insert object in an NSMutableArray saved with NSUserDefaults

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开发者_StackOverflow社区];
}

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!

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

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!

Another option is:

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

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)];

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];

Quick fix:

_retrivedArray = [_mutableArray mutableCopy];