Using FileWrappers as File Containers

An NSFileWrapper instance holds a file’s contents in dynamic memory. In this role it enables a document object to embed a file, treating it as a unit of data that can be displayed as an image (and possibly edited in place), saved to disk, or transmitted to another app. It can also store an icon for representing the file in a document or in a dragging operation.

Instances of this class are referred to as file wrapper objects, and when no confusion will result, merely as file wrappers. A file wrapper can be one of three specific types: a regular file wrapper, which holds the contents of a single actual file; a directory wrapper, which holds a directory and all of the files or directories within it; or a link wrapper, which simply represents a symbolic link in the file system.

Because the purpose of a file wrapper is to represent files in memory, it’s very loosely coupled to any disk representation. A file wrapper doesn’t record the path to the disk representation of its contents. This allows you to save the same file wrapper with different URLs, but it also requires you to record those URLs if you want to update the file wrapper from disk later.

Working with File Wrappers

You can create a file wrapper from data in memory using the initWithSerializedRepresentation: method or from data on disk using the initWithURL:options:error: method. Both create the appropriate type of file wrapper based on the nature of the serialized representation or of the file on disk.

Three convenience methods each create a file wrapper of a specific type: initRegularFileWithContents:, initDirectoryWithFileWrappers:, and initSymbolicLinkWithDestination:. Because each initialization method creates file wrappers of different types or states, they’re all designated initializers for this class—subclasses must meaningfully override them all as necessary.

Some file wrapper methods apply only to a specific wrapper type, and an exception is raised if a method sent to a file wrapper of the wrong type. To determine the type of a file wrapper, use the isRegularFile, isDirectory, and isSymbolicLink methods.

A file wrapper stores file system information (such as modification time and access permissions), which it updates when reading from disk and uses when writing files to disk. The fileAttributes method returns this information in the format described in the NSFileManager method attributesOfItemAtPath:error:. You can also set the file attributes using the setFileAttributes: method.

The NSFileWrapper class allows you to set a preferred filename for save operations, and it records the last filename it was actually saved to; the preferredFilename and filename methods return these names. This feature is most important for directory wrappers, though, and so is discussed under “Working with Directory Wrappers.”

When saving a file wrapper to disk, you typically determine the directory you want to save it in, then append the preferred filename to that directory URL, and use the writeToURL:options:originalContentsURL:error: method, which saves the file wrapper’s contents and updates the file attributes. You can save a file wrapper under a different name if you like, but this doing so may result in the recorded filename differing from the preferred filename, depending on how you invoke the writeToURL:options:originalContentsURL:error: method.

Besides saving its contents to disk, a file wrapper can reread them from disk when necessary. The matchesContentsOfURL: method determines whether a disk representation may have changed, based on the file attributes stored the last time the file was read or written. If the file wrapper’s modification time or access permissions are different from those of the file on disk, this method returns YES. You can then use readFromURL:options:error: to re-read the file from disk.

Finally, to transmit a file wrapper to another process or system (for example, using the pasteboard), you use the serializedRepresentation method to get an NSData object containing the file wrapper’s contents in the NSFileContentsPboardType format. You can safely transmit this representation over whatever channel you choose. The recipient of the representation can then reconstitute the file wrapper using the initWithSerializedRepresentation: method.

Working with Directory Wrappers

A directory wrapper contains other file wrappers (of any type), and allows you to access them by keys derived from their preferred filenames. You can add any type of file wrapper to a directory wrapper with the addFileWrapper: method and remove it with the removeFileWrapper: method. The convenience methods addRegularFileWithContents:preferredFilename: and addSymbolicLinkWithDestination:preferredFilename: allow you to add regular file and link wrappers while also setting their preferred names.

A directory wrapper stores its contents in an NSDictionary object, which you can retrieve using the fileWrappers method. The keys of this dictionary are based on the preferred filenames of each file wrapper contained in the directory wrapper. There exist, then, three identifiers for a file wrapper within a directory wrapper:

When working with the contents of a directory wrapper, you can use a dictionary enumerator to retrieve each file wrapper and perform whatever operation you need. With the exceptions of saving and updating, a directory file wrapper defines no recursive operations for its contents. To set the file attributes for all contained file wrappers, or to perform any other such operation, you must define a recursive method that examines the type of each file wrapper and invokes itself again for any directory wrapper it encounters.