Core Data with Array Attribute

Question

The following is the entity Player header file.

@interface Player : NSManagedObject

@property (nonatomic, retain) NSNumber * experience;
@property (nonatomic, retain) id items;
@property (nonatomic, retain) NSNumber * level;

@end

@interface items : NSValueTransformer
@end

items is essentially an NSMutableArray with NSNumber elements. And inside one function, I'm updating this array:

- (void)itemWasDropped:(ItemIndex)item
{ // _player has been correctly retrieved from the database
    // the current number of this item
    int nNum = [[_player.items objectAtIndex:item] intValue];
    // to increment the number
    [_player.items replaceObjectAtIndex:item withObject:[NSNumber numberWithInt:nNum + 1]];
}

This function gets called perfectly. I save the context in the viewWillDisappear function.

- (void)viewWillDisappear:(BOOL)animated
{ // _moc is an NSManagedObjectContext instance that has been correctly initialised.
    // to update the database
    NSError* err = nil;
    BOOL bSucc = [_moc save:&err];
    if (err || !bSucc)
    {
        ...
    }
}

The problem is that the updates are visible in other views as long as I do not shut down the app in the task bar. What's the problem? Anyone can help?

No, the context is the same. Every view asks the AppDelegate for the context. If the context hasn't been initialised, it's initialised and returned. If it has, the AppDelegate simply returns it. — user2226125
– user2226125, Commented Oct 19, 2013 at 10:08
Just solved it. In order to update an array attribute inside a Core Data entity, that array has to be assigned to another @property array in the view controller class or whatever, and update the @property array. Before database updating, the @property array has to be assigned back to the entity attribute. — user2226125
– user2226125, Commented Oct 19, 2013 at 11:29

Mundi · Accepted Answer · 2015-12-03 08:25:16Z

2

Core data can store arrays, but you will have to use a "transformable" attribute. In general it is not very good design. One easy solution is to transform your numbers array into something that can be stored. For example, in many cases it is sufficient to just use a string.

Encode:

player.items = [numbers componentsJoinedByString:@","];
// Swift
player.items = numbers.joinWithSeparator(",")

Decode:

NSMutableArray* numbers = [NSMutableArray array];
for (NSString *s in [player.items componentsSeparatedByString:@","]) {
  [numbers addObject:@([s integerValue])];
}
// Swift
let numbers = player.items.characters.split { $0 == "," } .map { Int($0!) }

edited Dec 3, 2015 at 8:25

answered Oct 19, 2013 at 10:08

Mundi

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4 Comments

user2226125 Over a year ago

Thanks but Core Data can do it by Transformable. I've tried this and it works.

user2226125 Over a year ago

Just solved it. In order to update an array attribute inside a Core Data entity, that array has to be assigned to another @property array in the view controller class or whatever, and update the @property array. Before database updating, the @property array has to be assigned back to the entity attribute.

Holly Over a year ago

If you've solved it, you should post your answer in detail so that others can read and understand your solution. Also, why didn't you use a one-to-many relationship with a different entity type?

user2226125 Over a year ago

The website said I can't answer my own questions within some tens of hours.Because the array has a predefined length and stores only predefined values, I didn't bother to make up a relationship.

user2226125 · Accepted Answer · 2013-10-19 21:54:12Z

Just solved it. In order to update an array attribute inside a Core Data entity, that array has to be assigned to another @property array in the view controller class or, and update the @property array. Before database updating, the @property array has to be assigned back to the entity attribute.

// to hold the player items
// _player.items cannot be directly updated
@property (strong, nonatomic) NSMutableArray* items;

- (void)viewDidLoad
{
    [super viewDidLoad];
    ...
    NSError* err = nil;
    NSMutableArray* results = [[_moc executeFetchRequest:fr error:&err] mutableCopy];
    if (err || results.count == 0)
    {
        ...
    } else
    {
        _player = [results objectAtIndex:0];
        _items = _player.items;
    }
}

// events
- (void)itemWasDropped:(ItemIndex)item
{
    // the current number of this item
    int nNum = [[_items objectAtIndex:item] intValue];
    // to increment the number
    [_items replaceObjectAtIndex:item withObject:[NSNumber numberWithInt:nNum + 1]];
}

- (void)viewWillDisappear:(BOOL)animated
{
    // to update the database
    _player.items = _items;
    NSError* err = nil;
    BOOL bSucc = [_moc save:&err];
    if (err || !bSucc)
    {
         ...
    }
}

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