Iphone – insert object in an NSMutableArray saved with NSUserDefaults

iphonensmutablearraynsuserdefaults

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!

Best Answer

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

Related Solutions

Ios – inconsistant defaults (NSUserDefaults) behavior in iPhone simulator defaults

You should be using an NSKeyedArchiver for saving your arrays, such as:

[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:spells] forKey:@"spells"];
[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:locs] forKey:@"locs"];
[[NSUserDefaults standardUserDefaults] synchronize];

You should also make sure your spells class implements the NSCoding protocol (encodeWithCoder: and initWithCoder:), if it's a custom class. It looks like your locs are NSStrings, which will archive just fine.

You'll also need to do something like

NSData *dataRepresentingSavedSpells = [currentDefaults objectForKey:@"spells"];
if (dataRepresentingSavedSpells != nil)
{
    NSArray *oldSpells = [NSKeyedUnarchiver unarchiveObjectWithData:dataRepresentingSavedSpells];
}

To load the old values from the defaults.

I use synchronize to write to disk on exit, and it's been very reliable in my experience.

Ios – How to sort an NSMutableArray with custom objects in it

Compare method

Either you implement a compare-method for your object:

- (NSComparisonResult)compare:(Person *)otherObject {
    return [self.birthDate compare:otherObject.birthDate];
}

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];

NSSortDescriptor (better)

or usually even better:

NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
                                           ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];

You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.

Blocks (shiny!)

There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
    return [a.birthDate compare:b.birthDate];
}];

Performance

The -compare: and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor as the latter relies on KVC. The primary advantage of the NSSortDescriptor method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView by clicking on the header row.