch images in your application’s language-specific project subdirectories. For more information about including localized resources in your application, see Localized Resources in Bundles.

Resources in an iOS Application

In an iOS application, nonlocalized resources are located at the top-level of the bundle directory, along with the application’s executable file and the Info.plist file. Most iOS applications have at least a few files at this level, including the application’s icon, launch image, and one or more nib files. Although you should place most nonlocalized resources in this top-level directory, you can also create subdirectories to organize your resource files. Localized resources must be placed in one or more language-specific subdirectories, which are discussed in more detail in Localized Resources in Bundles.

Listing 2-2 shows a fictional application that includes both localized and nonlocalized resources. The nonlocalized resources include Hand.png, MainWindow.nib, MyAppViewController.nib, and the contents of the WaterSounds directory. The localized resources include everything in the en.lproj and jp.lproj directories.

Listing 2-2 An iOS application with localized and nonlocalized resources

For information about finding resource files in your application bundle, see Accessing a Bundle's Contents. For information about how to load resource files and use them in your program, see Resource Programming Guide.

Anatomy of a macOS Application Bundle

The project templates provided by Xcode do most of the work necessary for setting up your Mac app bundle during development. However, understanding the bundle structure can help you decide where you should place your own custom files. macOS bundles use a highly organized structure to make it easier for the bundle-loading code to find resources and other important files in the bundle. The hierarchical nature also helps the system distinguish code bundles such as applications from the directory packages used by other applications to implement document types.

The Structure of a macOS Application Bundle

The basic structure of a Mac app bundle is very simple. At the top-level of the bundle is a directory named Contents. This directory contains everything, including the resources, executable code, private frameworks, private plug-ins, and support files needed by the application. While the Contents directory might seem superfluous, it identifies the bundle as a modern-style bundle and separates it from document and legacy bundle types found in earlier versions of Mac OS.

Listing 2-3 shows the high-level structure of a typical application bundle, including the immediate files and directories you are most likely to find inside the Contents directory. This structure represents the core of every Mac app.

Listing 2-3 The basic structure of a Mac app

Table 2-5 lists some of the directories that you might find inside the Contents directory, along with the purpose of each one. This list is not exhaustive but merely represents the directories in common usage.

Application bundles have evolved significantly over the years but the overall goal has been the same. The bundle organization makes it easier for the application to find its resources while making it harder for users to interfere with those resources. Because the Finder treats most bundles as opaque entities, it is difficult for casual users to move or delete the resources an application might need.

The Information Property List File

For the Finder to recognize an application bundle as such, you need to include an information property list (Info.plist) file. This file contains XML property-list data that identifies the configuration of your bundle. For a minimal bundle, this file would contain very little information, most likely just the name and identifier of the bundle. For more complex bundles, the Info.plist file includes much more information.

Important: Bundle resources are located using a case-sensitive search. Therefore, the name of your information property list file must start with a capital “I”.

Table 2-6 lists the keys that you should always include in your Info.plist file. Xcode provides all of these keys automatically when you create a new project. (Xcode obscures actual key names by default, so the string displayed by Xcode is also listed in parenthesis. You can see the real key names for all keys by Control-clicking the Information Property List key in the editor and choosing Show Raw Keys/Values from the contextual menu that appears.)

Table 2-7 lists the keys that you should also consider including in your Info.plist file.

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The exact information you put into your Info.plist file is dependent on your bundle’s needs and can be localized as necessary. For more information on this file, see Runtime Configuration Guidelines.

The Resources Directory

The Resources directory is where you put all of your images, sounds, nib files, string resources, icon files, data files, and configuration files among others. The contents of this directory are further subdivided into areas where you can store localized and nonlocalized resource files. Non-localized resources reside at the top level of the Resources directory itself or in a custom subdirectory that you define. Localized resources reside in separate subdirectories called language-specific project directories, which are named to coincide with the specific localization.

The best way to see how the Resources directory is organized is to look at an example. Listing 2-4 shows a fictional application that includes both localized and nonlocalized resources. The nonlocalized resources include Hand.tiff, MyApp.icns and the contents of the WaterSounds directory. The localized resources include everything in the en.lproj and jp.lproj directories or their subdirectories.

Listing 2-4 A Mac app with localized and nonlocalized resources

Each of your language-specific project directories should contain a copy of the same set of resource files, and the name for any single resource file must be the same across all localizations. In other words, only the content for a given file should change from one localization to another. When you request a resource file in your code, you specify only the name of the file you want. The bundle-loading code uses the current language preferences of the user to decide which directories to search for the file you requested.

For information about finding resource files in your application bundle, see Accessing a Bundle's Contents. For information about how to load resource files and use them in your program, see Resource Programming Guide.

The Application Icon File

One special resource that belongs in your top-level Resources directory is your application icon file. By convention, this file takes the name of the bundle and an extension of .icns; the image format can be any supported type, but if no extension is specified, the system assumes .icns.

Localizing the Information Property List

Because some of the keys in an application’s Info.plist file contain user-visible strings, macOS provides a mechanism for specifying localized versions of those strings. Inside each language-specific project directory, you can include an InfoPlist.strings file that specifies the appropriate localizations. This file is a strings file (not a property list) whose entries consist of the Info.plist key you want to localize and the appropriate translation. For example, in the TextEdit application, the German localization of this file contains the following strings:

Creating an Application Bundle

The simplest way to create an application bundle is using Xcode. All new application projects include an appropriately configured application target, which defines the rules needed to build an application bundle, including which source files to compile, which resource files to copy to the bundle, and so on. New projects also include a preconfigured Info.plist file and typically several other files to help you get started quickly. You can add any custom files as needed using the project window and configure those files using the Info or Inspector windows. For example, you might use the Info window to specify custom locations for resource files inside your bundle.

For information on how to configure targets in Xcode, see Xcode Build System Guide.

Framework Bundles

A framework is a hierarchical directory that encapsulates a dynamic shared library and the resource files needed to support that library. Frameworks provide some advantages over the typical dynamic shared library in that they provide a single location for all of the framework’s related resources. For example, most frameworks include the header files that define the symbols exported by the framework. Grouping these files with the shared library and its resources makes it easier to install and uninstall the framework and to locate the framework’s resources.

Just like a dynamic shared library, frameworks provide a way to factor out commonly used code into a central location that can be shared by multiple applications. Only one copy of a framework’s code and resources reside in-memory at any given time, regardless of how many processes are using those resources. Applications that link against the framework then share the memory containing the framework. This behavior reduces the memory footprint of the system and helps improve performance.

Note: Only the code and read-only resources of a framework are shared. If a framework defines writable variables, each application gets its own copy of those variables to prevent it from affecting other applications.

Although you can create frameworks of your own, most developers’ experience with frameworks comes from including them in their projects. Frameworks are how macOS delivers many key features to your application. The publicly available frameworks provided by macOS are located in the /System/Library/Frameworks directory. In iOS, the public frameworks are located in the System/Library/Frameworks directory of the appropriate iOS SDK directory. For information about adding frameworks to your Xcode projects, see Xcode Build System Guide.

Note: The creation of custom frameworks is not supported in iOS.

For more detailed information about frameworks and framework bundles, see Framework Programming Guide.

Anatomy of a Framework Bundle

The structure of framework bundles differs from that used by applications and plug-ins. The structure for frameworks is based on a bundle format that predates macOS and supports the inclusion of multiple versions of the framework’s code and resources in the framework bundle. This type of bundle is known as a versioned bundle. Supporting multiple versions of a framework allows older applications to continue running even as the framework shared library continues to evolve. The bundle’s Versions subdirectory contains the individual framework revisions while symbolic links at the top of the bundle directory point to the latest revision.

The system identifies a framework bundle by the .framework extension on its directory name. The system also uses the Info.plist file inside the framework’s Resources directory to gather information about the configuration of the framework. Listing 2-5 shows the basic structure of a framework bundle. The arrows (->) in the listing indicate symbolic links to specific files and subdirectories. These symbolic links provide convenient access to the latest version of the framework.

Listing 2-5 A simple framework bundle

Frameworks are not required to include a Headers directory but doing so allows you to include the header files that define the framework’s exported symbols. Frameworks can store other resource files in both standard and custom directories.

Creating a Framework Bundle

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If you are developing software for macOS, you can create your own custom frameworks and use them privately or make them available for other applications to use. You can create a new framework using a separate Xcode project or by adding a framework target to an existing project.

For information about how to create a framework, see Framework Programming Guide.

Loadable Bundles

Plug-ins and other types of loadable bundles provide a way for you to extend the behavior of an application dynamically. A loadable bundle consists of executable code and any resources needed to support that code stored in a bundle directory. You can use loadable bundles to load code lazily into your application or to allow other developers to extend the basic behavior of your application.

Note: The creation and use of loadable bundles is not supported in iOS.

Anatomy of a Loadable Bundle

Loadable bundles are based on the same structure as application bundles. At the top-level of the bundle is a single Contents directory. Inside this directory are several subdirectories for storing executable code and resources. The Contents directory also contains the bundle’s Info.plist file with information about the bundle’s configuration.

Unlike the executable of an application, loadable bundles generally do not have a main function as their main entry point. Instead, the application that loads the bundle is responsible for defining the expected entry point. For example, a bundle could be expected to define a function with a specific name or it could be expected to include information in its Info.plist file identifying a specific function or class to use. This choice is left to the application developer who defines the format of the loadable bundle.

Listing 2-6 shows the layout of a loadable bundle. The top-level directory of a loadable bundle can have any extension, but common extensions include .bundle and .plugin. macOS always treats bundles with those extensions as packages, hiding their contents by default.

Listing 2-6 A simple loadable bundle

In addition to the MacOS and Resources directories, loadable bundles may contain additional directories such as Frameworks, PlugIns, SharedFrameworks, and SharedSupport—all the features supported by full-fledged application packages.

The basic structure of a loadable bundle is the same regardless of which language that bundle uses in its implementation. For more information about the structure of loadable bundles, see Code Loading Programming Topics.

Creating a Loadable Bundle

If you are developing software for macOS, you can create your own custom loadable bundles and incorporate them into your applications. If other applications export a plug-in API, you can also develop bundles targeted at those APIs. Xcode includes template projects for implementing bundles using either C or Objective-C, depending on the intended target application.

For more information about how to design loadable bundles using Objective-C, see Code Loading Programming Topics. For information about how to design loadable bundles using the C language, see Plug-in Programming Topics.

Localized Resources in Bundles

Within the Resources directory of a macOS bundle (or the top-level directory of an iOS application bundle), you can create one or more language-specific project subdirectories to store language- and region-specific resources. The name of each directory is based on the language and region of the desired localization followed by the .lproj extension. To specify the language and region, you use the following format:

• language_region.lproj

The language portion of the directory name is a two-letter code that conforms to the ISO 639 conventions. The region portion is also a two-letter code but it conforms to the ISO 3166 conventions for designating specific regions. Although the region portion of the directory name is entirely optional, it can be a useful way to tune your localizations for specific parts of the world. For example, you could use a single en.lproj directory to support all English speaking nations. However, providing separate localizations for Great Britain (en_GB.lproj), Australia (en_AU.lproj), and the United States (en_US.lproj) lets you tailor your content for each of those countries.

Note: For backwards compatibility, the NSBundle class and CFBundleRef functions also support human-readable directory names for several common languages, including English.lproj, German.lproj, Japanese.lproj, and others. Although the human-readable names are supported, the ISO names are preferred.

If most of your resource files are the same for all regions of a given language, you can combine a language-only resource directory with one or more region-specific directories. Providing both types of directories alleviates the need to duplicate every resource file for each region you support. Instead, you can customize only the subset of files that are needed for a particular region. When looking for resources in your bundle, the bundle-loading code looks first in any region-specific directories, followed by the language-specific directory. And if neither localized directory contains the resource, the bundle-loading code looks for an appropriate nonlocalized resource.

Important: Do not store code in your lproj folders, because the system does not load or execute code stored in lproj folders. To learn more about where code and other types of data should be stored in your app bundle, see macOS Code Signing In Depth.

Listing 2-7 shows the potential structure of a Mac app that contains both language- and region-specific resource files. (In an iOS application, the contents of the Resources directory would be at the top-level of the bundle directory.) Notice that the region-specific directories contain only a subset of the files in the en.lproj directory. If a region-specific version of a resource is not found, the bundle looks in the language-specific directory (in this case en.lproj) for the resource. The language-specific directory should always contain a complete copy of any language-specific resource files.

Listing 2-7 A bundle with localized resources

For more information on language codes and the process for localizing resources, see Internationalization and Localization Guide.

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