insert object in an NSMutableArray saved with NSUserDefaults

Question

I have an NSMutableArray saved with NSUserDefaults. This array is my "favourite" items that the user can saves, so when i want to add one item, i need to read the array (from NSuserDefault) and save in the first free position.

I'm using this method to add a value in the NSMutableArray

-(IBAction)save{
   NSMutableArray *abc = [[NSUserDefaults standardUserDefaults] objectForKey:@"12345"];
   int n = [abc count];
   [abc insertObject:@"aaa" atIndex:n];
   NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
   [[NSUserDefaults standardUserDefaults] setObject:abc forKey:@"12345"];
   [defaults synchronize];
   [abc release];
}

what's the deal? That if the user call this method two times, the second time the app crashes with this log:

* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'

why? and why just the second time? The first time works fine!

Answer 1

NSUserDefaults always returns immutable objects, even if the original object was mutable. It's in the documentation for objectForKey:

The returned object is immutable, even if the value you originally set was mutable.

You will need to create a copy of the returned object before you modify it, using [NSMutableArray arrayWithArray:]

Probably also best to use the arrayForKey method of NSUserDefaults if you're retrieving an array. Docs here: https://developer.apple.com/documentation/foundation/userdefaults

Answer 2

Here is a complete working example:

- (void)saveScore:(NSNumber *)score
{
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSMutableArray *scoreList = [[NSMutableArray alloc] initWithArray: [defaults objectForKey:@"ScoreList"]];
    [scoreList addObject:score];
    [defaults setObject:scoreList forKey:@"ScoreList"];
    [defaults synchronize];
}

First we read in the stored data in a mutable array, update the array, save the new array, and then synchronize the user defaults to permanently record the changes.

Hope this helps!

Answer 3

Another option is:

[[NSUserDefaults standardUserDefaults] setObject:[[[NSUserDefaults standardUserDefaults] objectForKey:@"12345"] arrayByAddingObject:object] forKey:@"12345"];

Answer 4

Copy Array from an NSUserDefault like this way :

[YourArray addObjectsFromArray:[[NSUserDefaults standardUserDefaults] 
                  objectForKey:@"NSUSD"]];

or

YourArray = [[NSMutableArray alloc] 
             initWithArray:[[NSUserDefaults standardUserDefaults]  
              objectForKey:@"NSUSD"]];
[YourArray addObject: (currentObject)];

Answer 5

It is because you NSMutablepointer points an NSUserDefault object directly. You should first copy NSUserDefault object to an array than process it.

NSMutableArray *arr= [NSMutableArray arrayWithObjects:@"asd",@"dsa",nil];
    [[NSUserDefaults standardUserDefaults] setObject:arr forKey:@"12345"];
NSMutableArray *abc = [NSMutableArray arrayWithArray:[[NSUserDefaults standardUserDefaults] objectForKey:@"12345"]];
//[[NSUserDefaults standardUserDefaults] objectForKey:@"12345"];
    int n = [abc count];
    [abc insertObject:@"aaa" atIndex:n];
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    [[NSUserDefaults standardUserDefaults] setObject:abc forKey:@"12345"];
    [defaults synchronize];
    [abc release];

Answer 6

Quick fix:

_retrivedArray = [_mutableArray mutableCopy];