Saving a NSMutableArray using NSUserDefaults not working

Question

I have this mutable array named myWallet with its property declaration:

@property (nonatomic, strong) NSMutableArray *myWallet;

This array is being filled up by Cards, which is an object. I create a new instance of a Card from another View Controller so I use a delegate method in able to pass this created card on my initial View Controller which is a Table View Controller. Below is the delegate method

- (void)addCardViewController:(AddCardViewController *)sender didCreateCard:(Card *)newCard
{
    // insert a new card

    self.myCard = newCard;
    [self.myWallet addObject:self.myCard];

    [self.tableView reloadData];
}

So far, I can create new instances of Card, and when I do, a new table cell appears in my initial view controller (Card Wallet VC).

What I want is when the user is done using the app, the created instance(s) of Card he / she previously created still appears on next run time of the app. So I used NSUserDefaults to do this. The method below is how I'm using NSUserDefaults to save.

- (void)saveMyWallet
{

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

    [defaults setObject:self.myWallet forKey:@"myWalletArray"];

    [defaults synchronize];
    NSLog(@"I am saved");
}

I call this method inside the didSelectRowAtIndexPath. When I run the app, then create a new instance of a Card, click the table cell, this appears on my log.

2012-05-04 14:51:00.900 CardWallet[24998:f803] *** -[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value '("<Card: 0x6aa02f0>")' of class '__NSArrayM'.  
Note that dictionaries and arrays in property lists must also contain only property values.

What could be a possible reason to this? And can you suggest me ways I can go about it.

Thanks in advance.

Answer 1

Your Card class must also conform to the NSCoding Protocol because you can only store NSData, NSString, NSArray, NSDictionary and NSNumber with NSUserDefaults. That means, you need to add two methods to your class:

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

    [enCoder encodeObject:instanceVariable forKey:INSTANCEVARIABLE_KEY];

    // Similarly for the other instance variables.
    ....
}

- (id)initWithCoder:(NSCoder *)aDecoder {

   if(self = [super initWithCoder:aDecoder]) {
       self.instanceVariable = [aDecoder decodeObjectForKey:INSTANCEVARIABLE_KEY];

       // similarly for other instance variables
       ....
   }

   return self;
}

Then create a NSData NSKeyedArchiver before storing it to NSUserDefaults. Like this:

 NSUserDefaults *ud = [NSUserDefaults standardUserDefaults];
 NSData *yourArrayAsData = [NSKeyedArchiver archivedDataWithRootObject:yourArray];
 [ud setObject:sharesAsData forKey:kA_KEY_FOR_USERDEFAULTS];