replaceItemAtURL fails without error on iOS but works fine on OSX

Question

I'm implementing a manually-triggered migration process for a CoreData-based app, and after the migration completes successfully, I'm trying to move the migrated DB back over the top of the original one using replaceItemAtURL:withItemAtURL:backupItemName:options:resultingItemURL:error:.

The problem is that on iOS, nothing I do will make this method return YES, however it also never puts anything into the error pointer to allow you to see what's going wrong.

I'd read things elsewhere (e.g. http://www.cocoabuilder.com/archive/cocoa/287790-nsdoc-magic-file-watcher-ruins-core-data-migration.html) indicating that not shutting down all the CoreData objects (e.g. NSMigrationManager, NSManagedObjectModel etc) before attempting the replace might be the cause, but that wasn't it. I even implemented a little two file create-and-swap thing that didn't involve CoreData DBs at all to verify that the CoreData stuff didn't have anything to do with it.

I then noticed in the official documentation that the newitemURL is supposed to be in a directory deemed appropriate for temporary files. I assumed that that meant a directory returned by URLForDirectory:inDomain:appropriateForURL:create:error: using NSItemReplacementDirectory as the search path.

That didn't work either! I ended up falling back to implementing the replacement logic using separate operations, but this is non-atomic and unsafe and all that bad stuff.

Does anyone have a working snippet of code that runs on iOS that either return YES from a call to replaceItemAtURL or actually puts error information into the error pointer?

Any help much appreciated.

EDIT - Test code included below. This runs in application:didFinishLaunchingWithOptions: on the main thread.

NSFileManager *fm = [[NSFileManager alloc] init];
NSError *err = nil;
NSURL *docDir = [NSURL fileURLWithPath:[self applicationDocumentsDirectory]];

NSURL *tmpDir = [fm URLForDirectory:NSItemReplacementDirectory
                           inDomain:NSUserDomainMask
                  appropriateForURL:docDir
                             create:NO
                              error:&err];

NSURL *u1 = [docDir URLByAppendingPathComponent:@"f1"];
NSURL *u2 = [tmpDir URLByAppendingPathComponent:@"f2"];
NSURL *repl = nil;

[fm createFileAtPath:[u1 path]
            contents:[[NSString stringWithString:@"Hello"]
                      dataUsingEncoding:NSUTF8StringEncoding]
          attributes:nil];

[fm createFileAtPath:[u2 path]
            contents:[[NSString stringWithString:@"World"]        
                      dataUsingEncoding:NSUTF8StringEncoding]
          attributes:nil];

BOOL test = [fm replaceItemAtURL:u1 withItemAtURL:u2 backupItemName:@"f1backup"
                         options:0 resultingItemURL:&repl error:&err];

// At this point GDB shows test to be NO but error is still nil

Answer 1

I have experienced issues with all the NSFileManager methods using an URL on iOS. However, all the methods using Path work. So I think you should use removeItemAtPath:error:and copyItemAtPath:toURL:error: for that purpose.

Hope it helps