Check if file is updated

Question

I have iOS application. There some files in the documents directory. I need to check if these files were edited or replaced. What is the proper way to do it? Shall i some hash of the file or last edited time(how to get this time in iOS) or there is some better way to implement this?

You can surely check with its updated time. Go on to save the updated time to some plist and check it. — Anoop Vaidya, Mar 03 '13 at 18:45
who else can edit/replace the files in the `Documents` directory in runtime beside your application...? — holex, Mar 03 '13 at 18:45
Itunes synchronization, you can add some files to documets directory from itunes — B.S., Mar 03 '13 at 18:47
@George, in that case try to implement an existing solution for it, like **[this](http://stackoverflow.com/questions/3181821/notification-of-changes-to-the-iphones-documents-directory)** — holex, Mar 03 '13 at 19:10

score 4 · Accepted Answer · answered Mar 03 '13 at 18:59

you can use this code to get the file modification date:

NSFileManager* fileManager = [NSFileManager defaultManager];
NSDictionary* attributes = [fileManager attributesOfItemAtPath:path error:nil];

if (attributes != nil) {
    NSDate *date = (NSDate*)[attributes objectForKey:NSFileModificationDate];
    NSLog(@"Last modification date: %@", [date description]);
} else {
    NSLog(@"Not found");
}

here you can find the documentation of the attributesOfItemAtPath:error: function, and here the list of file properties you can get

score 1 · Answer 2 · answered Mar 03 '13 at 19:45

Hashing the entire file will work, but is a bit overkill for most purposes. I'd combine a few things which are quick to access:

NSFileModificationDate
NSFileSize
NSFileCreationDate
NSFileSystemFileNumber (inode number)

If you are using the NSURL-based API (iOS 4.0+), I'd use

NSURLContentModificationDateKey (modification time)
NSURLFileSizeKey
NSURLCreationDateKey
NSURLFileAllocatedSizeKey
NSURLAttributeModificationDateKey (st_ctime?)

I'd at least use the the modification time and file size. The other things are things that generally do not change unless the contents change, with a couple of caveats:

NSFileSystemFileNumber (the inode number) will likely change if the user restores from a backup.
NSURLAttributeModificationDateKey is supposed to change when attributes change. If it is st_ctime (and I'm not entirely sure it is), it is almost guaranteed to catch all changes to the file, but may also catch a lot of changes that don't represent changes to the file contents. I don't know if such changes happen externally (e.g. does iTunes/iCloud backup set attributes as part of the backup process?).

Check if file is updated

2 Answers2