Working with Directories in Objective-C
From Techotopia
Revision as of 21:05, 1 February 2016 by Neil (Talk | contribs) (Text replacement - "<google>BUY_OBJC</google>" to "<htmlet>objc<htmlet>")
| Previous | Table of Contents | Next |
| Objective-C Dictionary Objects | Working with Files in Objective-C |
Cannot find HTML file objc<htmlet>
A key element of gaining proficiency in any programming language involves the ability to work with files and file systems. The level of difficulty in working with files varies from very easy to hard, depending on the programming language concerned. The C programming language, on which Objective-C is based, tended to make file handling a little hard relative to the standards of today's programmer friendly object oriented languages. The good news for Objective-C developers is that the Foundation Framework includes a number of classes designed specifically to make the task of working with files and directories as straightforward as possible.
In this chapter we will look at these classes and provide examples of how use them to perform some basic directory operations from within an Objective-C program. In [[Working with Files in Objective-C]] we will take a close look at working with files using these classes.
== The Objective-C NSFileManager, NSFileHandle and NSData Classes ==
<google>IOSBOX<google>
The Foundation Framework provides three classes that are indispensable when it comes to working with files and directories:
* '''NSFileManager''' - The ''NSFileManager'' class can be used to perform basic file and directory operations such as creating, moving, reading and writing files and reading and setting file attributes. In addition, this class provides methods for, amongst other tasks, identifying the current working directory, changing to a new directory, creating directories and listing the contents of a directory.
* '''NSFileHandle''' - The ''NSFileHandle'' class is provided for performing lower level operations on files, such as seeking to a specific position in a file and reading and writing a file's contents by a specified number of byte chunks and appending data to an existing file.
* '''NSData''' - The ''NSData'' class provides a useful storage buffer into which the contents of a file may be read, or from which data may be written to a file.
== Understanding Pathnames in Objective-C ==
When using the above classes, pathnames are defined using the UNIX convention. As such each component of a path is separated by a forward slash (). Paths that do not begin with a slash are interpreted to be relative to a current working directory. For example, if the current working directory is ''homeobjc'' and the path name is ''myappexample.m'' then the file is considered to have a full pathname of ''homeobjcmyappexample.m''.
In addition, the home directory of the current user can be represented using the tilde (~) character. For example the pathname ''~example.m'' references a file named ''example.m'' located in the home directory of the current user. The home directory of another user may be referenced by prefixing the user name with a ~. For example, ''~johndemo.m'' references a file located in the home directory of a user named ''john''.
== Obtaining a Reference to the Default NSFileManager Object ==
The NSFileManager class contains a class method named defaultManager that is used to obtain a reference to the application’s default file manager instance:
<pre>
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
<pre>
In the above example we have declared a variable named filemgr to point to an object of type NSFileManager, and then requested a pointer to the application’s file manager and assigned it to the variable. Having obtained the object reference we can begin to use it to work with files and directories.
== Identifying the Current Working Directory ==
The current working directory may be identified using the ''currentDirectoryPath'' instance method of our NSFileManager object. The current path is returned from the method in the form of an NSString object:
<pre>
NSFileManager *filemgr;
NSString *currentpath;
filemgr = [NSFileManager defaultManager];
currentpath = [filemgr currentDirectoryPath];
NSLog (@"Current directory is %@", currentpath);
<pre>
== Changing to a Different Directory ==
The current working directory of a running Objective-C program can be changed with a call to the ''changeCurrentDirectoryPath'' method. The destination directory path is passed as an argument to the instance method in the form of an NSString object. Note that this method returns a boolean ''YES'' or ''NO'' result to indicate if the requested directory change was successful for not:
<pre>
NSFileManager *filemgr;
NSString *currentpath;
filemgr = [NSFileManager defaultManager];
currentpath = [filemgr currentDirectoryPath];
NSLog (@"Current directory is %@", currentpath);
if ([filemgr changeCurrentDirectoryPath: @"tempmydir"] == NO)
NSLog (@"Cannot change directory.");
currentpath = [filemgr currentDirectoryPath];
NSLog (@"Current directory is %@", currentpath);
<pre>
== Creating a New Directory ==
A new directory is created using the createDirectoryAtURL instance method, this time passing through the through the pathname of the new directory as an argument in the form of an NSURL object. This method also takes additional arguments in the form of a set of attributes for the new directory and a Boolean value indicating whether or not intermediate directories should be created if they do not already exist. Specifying nil will use the default attributes:
<pre>
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
NSURL *newDir = [NSURL fileURLWithPath:@"tmpmynewdir"];
[filemgr createDirectoryAtURL: newDir withIntermediateDirectories:YES attributes: nil error:nil];
<pre>
The createDirectoryAtURL method returns a Boolean result indicating the success or otherwise of the operation.
== Deleting a Directory ==
An existing directory may be removed from the file system using the ''removeItemAtPath'' method, passing though the path of the directory to be removed as an argument:
<pre>
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
[filemgr removeItemAtPath: @"tmpmynewdir" handler: nil];
<pre>
== Renaming or Moving a File or Directory ==
An existing file or directory may be moved (also known as renaming) using the moveItemAtURL method. This method takes the source and destination pathnames as arguments in the form of NSURL objects and requires that the destination path not already exist. If the target exists, a boolean NO result is returned by the method to indicate failure of the operation:
<pre>
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
NSURL *oldDir = [NSURL fileURLWithPath:@"tmpmynewdir"];
NSURL *newDir = [NSURL fileURLWithPath:@"tmpmynewdir2"];
[filemgr moveItemAtURL: oldDir toURL: newDir error: nil];
<pre>
== Getting a Directory File Listing ==
A listing of the files contained within a specified directory can be obtained using the contentsOfDirectoryAtPath method. This method takes the directory pathname as an argument and returns an NSArray object containing the names of the files and sub-directories in that directory:
<pre>
NSFileManager *filemgr;
NSString *currentpath;
NSArray *filelist;
int count;
int i;
filemgr = [NSFileManager defaultManager];
filelist = [filemgr contentsOfDirectoryAtPath: @"tmp" error: nil];
count = [filelist count];
for (i = 0; i < count; i++)
NSLog (@"%@", [filelist objectAtIndex: i]);
<pre>
When executed as part of a program, the above code excerpt will display a listing of all the files located in the tmp directory.
== Getting the Attributes of a File or Directory ==
The attributes of a file or directory can be obtained using the ''attributesOfItemAtPath'' method. This takes as arguments the path of the directory and an optional NSError object into which information about any errors will be placed (may be specified as NULL if this information is not required). The results are returned in the form of an ''NSDictionary'' dictionary object (for details of working with dictionary objects refer to [[Objective-C Dictionary Objects]]). The keys for this dictionary are as follows:
<google>ADSDAQBOX_FLOW<google>
<tt>NSFileType<tt><br>
<tt>NSFileTypeDirectory<tt><br>
<tt>NSFileTypeRegular<tt><br>
<tt>NSFileTypeSymbolicLink<tt><br>
<tt>NSFileTypeSocket<tt><br>
<tt>NSFileTypeCharacterSpecial<tt><br>
<tt>NSFileTypeBlockSpecial<tt><br>
<tt>NSFileTypeUnknown<tt><br>
<tt>NSFileSize<tt><br>
<tt>NSFileModificationDate<tt><br>
<tt>NSFileReferenceCount<tt><br>
<tt>NSFileDeviceIdentifier<tt><br>
<tt>NSFileOwnerAccountName<tt><br>
<tt>NSFileGroupOwnerAccountName<tt><br>
<tt>NSFilePosixPermissions<tt><br>
<tt>NSFileSystemNumber<tt><br>
<tt>NSFileSystemFileNumber<tt><br>
<tt>NSFileExtensionHidden<tt><br>
<tt>NSFileHFSCreatorCode<tt><br>
<tt>NSFileHFSTypeCode<tt><br>
<tt>NSFileImmutable<tt><br>
<tt>NSFileAppendOnly<tt><br>
<tt>NSFileCreationDate<tt><br>
<tt>NSFileOwnerAccountID<tt><br>
<tt>NSFileGroupOwnerAccountID<tt><br>
For example, we can extract the creation date, file type and POSIX permissions for the ''tmp'' directory using the following code excerpt:
<pre>
NSFileManager *filemgr;
NSDictionary *attribs;
filemgr = [NSFileManager defaultManager];
attribs = [filemgr attributesOfItemAtPath: @"tmp" error: NULL];
NSLog (@"Created on %@", [attribs objectForKey: NSFileCreationDate]);
NSLog (@"File type %@", [attribs objectForKey: NSFileType]);
NSLog (@"POSIX Permissions %@", [attribs objectForKey: NSFilePosixPermissions]);
<pre>
When executed on a Mac OS X system, we can expect to see the following output (note that ''tmp'' on a Mac OS X system is a symbolic link to ''privatetmp''):
<pre>
Created on 2009-01-14 07:34:32 -0500
File type NSFileTypeSymbolicLink
POSIX Permissions 493
<pre>
<google>BUY_OBJC_BOTTOM<google>
<hr>
<table border="0" cellspacing="0" width="100%">
<tr>
<td width="20%">[[Objective-C Dictionary Objects|Previous]]<td align="center">[[Objective-C 2.0 Essentials|Table of Contents]]<td width="20%" align="right">[[Working with Files in Objective-C|Next]]<td>
<tr>
<td width="20%">Objective-C Dictionary Objects<td align="center"><td width="20%" align="right">Working with Files in Objective-C<td>
<table>.html
