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File Properties
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A file s properties, or metadata, are information about a file. Java s java.io.File object is the principal interface for obtaining the properties of a file. It implements a variety of specific methods (i.e., boolean isFile(), long lastModified(), long length()) that describe the various properties of the file. Objective-C has a single -[NSFileManager fileAttributesAtPath:traverseLink:] method that accepts a file path and returns an immutable dictionary containing all salient properties of the file. The traverseLink: argument determines what to do if the path specifies a symbolic link file. If YES, the attributes returned will be for the file the symbolic link refers to; if NO, the attributes describe the symbolic link file. To examine a particular property, retrieve its value from the dictionary collection. An example using the attributes of a file was already demonstrated listing 11-6. The keys are string constants to values that are NSNumber, NSDate, or NSString objects. Table 11-8 lists the property methods in java.io.File and their Objective-C equivalent. Some are methods, while others are keys into the attribute dictionary.
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CHAPTER 11 FILES
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Table 11-8. File Attribute Keys
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java.io.File Method
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exists() canRead()
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-[NSFileManager fileExistsAtPath:] -[NSFileManager isReadableFileAtPath:] -[NSFileManager isWritableFileAtPath:] NSFilePosixPermissions
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File exists File is readable
canWrite()
File is writable
integer containing user, group, and global read/write/execute permission bits True if type is NSFileTypeRegular True if type is NSFileTypeDirectory Launch Services API provides display information NSDate file was last modified logical length of file NSDate file was created Account number of file s owner Account name of file s owner Group number of file s group Group name of file s group
isFile() isDirectory() isHidden()
NSFileType NSFileType LSCopyItemInfoForURL( )
lastModified() length()
NSFileModificationDate NSFileSize NSFileCreationDate NSFileOwnerAccountID NSFileOwnerAccountName NSFileGroupOwnerAccountID NSFileGroupOwnerAccountName
The type of a file is determined by examining its NSFileType attribute. The attribute value will be one of NSFileTypeDirectory, NSFileTypeRegular, NSFileTypeSymbolicLink, NSFileTypeSocket, NSFileTypeCharacterSpecial, NSFileTypeBlockSpecial, or NSFileTypeUnknown. Detailed information about an item s display name, whether the file s extension should be hidden from the user, or if the entire file should be hidden, is available through various Launch Services functions. See the NSFileManager documentation for a complete list of file attribute keys. Most file attributes can be modified using the -[NSFileManager setAttributes:ofItemAtPath: error:] method. You provide a dictionary containing the attributes you want modified and a file path. Some attributes, such as NSFileType and NSFileSize, cannot be modified using -setAttributes: . The security policy may conditionally inhibit modification of other attributes, such as NSFileOwnerAccountID.
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CHAPTER 11 FILES
The method returns YES if all attributes changes were effected successfully. If it returns NO, the changes were indeterminate.
High-Level File Operations
Objective-C applications tend to deal with files in a holistic fashion, rather than the more traditional approach of opening the file, reading some or all of its data, and closing it again. Application classes, like the document management classes, tend to favor reading an entire file into a single data object. The typical life cycle of a document file is to be read in its entirety into a single NSData object, edited, and then written back out, overwriting the original file. In modern file systems, it is also difficult to correctly perform even simple operations, such as copying a file. Extended metadata, access control lists, file ownership, multiple data forks, display properties, and remote file copying protocols are just a few of the seemingly endless details that must be considered just to duplicate a file. To spare you from these burdens, the Cocoa framework provides a number of high-level methods that correctly perform a variety of atomic actions on whole files. The common ones are listed in Table 11-9. Table 11-9. Common High-Level File Operations
Met ho d
-[NSFileManager contentsAtPath:]
Des cripti on
Returns the contents of the entire file as an NSData object Creates a new NSData object with the contents of a file Creates a virtual memory region mapped to the data in a file Writes the contents of the data object to a file Creates a new NSString object with the contents of a file Writes the contents of a string to a file Copies a file Moves or rename a file Deletes a file Compares the contents of two files or two directories Moves, copies, links, or trashes a set of files
+[NSData dataWithContentsOfFile:]
+[NSData dataWithContentsOfMappedFile:]
-[NSData writeToFile:atomically:] +[NSString stringWithContentsOfFile: encoding:error:] -[NSString writeToFile:atomically:encoding:error:] -[NSFileManager copyItemAtPath:toPath:error:] -[NSFileManager moveItemAtPath:toPath:error:] -[NSFileManager removeItemAtPath:error:] -[NSFileManager contentsEqualsAtPath:andPath:]
-[NSWorkspace performFileOperation:source: destination:files:tag:]
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