NSJSONSerialization not creating mutable container

Question

Here's the code:

NSError *parseError;
NSMutableArray *listOfObjects = [NSJSONSerialization JSONObjectWithData:[@"[]" dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:&parseError];
NSLog(@"Is mutable? %li", [listOfObjects isKindOfClass:[NSMutableArray class]]);

listOfObjects = [NSJSONSerialization JSONObjectWithData:[@"[[],{}]" dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingMutableContainers error:&parseError];
NSLog(@"Is mutable? %li", [listOfObjects isKindOfClass:[NSMutableArray class]]);

As you can see, I'm calling exactly the same method for parsing the JSON both times, one with an empty list in the JSON, and then a list with an object inside. Here's the result:

Is mutable? 0
Is mutable? 1 

The problem is that the NSJSONSerialization doesn't seem to follow the option to create mutable containers for empty lists. Seems like a bug to me, but maybe I just misunderstanding things.

Any ideas?

Answer 1

Here is what I do:

BOOL needsWorkaround = YES;
if (needsWorkaround)
{
    NSMutableDictionary* appState2 = 
        (__bridge_transfer NSMutableDictionary*) 
        CFPropertyListCreateDeepCopy (
            kCFAllocatorDefault, (__bridge void*)appState,
            kCFPropertyListMutableContainersAndLeaves
        );
    appState = appState2;
}

Answer 2

Here's my workaround for this problem:

#import "NSJSONSerialization+MutableBugFix.h"

@implementation NSJSONSerialization (NSJSONSerialization_MutableBugFix)

+ (id)JSONObjectWithDataFixed:(NSData *)data options:(NSJSONReadingOptions)opt error:(NSError **)error {
    id object = [NSJSONSerialization JSONObjectWithData:data options:opt error:error];

    if (opt & NSJSONReadingMutableContainers) {
        return [self JSONMutableFixObject:object];
    }

    return object;
}

+ (id)JSONMutableFixObject:(id)object {
    if ([object isKindOfClass:[NSDictionary class]]) {
        // NSJSONSerialization creates an immutable container if it's empty (boo!!)
        if ([object count] == 0) {
            object = [object mutableCopy];
        }

        for (NSString *key in [object allKeys]) {
            [object setObject:[self JSONMutableFixObject:[object objectForKey:key]] forKey:key];
        }
    } else if ([object isKindOfClass:[NSArray class]]) {
        // NSJSONSerialization creates an immutable container if it's empty (boo!!)
        if (![object count] == 0) {
            object = [object mutableCopy];
        }

        for (NSUInteger i = 0; i < [object count]; ++i) {
            [object replaceObjectAtIndex:i withObject:[self JSONMutableFixObject:[object objectAtIndex:i]]];
        }
    }

    return object;
}

@end

So I call:

NSDictionary *object = [NSJSONSerialization JSONObjectWithDataFixed:jsonData options:NSJSONReadingMutableContainers error:&err];

Instead of the usual:

NSDictionary *object = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&err];

Answer 3

Others also take this for a bug, see

1. https://github.com/couchbaselabs/TouchDB-iOS/issues/44 or
2. https://github.com/johnlabarge/jlbiosutils/blob/master/jlbiosutils/DynamicProperties.m, for example.

In the latter case you can see complete workaround also for empty dictionaries (see DynamicGetter(...) method).

Answer 4

This works just as expected:

NSString *s = @"{ \"objs\": [ \"a\", \"b\" ] }";    
NSData *d = [NSData dataWithBytes:[s UTF8String] length:[s length]];
id dict = [NSJSONSerialization JSONObjectWithData:d options:NSJSONReadingMutableContainers error:NULL];

NSLog(@"%@", dict);

[[dict objectForKey:@"objs"] addObject:@"c"];

NSLog(@"%@", dict);
NSLog(@"%@", [[dict objectForKey:@"objs"] class]);

Here's the console output:

2012-03-28 13:49:46.224 ExampleRunner[42526:707] {
    objs =     (
        a,
        b
    );
}
2012-03-28 13:49:46.225 ExampleRunner[42526:707] {
    objs =     (
        a,
        b,
        c
    );
}
2012-03-28 13:49:46.225 ExampleRunner[42526:707] __NSArrayM

EDIT

Note that if we append the following line to the code above...

NSLog(@"%@", [[dict objectForKey:@"objs"] superclass]);

...we get the following output on the console:

2012-03-28 18:09:53.770 ExampleRunner[42830:707] NSMutableArray

...just in case it wasn't clear that __NSArrayM is a private subclass of NSMutableArray, thus proving that the OP's code did indeed work as expected (except for his NSLog statement).

EDIT

Oh, and by the way, the following line of code...

NSLog(@"%d", [[dict objectForKey:@"objs"] isKindOfClass:[NSMutableArray class]]);

...results in the following console output:

2012-03-28 18:19:19.721 ExampleRunner[42886:707] 1

EDIT (responding to changed question)

Interesting...looks like a bug. Given the following code:

NSData *dictData2 = [@"{ \"foo\": \"bar\" }" dataUsingEncoding:NSUTF8StringEncoding];
id dict2 = [NSJSONSerialization JSONObjectWithData:dictData2 options:NSJSONReadingMutableContainers error:NULL];
NSLog(@"%@", [dict2 class]);
NSLog(@"%@", [dict2 superclass]);
NSLog(@"%d", [dict2 isKindOfClass:[NSMutableDictionary class]]);

// This works...
[dict2 setObject:@"quux" forKey:@"baz"];
NSLog(@"%@", dict2);

NSData *dictData = [@"{}" dataUsingEncoding:NSUTF8StringEncoding];
id emptyDict = [NSJSONSerialization JSONObjectWithData:dictData options:NSJSONReadingMutableContainers error:NULL];
NSLog(@"%@", [emptyDict class]);
NSLog(@"%@", [emptyDict superclass]);
NSLog(@"%d", [emptyDict isKindOfClass:[NSMutableDictionary class]]);

//...but this fails:
[emptyDict setObject:@"quux" forKey:@"baz"];
NSLog(@"%@", emptyDict);

Here's the console output:

2012-03-29 09:40:52.781 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.782 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.782 ExampleRunner[43816:707] __NSCFDictionary
2012-03-29 09:40:52.782 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.783 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.783 ExampleRunner[43816:707] {
    baz = quux;
    foo = bar;
}
2012-03-29 09:40:52.784 ExampleRunner[43816:707] __NSCFDictionary
2012-03-29 09:40:52.784 ExampleRunner[43816:707] NSMutableDictionary
2012-03-29 09:40:52.784 ExampleRunner[43816:707] 1
2012-03-29 09:40:52.785 ExampleRunner[43816:707] NSException: -[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object

So empty arrays and dictionaries created this way don't seem to behave as expected.