Can't write to document directory - ios

Question

I'm trying to copy a json file from the application bundle to the Document directory, but surprisingly i'm not able to do that. Here's the code:
NSArray *array = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docDir = [array lastObject];
NSString *path = [docDir stringByAppendingPathExtension:#"themes.json"];
if (![[NSFileManager defaultManager] fileExistsAtPath:path])
{
NSString *themesPath = [[NSBundle mainBundle] pathForResource:#"themes" ofType:#"json"];
NSError *error = nil;
[[NSFileManager defaultManager] copyItemAtPath:themesPath toPath:path error:&error];
if (error)
NSLog(#"Error %#", error);
}
It produces the following error:
Error Domain=NSCocoaErrorDomain Code=513 "The operation couldn’t be completed. (Cocoa error 513.)" UserInfo=0x194a0e90 {NSSourceFilePathErrorKey=/var/mobile/Applications/ACED3EF9-B0D8-49E8-91DE-37128357E509/Frinder.app/themes.json, NSUserStringVariant=(
Copy
), NSFilePath=/var/mobile/Applications/ACED3EF9-B0D8-49E8-91DE-37128357E509/Frinder.app/themes.json, NSDestinationFilePath=/var/mobile/Applications/ACED3EF9-B0D8-49E8-91DE-37128357E509/Documents.themes.json, NSUnderlyingError=0x194a0400 "The operation couldn’t be completed. Operation not permitted"}
After a search i've found this question and tried to modify my code as follows:
if (![[NSFileManager defaultManager] fileExistsAtPath:path])
{
NSString *themesPath = [[NSBundle mainBundle] pathForResource:#"themes" ofType:#"json"];
NSData *data = [NSData dataWithContentsOfFile:themesPath];
BOOL success = [[NSFileManager defaultManager] createFileAtPath:path contents:data attributes:nil];
if (!success)
NSLog(#"Fail");
}
but it doesn't work either. success variable is NO. The last thing i tried was:
[data writeToFile:path atomically:YES];
but still in vain. it returns NO.
I need to note that the issue arises on a device only. On a simulator it works all right.
Can anybody give me a clue?

Answer 1

The first example is correct on one thing:
[docDir stringByAppendingPathExtension:#"themes.json"];
should be:
[docDir stringByAppendingPathComponent:#"themes.json"];
This becomes clear we you read the error message, you see that it tries to write the file to /var/mobile/Applications/ACED3EF9-B0D8-49E8-91DE-37128357E509/Documents.themes.json. Notice that there a . where there should be a /.
The stringByAppendingPathExtension: is used for adding a extension to a file, jpg, txt, html,.....
stringByAppendingPathComponent will add a file to a path, by adding the correct directory separator, you case /.
You second example will definitely fail since the app bundle is readonly.