Question

I know this sounds like an odd question, but I need to keep a copy of my NSUserDefaults in to a database (my aim is provide a database backup / restore feature, using one file, the database).

So I think I’ve figured out how to load to a file (although I haven’t tried this in xcode).

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults registerDefaults:[NSDictionary dictionaryWithContentsOfFile:
   [[NSBundle mainBundle] pathForResource:@"UserDefaults" ofType:@"plist"]]];

I’ve googled how to save NSUserDefaults to a plist and to a string and back, but haven’t found anything.

Answer 1

You can use the asynchronous NSPropertyListSerialization API or just the synchronous convenience methods on NSDictionary.

Checkout the discussion in the NSDictionary Apple Docs on the writeToFile:automatically method for more info on how it works

Also, This article has some good info on Serialization in cocoa generally.

Use the following code should get you on your way.

//Get the user documents directory
NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
//Create a path to save the details
NSString *backedUpUserDefaultsPath = [documentsDirectory stringByAppendingPathComponent:@"NSUserDefaultsBackup.plist"];
//Get the standardUserDefaults as an NSDictionary
NSDictionary *userDefaults = [[NSUserDefaults standardUserDefaults] dictionaryRepresentation];

//The easiest thing to do here is just write it to a file
[userDefaults writeToFile:backedUpUserDefaultsPath atomically:YES];

//Alternatively, you could use the Asynchronous version
NSData *userDefaultsAsData = [NSKeyedArchiver archivedDataWithRootObject:userDefaults];

//create a property list object
id propertyList = [NSPropertyListSerialization propertyListFromData:userDefaultsAsData
                                                   mutabilityOption:NSPropertyListImmutable
                                                             format:NULL
                                                   errorDescription:nil];
//Create and open a stream    
NSOutputStream *outputStream = [[NSOutputStream alloc] initToFileAtPath:backedUpUserDefaultsPath append:NO];
[outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
outputStream.delegate = self; //you'll want to close, and potentially dealloc your stream in the delegate callback
[outputStream open];

//write that to the stream!
[NSPropertyListSerialization writePropertyList:propertyList 
                                      toStream:outputStream 
                                        format:NSPropertyListImmutable 
                                       options:NSPropertyListImmutable 
                                         error:nil];

When you want to go backwards you can simply do something like:

NSDictionary *dictionaryFromDisk = [NSDictionary dictionaryWithContentsOfFile:backedUpUserDefaultsPath];    

Or you could use the stream/NSData approach from NSPropertyListSerialization, which is similar to the way you save it.