By: Jonathan Zdziarski
Apple came up with an elegant way to contain applications in their operating system. As OS X is a Unix-based platform, Apple wanted to make it adhere to basic Unix file conventions, and so the resource forks of olde were no longer sufficient (or efficient, for that matter). The challenge was to design a structure that would allow an application to remain self-contained while surviving on a file system that didn't believe in cheapening its architecture with proprietary workarounds. The answer came from an older ancestor of Mac OS X named NeXT, which treated an application as a bundle represented within a directory. The bundle concept introduces an approach to group application resources, binaries, and other related files.
If you look at any Mac application, you'll find that the .app extension denotes not a file, but a directory. This is the application's program directory. Inside it is an organized structure containing resources the application needs to run, property lists containing information about the application, and the application's executable binaries. The iPhone SDK builds the binary executable for your program and deposits files it needs into this program directory structure. So to build a complete application, it's up to the developer to tell the Xcode IDE which supporting files should be installed. Applications are executed from within a sandbox on the iPhone. A sandbox is a restricted environment that prevents applications from accessing unauthorized resources. One of its functions is to prohibit any read or write operations outside of the application's designated home directory. Everything your application needs to run must be self-contained within this directory structure. In addition to this, your application won't know where it is installed, as a unique identifier is added to your application's path at each installation. You'll only be able to find your path by using functions like NSHomeDirectory and classes like NSBundle, which you'll learn about in the coming chapters.
Each iPhone application has its own home directory containing Library and Documents folders, and a tmp directory for storing temporary files. The program directory for an iPhone application is much less structured than desktop Mac applications, and all of the application's resources are stored in the root of the .app program folder. The following is an example of a single application's complete home directory, as it might appear on the iPhone's file system:
drwxr-xr-x mobile mobile Documents/ drwxr-xr-x mobile mobile Library/ drwxr-xr-x mobile mobile Preferences/ drwxr-xr-x mobile mobile MyApp.app/ drw-r--r-- mobile mobile _CodeSignature -rw-r--r-- mobile mobile Default.png -rw-r--r-- mobile mobile icon.png -rw-r--r-- mobile mobile Info.plist -rwxr-xr-x mobile mobile MyApp -rw-r--r-- mobile mobile pie.png -rw-r--r-- mobile mobile PkgInfo -rw-r--r-- mobile mobile ResourceRules.plist drwxr-xr-x mobile mobile tmp/
This list reflects a very basic simple iPhone application named MyApp:
A folder in which your application may store settings and other resources created after installation. Inside this folder is another folder named Preferences, which will store your application's preferences.
A directory containing code signatures for each file bundled with your application. These ensure that the application has not been modified from its original form. All applications must be signed in order to run on the iPhone.
A PNG (portable network graphics) image file containing the application's default title screen. When the user runs your application, the iPhone animates it to give the appearance that it's zooming to the front of the screen. The application's Default.png file is loaded and scaled up until it fills the entire screen. This 320x480 image zooms to the front and remains on the screen until the application finishes launching. Applications generally use a solid black or white background, a logo, or a background resembling the title screen that an application will display after initializing.
A PNG image containing the application's icon. The icon is displayed on the iPhone's home screen. Apple recommends that icons be 57x57 pixels. This file can be named anything you like, as long as it's specified in the Info.plist manifest, explained below. Icons are automatically given a "shine" when displayed, so you won't need to worry about drawing rounded edges or lighting effects on your icon.
A property list containing information about the application. This includes the name of its binary executable and a bundle identifier, which is read when the application is launched. You'll see an example property list later in this chapter.
The actual binary executable that is called when the application is launched. This is what Xcode outputs when it builds your application. Xcode will automatically place the binary in the application folder when performing a Build and Go.
An example image resource used by this sample application. The iPhone framework provides many methods to fetch resources, so you don't need to access them directly by path. This is consistent with Apple's effort to keep applications self-contained. The Xcode IDE will take any files you've dragged into your project's Resources folder (on the desktop) and will place them into the application's program folder (on the iPhone) when the project is installed.
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