Difference between revisions of "Working with Files in Objective-C"

Revision as of 17:46, 11 November 2009

In Working with Directories in Objective-C we looked at the NSFileManager, NSFileHandle and NSData Foundation Framework classes and discussed how the NSFileManager class in particular enables us to work with directories in Objective-C. In this chapter we move on from working with directories to covering the details of working with files using all three of these classes.

Creating an NSFileManager Instance

First we need to recap the steps necessary to create an instance of the NSFileManager class. As discussed in the previous chapter, the NSFileManager class contains a class method named defaultManager that is used to create an instance of the class. For example:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

Checking if a File Exists

The NSFileManager class contains an instance method named fileExistsAtPath that checks whether a specified file already exists. The method takes as an NSString object containing the path to file and returns a boolean YES or No value indicating the presence or otherwise of that file:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr fileExistsAtPath: @"/tmp/myfile.txt" ] == YES)
        NSLog (@"File exists");
else
        NSLog (@"File not found");

Comparing the Contents of Two Files

The contents of two files can be compared for equality using the contentsEqualAtPath method. This methods takes as arguments the paths to the two files to be compared and returns a boolean YES or NO to indicate whether the file contents match:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr contentsEqualAtPath: @"/tmp/myfile.txt" andPath: @"/tmp/sales.txt"] == YES)
        NSLog (@"File contents match");
else
        NSLog (@"File contents do not match");

Checking if a File is Readable/Writable/Executable/Deletable

Most operating systems provide some level of file access control. These typically take the form of attributes that control the level of access to a file for each user or user group. As such, it is not a certainty that your program will have read or write access to a particular file, or the appropriate permissions to delete or execute it. The quickest way to find out if your program has a particular access permission is to use the isReadableFileAtPath, isWritableFileAtPath, isExecutableFileAtPath and isDeletableFileAtPath methods. Each method takes a single argument in the form of the path to the to be checked and returns a boolean YES or NO result. For example, the following code excerpt checks to find out if a file is writable:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr isWritableFileAtPath: @"/tmp/myfile.txt"]  == YES)
        NSLog (@"File is writable");
else
        NSLog (@"File is read only");

To check for other access permissions simply substitute the corresponding method name in place of isWritableFileAtPath in the above example.

Moving/Renaming a File

A file may be renamed (assuming adequate permissions) using the moveItemAtPath method. This method returns a boolean YES or NO result and takes as arguments the pathname for the file to moved, the destination path and an optional NSError object into which information describing any errors encountered during the operation will be placed. If no error description information is required, this argument may be set to NULL. Note that if the destination file path already exists this operation will fail.

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr moveItemAtPath: @"/tmp/myfile.txt" toPath: @"/tmp/newfile.txt" error: NULL]  == YES)
        NSLog (@"Move successful");
else
        NSLog (@"Move failed");

Copying a File

File copying can be achieved using the copyItemAtPath method. As with the move method, this takes as arguments the source and destination pathnames and an optional NSError object. Success of the operation is indicated by the returned boolean value:

if ([filemgr copyItemAtPath: @"/tmp/myfile.txt" toPath: @"/Users/demo/newfile.txt" error: NULL]  == YES)
        NSLog (@"Copy successful");
else
        NSLog (@"Copy failed");

Removing a File

The removeItemAtPath removes the specified file from the file system. The method takes as arguments the pathname of the file to be removed and an optional NSError object. The success of the operation is, as usual, reported in the form of a boolean YES or NO return value:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr removeItemAtPath: @"/tmp/myfile.txt" error: NULL]  == YES)
        NSLog (@"Remove successful");
else
        NSLog (@"Remove failed");

Creating a Symbolic Link

A symbolic to a particular file may be created using the createSymbolicLinkAtPath method. This takes arguments the path of the symbolic link, the path to the file to which the link is to refer and an optional NSError object. For example, the following code creates a symbolic link from /Users/demo/file1.txt that links to the pre-existing file /tmp/myfile.txt:

NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr createSymbolicLinkAtPath: @"/Users/demo/file1.txt" 
                withDestinationPath: @"/tmp/myfile.txt" error: NULL]  == YES)
        NSLog (@"Remove successful");
else
        NSLog (@"Remove failed");

Reading and Writing Files with NSFileManager

The NSFileManager class includes some basic file reading and writing capabilities. these capabilities are somewhat limited when compared to the options provided by the NSFileHandle class, but can be useful for nonetheless.

Firstly, the contents of a file may be read and stored in an NSData object through the use of the contentsAtPath method:

NSFileManager *filemgr;
NSData *databuffer;

filemgr = [NSFileManager defaultManager];

databuffer = [filemgr contentsAtPath: @"/tmp/myfile.txt" ];

Having stored the contents of a file in an NSData object, that data may subsequently be written out to a new file using the createFileAtPath method:

databuffer = [filemgr contentsAtPath: @"/tmp/myfile.txt" ];

[filemgr createFileAtPath: @"/tmp/newfile.txt" contents: databuffer attributes: nil];

In the above example we have essentially copied the contents from an existing file to a new file. This, however, gives us no control over how much data is to be read or written and does not allow us to append data to the end of an existing file. If the file /tmp/newfile.txt in the above example had already existed it, and any data it contained, would have been overwritten by the contents of the source file. Clearly some more flexible mechanism is required. This is provided by the Foundation Framework in the form of the NSFileHandle class.

Working with Files using the NSFileHandle Class

The NSFileHandle class provides a range of methods designed to provide a more advanced mechanism for working with files. In addition to files, this class can also be used for working with devices and network sockets. In the following sections we will look at some of the more common uses for this class.

Creating an NSFileHandle Object

An NSFileHandle object can be created when opening a file for reading, writing or updating (reading and writing). This is achieved using the fileHandleForReadingAtPath, fileHandleForWritingAtPath and fileHandleForUpdatingAtPath respectively. Having opened a file, it must subsequently be closed when we have finsihed working with it using the closeFile method. If an attempt to open a file fails, for example because an attempt is made to open a non-existent file reading, these methods return nil.

For example, the following code excerpt opens a file for reading and writing and then closes it without actually doing anything to the file:

NSFileHandle *file;

file = [NSFileHandle fileHandleForWritingAtPath: @"/tmp/myfile.txt"];

if (file == nil)
        NSLog(@"Failed to open file");

[file closeFile];

NSFileHandle File Offsets and Seeking

NSFileHandle objects maintain a pointer to the current position in a file. This is referred to as the offset. When a file is first opened the offset is set to 0 (the beginning of the file). This means that any read or write operations we perform using the NSFileHandle methods will take place at offset 0 in the file. To perform operations at different locations in a file (for example to append data to the end of the file) it is first necessary to seek to the required offset. For example to move the current offset to the end of the file, use the seekToEndOfFile method. Alternatively, seekToFileOffset allows you to specifiy the precise location in the file to which the offset is to be positioned. Finally, the current offset may be identified using the offsetInFile method. in order to accommodate large files, the offset is stored in the form of an unsigned long long.

The following example opens a file for reading and then performs a number of method calls to move the offset to different positions, outputting the current offset after each move:

file = [NSFileHandle fileHandleForUpdatingAtPath: @"/tmp/myfile.txt"];

if (file == nil)
        NSLog(@"Failed to open file");

NSLog (@"Offset = %llu", [file offsetInFile]);

[file seekToEndOfFile];

NSLog (@"Offset = %llu", [file offsetInFile]);

[file seekToFileOffset: 30];

NSLog (@"Offset = %llu", [file offsetInFile]);

[file closeFile];

File offsets are a key aspect of working with files using the NSFileHandle class so it is worth taking extra time to make sure you understand the concept. Without knowing where the current offset is in a file it is impossible to know where in the file data will be read or written.

Reading Data from a File

Once a file has been opened and assigned a file handle, the contents of that file may be read either in its entirety, or as a specified number of bytes starting at any location in the file.