使用ObjectiveC进行JSON解析的弹性

Question

A JSON API feed used by our iOS ObjectiveC app is a bit flaky, so sometimes a field is null.

When parsing JSON we use

NSDictionary *json = [self JSONFromResponseObject:responseObject];

Then try to use the fields with eg

[widgetIDArray addObject:widget[@"name"][@"id"]];

Where sometimes the "name" field will be a null. Do we:

1) Ask the API provider to clean up their flaky API code

2) Check for null each and every time we try to use something from the json dict

if ( ![widget[@"name"] isKindOfClass:[NSNull class]] )

3) Use try - catch

 @try {
      [widgetIDArray addObject:widget[@"name"][@"id"]];
     }
 @catch (NSException *exception) 
    {
     NSLog(@"Exception %@",exception);
   }

ANSWER:

Thanks for the answers, below. Here is the extension to NSObject I added that allows me to get deeply nested JSON items that may or may not be present.

First call with something like

self.item_logo = [self valueFromJSONWithKeyArray:event withKeyArray:@[@"categories",@"bikes",@"wheels",@"model",@"badge_uri"]];

Here is the code in NSObject+extensions.m

- (id) valueFromJSONWithKeyArray:(id)json withKeyArray:(NSArray *)keyArray
{
    for (NSString * keyString in keyArray)
    {
        if ([json[keyString] isKindOfClass:[NSObject class]])
        {
            json = json[keyString]; // go down a level
        }
        else
        {
            return nil; // we didn't find this key
        }
    }  
    return json; // We successfully found all the keys, return the object
}

Answer 1

null in a JSON response isn't "flaky", it is absolutely standard.

Even if it was "flaky", any message that you receive from the outside is an attack vector that could allow an attacker to hack into your program, so resilience is required. Crashing when your receive a null allows a DOS attack against your application.

@try / @catch is awful. Exceptions are thrown in response to programming errors. You don't catch them, you fix your code.

How do you fix your code? Simple. Write a few helper methods in an NSDictionary extension.

First you don't know that json is a dictionary. So you add an NSDictionary class method where you pass in anything and it returns what you passed if it is a dictionary and nil (with appropriate logging) if it is anything else.

Next you assume that there is a dictionary under the key "name". So you write an extension "jsonDictionaryForKey" which returns a dictionary if there is one, and nil (with appropriate logging) if it is anything else.

And so on. Make your JSON parsing bullet proof if you want to call yourself a professional developer. For extra bonus points you add a method which will take a dictionary and list all keys that are present that you didn't ask for - so you know if your API is sending things that you don't expect.

Answer 2

You can delete all NSNULL values in your JSON object. Here is a function I used in my library to git rid of all null values in a JSON object.

id BWJSONObjectByRemovingKeysWithNullValues(id json, NSJSONReadingOptions options) {
    if ([json isKindOfClass:[NSArray class]]) {
        NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:[(NSArray *)json count]];
        for (id value in (NSArray *)json) {
            [mutableArray addObject:BWJSONObjectByRemovingKeysWithNullValues(value, options)];
        }

        return (options & NSJSONReadingMutableContainers) ? mutableArray : [NSArray arrayWithArray:mutableArray];
    } else if ([json isKindOfClass:[NSDictionary class]]) {
        NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithDictionary:json];
        for (id<NSCopying> key in [(NSDictionary *)json allKeys]) {
            id value = [(NSDictionary *)json objectForKey:key];

            if (isNullValue(value)) {
                [mutableDictionary removeObjectForKey:key];
            } else if ([value isKindOfClass:[NSArray class]] || [value isKindOfClass:[NSDictionary class]]) {
                [mutableDictionary setObject:BWJSONObjectByRemovingKeysWithNullValues(value, options) forKey:key];
            }
        }

        return (options & NSJSONReadingMutableContainers) ? mutableDictionary : [NSDictionary dictionaryWithDictionary:mutableDictionary];
    }

    return json;
}

After all null values have been cleared, perhaps the exceptions will gone too.

Answer 3

An approach that's often used is categories similar to these on NSDictionary and NSMutableDictionary , that ignore nil and NSNull .

@interface NSDictionary (nullnilsafe)
- (ObjectType)nullnilsafe_objectForKey:(KeyType)aKey;
@end    

@implementation NSDictionary

- (ObjectType)nullnilsafe_objectForKey:(KeyType)aKey
{
    id obj = [self objectForKey:aKey];
    if (obj && ![obj isKindOfClass:[NSNull class]] ){
        return obj;
    }
    return nil; 
}

@end

@interface NSMutalbeDictionary (nullnilsafe)
- (void)nullnilsafe_setObject:(ObjectType)anObject forKey:(id<NSCopying>)aKey;
@end    

@implementation NSMutalbeDictionary
- (void)nullnilsafe_setObject:(ObjectType)anObject forKey:(id<NSCopying>)aKey
{
    if (anObject && aKey && ![anObject isKindOfClass:[NSNull class]]){
        [self setObject:anObject forKey:aKey];
    }
}

@end