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NSString *paths = [[NSBundle mainBundle] resourcePath]; NSString *bundlePath = [paths stringByAppendingPathComponent:dbFile];
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In this example, mainBundle returns the directory path that corresponds to your application s bundle, and resourcePath expands that to be the directory path for the resources of your program (including, in this case, a database; but this could be anything else you added to your program). Finally, you use stringByAppendingPathComponent: to add your specific file to the path. This NSString method makes sure a path is constructed using slashes (//) as needed. The result is a complete path that can be handed to other objects as needed. You ll see how that works with a database in the next section. You can likewise use it for UImage s imageWithContentsOfFile: method or NSFileHandle s fileHandleForReadingAtPath method. We ll return to the latter shortly. But there s one fundamental problem with accessing files in the application bundle: you can t modify them. Apple generally suggests that you should treat the application bundle as read only, and there s a real penalty if you don t: your program will stop working because it won t checksum correctly. This means that the application bundle is great for files that don t change; but if you want to modify something (or create something new), you need to use the other directories we mentioned, starting with the Documents folder.
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When you re working with directories other than the bundle, you have to think about two things: how to access those files and how to move files among multiple directories.
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When a file is sitting in your Documents directory, you can retrieve it much as you retrieved files from the bundle directory:
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NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0]; NSString *docPath = [documentsDirectory stringByAppendingPathComponent:dbFile];
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The magic here occurs in the NSSearchPathForDirectoriesInDomains function. The first argument is usually NSDocumentDirectory or NSLibraryDirectory, depending on which directory you want to get to. The other two arguments should always be the same for the iPhone and iPad. The result is an array of strings, each containing a path. The first path in the NSArray is usually the right one, as shown here. You can then use the stringByAppendingPathComponent: method, as before, to build the complete path for your file. Voila! You ve now used some slightly different methods to access a file in your Documents directory rather than the bundle directory.
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COPYING A FILE
There s been a slight disconnect in our discussion of files and directories to date. When you compile your project, all of your files are placed into your application
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bundle. But if you ever want to edit a file, it must be placed in a different directory, such as Documents. So how do you get a file from one place to the other You use the NSFileManager:
NSFileManager *fileManager = [NSFileManager defaultManager]; success = [fileManager copyItemAtPath:bundlePath toPath:docPath error:&error];
The file manager is a class that allows you to easily manipulate files by creating them, moving them, deleting them, and otherwise modifying them. As is the case with many classes you ve seen, you initialize it by accessing a shared object. You can do lots of things with the file manager, including copying (as you ve done here) and checking for a file s existence (which we ll demonstrate shortly). You should look at the NSFileManager class reference for complete information. As you ll see, the NSFileManager is one of numerous classes that you can use to work with files.
Manipulating files
It s possible that after you ve built your file path, you ll be ready to immediately read the file s contents, using something like the UIImage methods (which we ll touch on in chapter 13) or the functions related to SQLite (which we ll cover in the next chapter). But it s also possible that you ll want to manipulate the raw files, reading and parsing them in your code, as soon as you ve created a file path. There are numerous ways to do this, as shown in table 8.4.
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