Error: Mutating method sent to immutable object for NSMutableArray from JSON file

Question

This seems to be a fairly common problem, but the solutions that I have looked at do not solve the error. I am trying to read an NSMutableArray from a JSON file. Many of the suggestions I have seen involve using mutableCopy or [NSMutableArray arrayWithArray:] but both of these solutions do not fix the problem when using the call replaceObjectAtIndex:withObject: seen below. Please let me know if you have any advice on how to solve this problem.

EDIT: I would also like to add that the inventory list is an NSMutableArray of NSMutableArray objects.

The exact error reads:

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

I have the property defined as follows at the top of my implementation file:

NSMutableArray *inventoryData;

I am trying to read it from a JSON file as follows:

- (void)readJSON
{
    //Code to get dictionary full of saves from JSON file (overworld.json) - includes the file path on the ipad as well as
    //the dictionary itself
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *localPath = [[NSString alloc] initWithString:[documentsDirectory stringByAppendingPathComponent:@"savedPaintGameData.json"]];
    NSString *filePath = [localPath mutableCopy];
    NSError *e = nil;

    // Read data from file saved previously - read the raw data from the path, then parse it into a dictionary using JSONObjectWithData
    NSData *RawJSON = [NSData dataWithContentsOfFile:filePath options:NSDataReadingMappedIfSafe error:&e];

    if (RawJSON == nil) {
        [self saveGameInitialize];
    } else {
        NSMutableDictionary *localDictionary = [[NSMutableDictionary alloc] initWithDictionary:[NSJSONSerialization JSONObjectWithData:RawJSON options:NSJSONReadingAllowFragments error:&e]];
        NSMutableDictionary *savedDataDictionary = [localDictionary mutableCopy];

        //inventoryData = [[savedDataDictionary objectForKey:@"inventory"] mutableCopy];
        inventoryData = [NSMutableArray arrayWithArray:[savedDataDictionary objectForKey:@"inventory"]];
    }
}

I am then trying to replace an object at the given index of the NSMutableArray as seen here:

- (void)setInventoryData: (NSString *) colorKey: (int) change
{
    // Check if inventory already contains the paint and change the amount
    bool foundPaint = false;
    int newAmount = 100;    // Magic number prevents crashing @ removal check
    for (int i = 0; i < [inventoryData count]; i++) {
        NSMutableArray *object = [inventoryData objectAtIndex:i];
        if ([[object objectAtIndex:0] isEqualToString:colorKey]) {
            newAmount = [[object objectAtIndex:1] integerValue] + change;
            [[inventoryData objectAtIndex:i] replaceObjectAtIndex:1 withObject:[NSNumber numberWithInt:newAmount]];
            foundPaint = true;
            break;
        }
    }

    if (newAmount == 0) {
        [self removeInventoryColor:colorKey];
    }
}

Answer 1

The issue appears to be surround the depth at which you are working... the mutable versions of containers you are creating only apply to that "level". You are later indexing into that level (i.e. accessing a container one level deeper) which is still immutable. Try passing the NSJSONReadingMutableContainers option when you first unserialize the JSON:

NSUInteger jsonReadingOptions = NSJSONReadingAllowFragments | NSJSONReadingMutableContainers;
NSMutableDictionary *localDictionary = [[NSMutableDictionary alloc] initWithDictionary:[NSJSONSerialization JSONObjectWithData:RawJSON options:jsonReadinOptions error:&e]];