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When you ve done that, you can use NSUserDefaults objectForKey: methods, such as arrayForKey:, integerForKey:, and stringForKey:, as appropriate to access the information from the settings. For example, the following code applies a string from the settings to a label:
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myLabel.text = [[NSUserDefaults standardUserDefaults] stringForKey:@"name_preference"];
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Similarly, you can save new settings by using the various setObjectForKey: methods although we don t think this is a particularly good idea if users are otherwise modifying these values in Settings. There is one considerable gotcha that you must watch for: if a user hasn t yet accessed the settings for your program, then all settings without default values have a value of nil. This means you either need to create your preferences by hand or build defaults into your program, as appropriate. Most of the time, you ll only need to retrieve the setting values, as described here; but if more is required, you should look at the class reference for NSUserDefaults. That concludes our look at the two ways to create preferences for your programs and also at how users can input data into your program; but user input represents just one part of the data puzzle. Certainly, a lot of important data comes from users, but data can also come from various files and databases built into your program or into the device. Retrieving data from those sources is the topic of the latter half of this chapter.
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When we talked about bundles earlier in this chapter, you saw how the iPhone and iPad arrange their internal information for programs. That arrangement becomes vitally important when you re trying to access files that you ve added to a project. Fortunately, for the iPhone, you can look at how your program s files are arranged when you re testing applications on the Simulator. Each time you run a program, the program is compiled to a directory under ~/Library/Application Support/iPhone Simulator/Users/Applications. The specific directory has a hexadecimal name, but you can search to find the right one. Figure 8.5 shows an example of the directory for the sample program that we used to set up the system preferences example (the subdirectories are the same for any basic program). The process is similar for the iPad. As shown, there are four directories of files for this one simple program. The majority of the content appears in the application bundle, which in this example is called systempreferences.app. There, you find everything you ve added to your project, including text files, pictures, and databases. The other three directories you can use are Documents, Library, and tmp. These are all intended to be used for files that are created or modified when the program is run. Documents should contain user-created information (including new or modified text files, pictures, and databases); Library should contain more programmatic items (like preferences); and tmp should contain temporary information. Each
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Compiled programs contain several directories full of files.
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of these directories starts out empty, other than the fact that the Library maintains a local copy of your system settings. We ll talk about how and why you fill them momentarily. First, let s look at how to access your bundle; later, we ll discuss how to access other directories and also how to manipulate files. At the end of the section, we ll put everything together with a concrete example.
Accessing your bundle
In previous chapters, we ve shown how easy it is to add files to your project. You drag the file into Xcode, and everything is correctly set up so that the file will become part of your program when it compiles. As you now know, that means the file is copied into your application bundle. For many bundled files, you don t have to worry about anything beyond that. For example, when you work with picture files, you enter the name of the file in Interface Builder, and the SDK automatically finds it for you. But if you want to access a file that doesn t have this built-in link, you need to do a bit more work. Whenever you re working with the filesystem on the iPhone or iPad, access is abstracted through objects. You send messages that tell the SDK what area of the filesystem you re looking for, and the SDK then gives you precise directory paths. The benefit of this abstraction is that Apple can reorganize the filesystem in future releases, and your program won t be affected at all. The first files you ll want to access will probably be in your bundle: files that you included when you compiled your program. Accessing a bundle file is usually a two-step process, as shown in this database example (which we ll return to in the next section):
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