NSFileManager unique file names

Create your own file name:

CFUUIDRef uuid = CFUUIDCreate(NULL);
CFStringRef uuidString = CFUUIDCreateString(NULL, uuid);
CFRelease(uuid);
NSString *uniqueFileName = [NSString stringWithFormat:@"%@%@", prefixString, (NSString *)uuidString];
CFRelease(uuidString);

A simpler alternative proposed by @darrinm in the comments:

NSString *prefixString = @"MyFilename";

NSString *guid = [[NSProcessInfo processInfo] globallyUniqueString] ;
NSString *uniqueFileName = [NSString stringWithFormat:@"%@_%@", prefixString, guid];

NSLog(@"uniqueFileName: '%@'", uniqueFileName);

NSLog output:
uniqueFileName: 'MyFilename_680E77F2-20B8-444E-875B-11453B06606E-688-00000145B460AF51'

Note: iOS6 introduced the NSUUID class which can be used in place of CFUUID.

NSString *guid = [[NSUUID new] UUIDString];

I use current date to generate random file name with a given extension. This is one of the methods in my NSFileManager category:

+ (NSString*)generateFileNameWithExtension:(NSString *)extensionString
{
    // Extenstion string is like @".png"

    NSDate *time = [NSDate date];
    NSDateFormatter* df = [NSDateFormatter new];
    [df setDateFormat:@"dd-MM-yyyy-hh-mm-ss"];
    NSString *timeString = [df stringFromDate:time];
    NSString *fileName = [NSString stringWithFormat:@"File-%@%@", timeString, extensionString];

    return fileName;
}

Super-easy Swift 4 1-liner:

fileName = "MyFileName_" + UUID().uuidString

or

fileName = "MyFileName_" + ProcessInfo().globallyUniqueString