Check if key of NSMutableDictionary is nil

Question

If got a NSMutableDictionary from Json Data

  NSMutableDictionary *returnedDict = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];

I know that this key returnedDict[@"data"][@"weather"][day][@"tides"] is NSNull in some cases. So I get -[NSNull objectForKeyedSubscript:]

So I try according to this answer How to check if an NSDictionary or NSMutableDictionary contains a key? to check if it is nil or not.

if (returnedDict[@"data"][@"weather"][day][@"tides"]){ some code }

and

if (returnedDict[@"data"][@"weather"][day][@"tides"]!=[NSNull null]){ some code}

does not avoid to run {some code}

How do I check this in the right way?

Answer 1

So your issue is:

Your server may return null to indicate that an object isn't present. NSJSONSerialization will convert that null into an instance of NSNull. In theory that means that instead of doing result[a][b][c] you need to check whether result[a] is a dictionary and, if so, whether result[a][b] is a dictionary, etc, etc, which is repetitious and error-prone?

Perhaps the easiest thing might be to remove from the dictionary any key with a value of NSNull, so that next time you ask for the value you'll get an ordinary nil, which is safe to message per the usual compound-messaging rules?

NSJSONSerialization won't do that for you but it's easy enough to add after the fact:

@interface NSDictionary(RemoveNullValues)
- (NSDictionary *)ty_collectionWithoutNullValues;
@end

@interface NSArray(RemoveNullValues)
- (NSArray *)ty_collectionWithoutNullValues;
@end

[...]

@implementation NSDictionary(RemoveNullValues)

- (NSDictionary *)ty_collectionWithoutNullValues {

    NSMutableDictionary *reducedDictionary = [self mutableCopy];

    // remove any keys for which NSNull is the direct value
    NSArray *keysEvaluatingToNull = [self allKeysForObject:[NSNull null]];
    [reducedDictionary removeObjectsForKeys:keysEvaluatingToNull];

    // ask any child dictionaries to do the same; note that it's safe
    // to mutate reducedDictionary in this array because allKeys is a 
    // copy property; what you're iterating is not reducedDictionary 
    // but a snapshot of its keys when the array first began
    for (id key in [reducedDictionary allKeys]) {
        id child = reducedDictionary[key];
        if ([child respondsToSelector:@selector(ty_collectionWithoutNullValues)]) {
            reducedDictionary[key] = [child ty_collectionWithoutNullValues];
        }
    }

    return [reducedDictionary copy];
}

@end

@implementation NSArray(RemoveNullValues)
- (NSArray *)ty_collectionWithoutNullValues {
    NSMutableArray *reducedArray = [NSMutableArray array];

    for (id child in self) {
        if ([child isKindOfClass:[NSNull class]]) continue;

        if ([child respondsToSelector:@selector(ty_collectionWithoutNullValues)]) {
            [reducedArray addObject:[child ty_collectionWithoutNullValues]];
        } else {
            [reducedArray addObject:child];
        }
   }

   return [reducedArray copy];
}
@end

Answer 2

You must read this answer in conjunction with the accepted answer and comments to the question Is there NSMutableDictionary literal syntax to remove an element?

Following on from the linked answer you can quietly remove all the NSNull's and return nil instead if you access the element using the literal syntax (i.e. not using objectForKey:) by adding the following to your application:

@implementation NSDictionary (ClobberNSNull)

- (id) objectForKeyedSubscript:(id<NSCopying>)key
{
   id result = [self objectForKey:key];
   return result == NSNull.null ? nil : result;
}

@end

Now when you use the syntax:

dictionary[key]

if the matching object is NSNull then nil will be returned just as if the key did not exist.

There are caveats, see the linked question, and you need to decide if this approach is suitable for your situation. But it is simple.

HTH

Note: Before someone comments, NSNull is a singleton so the == is OK.

Answer 3

use if(![returnedDict[@"data"][@"weather"][day][@"tides"] isKindOfClass:[NSNull class]]) { some code }