Question

I’m developing an app which will save data to the local file system. The data that will be saved will be mostly NSString and NSDate. The data will not be saved that often, perhaps new data will be entered 10 times at a typical usage. The data should also of course be retrievable (CRUD)

How should I save this data? First of all is it necessary to model these objects? If not should I use property lists? Or SQLLite3?

Else should I archive the class models? Use SQLLite3?

EDIT: I accidentally left out some vital information about the app. Actually my app will be having 2 data models which have an aggregated relationship. So my first data model (lets call it DataA) which will have a NSString and NSDate will also have a reference to the second data model (lets call it DataB) which itself will consist of a NSString and a NSArray. So it gets a little more complicated now. If an object from DataB gets deleted it should of course cease to exist in DataA (but the rest of DataA should be left untouched)

Answer 1

This kind of data seems to be very simple to store and retrieve and does not have any other dependencies such as a horridly complex object graph.

You should store this data in a flat file or in NSUserDefaults.

I’ll give you an example of both, using object archiving with the use of the NSCoding protocol:

@interface ApplicationData <NSCopying, NSCoding> {}

@property (nonatomic, strong) NSDate *someDate;
@property (nonatomic, strong) NSDate *someOtherDate;

@property (nonatomic, copy) NSString *someString;
@property (nonatomic, copy) NSString *someOtherString;

@end

@implementation ApplicationData

@synthesize someDate = _someDate, someOtherDate = _someOtherDate, someString = _someString, someOtherString = _someOtherString;

- (NSArray *)keys {
   static dispatch_once_t once;
   static NSArray *keys = nil;
   dispatch_once(&once, ^{
      keys = [NSArray arrayWithObjects:@"someString", @"someOtherString", @"someDate", @"someOtherDate", nil];
  });
   return keys;
}

- (id) copyWithZone:(NSZone *) zone {
    ApplicationData *data = [[[self class] allocWithZone:zone] init];
    if(data) {
        data.someString = _someString;
        data.someOtherString = _someOtherString;     

        data.someDate = _someDate;
        data.someOtherDate = _someOtherDate;
        //...
    }
    return data;
 }

 - (void) encodeWithCoder:(NSCoder *) coder {
     [super encodeWithCoder:coder];

     NSDictionary *pairs = [self dictionaryWithValuesForKeys:[self keys]];

     for(NSString *key in keys) {
        [coder encodeObject:[pairs objectForKey:key] forKey:key];
     }
  }


  - (id) initWithCoder:(NSCoder *) decoder {
     self = [super initWithCoder:decoder];
     if(self) {
        for(NSString *key in [self keys]) {
           [self setValue:[decoder decodeObjectForKey:key] forKey:key];
        }
     }
     return self;
  }

  @end

Then, say in your application delegate, you can do this:

@interface AppDelegate (Persistence)

@property (nonatomic, strong) ApplicationData *data;

- (void)saveApplicationDataToFlatFile;
- (void)loadApplicationDataFromFlatFile;
- (void)saveApplicationDataToUserDefaults;
- (void)loadApplicationDataFromUserDefaults;

@end

@implementation AppDelegate (Persistence) 
@synthesize data;

- (NSString *)_dataFilePath {
   static NSString *path = nil;
   static dispatch_once_t once;
   dispatch_once(&once, ^{
     path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) stringByAppendingPathComponent:@"xAppData.dat"];
   });
   return path;
}

- (void)loadApplicationDataFromUserDefaults {        
   NSData *archivedData = [[NSUserDefaults standardUserDefaults] objectForKey:@"appData"];
   self.data = [NSKeyedUnarchiver unarchiveObjectWithData:archivedData];
}

- (void)saveApplicationDataToUserDefaults {
   NSData *archivedData = [NSKeyedArchiver archivedDataWithRootObject:self.data];
   [[NSUserDefaults standardUserDefaults] setObject:archivedData forKey:@"appData"];
   [[NSUserDefaults standardUserDefaults] synchronize];
}

- (void)loadApplicationDataFromFlatFile {
   NSData *archivedData = [NSData dataWithContentsOfFile:[self _dataFilePath]];
   self.data = [NSKeyedUnarchiver unarchiveObjectWithData:archivedData];
}

- (void)saveApplicationDataToFlatFile {  
   NSData *archivedData = [NSKeyedArchiver archivedDataWithRootObject:self.data];
   [archivedData writeToFile:[self _dataFilePath] atomically:YES];
}

@end

Disclaimer: I have not tested this code.