问题
I'm setting values for properties of my NSManagedObject, these values are coming from a NSDictionary properly serialized from a JSON file. My problem is, that, when some value is [NSNull null], I can't assign directly to the property:
fight.winnerID = [dict objectForKey:@"winner"];
this throws a NSInvalidArgumentException
"winnerID"; desired type = NSString; given type = NSNull; value = <null>;
I could easily check the value for [NSNull null] and assign nil instead:
fight.winnerID = [dict objectForKey:@"winner"] == [NSNull null] ? nil : [dict objectForKey:@"winner"];
But I think this is not elegant and gets messy with lots of properties to set.
Also, this gets harder when dealing with NSNumber properties:
fight.round = [NSNumber numberWithUnsignedInteger:[[dict valueForKey:@"round"] unsignedIntegerValue]]
The NSInvalidArgumentException is now:
[NSNull unsignedIntegerValue]: unrecognized selector sent to instance
In this case I have to treat [dict valueForKey:@"round"] before making an NSUInteger value of it. And the one line solution is gone.
I tried making a @try @catch block, but as soon as the first value is caught, it jumps the whole @try block and the next properties are ignored.
Is there a better way to handle [NSNull null] or perhaps make this entirely different but easier?
回答1:
It might be a little easier if you wrap this in a macro:
#define NULL_TO_NIL(obj) ({ __typeof__ (obj) __obj = (obj); __obj == [NSNull null] ? nil : obj; })
Then you can write things like
fight.winnerID = NULL_TO_NIL([dict objectForKey:@"winner"]);
Alternatively you can pre-process your dictionary and replace all NSNulls with nil before even trying to stuff it into your managed object.
回答2:
Ok, I've just woke up this morning with a good solution. What about this:
Serialize the JSON using the option to receive Mutable Arrays and Dictionaries:
NSMutableDictionary *rootDict = [NSJSONSerialization JSONObjectWithData:_receivedData options:NSJSONReadingMutableContainers error:&error];
...
Get a set of keys that have [NSNull null] values from the leafDict:
NSSet *nullSet = [leafDict keysOfEntriesWithOptions:NSEnumerationConcurrent passingTest:^BOOL(id key, id obj, BOOL *stop) {
return [obj isEqual:[NSNull null]] ? YES : NO;
}];
Remove the filtered properties from your Mutable leafDict:
[leafDict removeObjectsForKeys:[nullSet allObjects]];
Now when you call fight.winnerID = [dict objectForKey:@"winner"]; winnerID is automatically going to be (null) or nil as opposed to <null> or [NSNull null].
Not relative to this, but I also noticed that it is better to use a NSNumberFormatter when parsing strings to NSNumber, the way I was doing was getting integerValue from a nil string, this gives me an undesired NSNumber of 0, when I actually wanted it to be nil.
Before:
// when [leafDict valueForKey:@"round"] == nil
fight.round = [NSNumber numberWithInteger:[[leafDict valueForKey:@"round"] integerValue]]
// Result: fight.round = 0
After:
__autoreleasing NSNumberFormatter* numberFormatter = [[NSNumberFormatter alloc] init];
fight.round = [numberFormatter numberFromString:[leafDict valueForKey:@"round"]];
// Result: fight.round = nil
回答3:
I wrote a couple of category methods to strip nulls from a JSON-generated dictionary or array prior to use:
@implementation NSMutableArray (StripNulls)
- (void)stripNullValues
{
for (int i = [self count] - 1; i >= 0; i--)
{
id value = [self objectAtIndex:i];
if (value == [NSNull null])
{
[self removeObjectAtIndex:i];
}
else if ([value isKindOfClass:[NSArray class]] ||
[value isKindOfClass:[NSDictionary class]])
{
if (![value respondsToSelector:@selector(setObject:forKey:)] &&
![value respondsToSelector:@selector(addObject:)])
{
value = [value mutableCopy];
[self replaceObjectAtIndex:i withObject:value];
}
[value stripNullValues];
}
}
}
@end
@implementation NSMutableDictionary (StripNulls)
- (void)stripNullValues
{
for (NSString *key in [self allKeys])
{
id value = [self objectForKey:key];
if (value == [NSNull null])
{
[self removeObjectForKey:key];
}
else if ([value isKindOfClass:[NSArray class]] ||
[value isKindOfClass:[NSDictionary class]])
{
if (![value respondsToSelector:@selector(setObject:forKey:)] &&
![value respondsToSelector:@selector(addObject:)])
{
value = [value mutableCopy];
[self setObject:value forKey:key];
}
[value stripNullValues];
}
}
}
@end
It would be nice if the standard JSON parsing libs had this behaviour by default - it's almost always preferable to omit null objects than to include them as NSNulls.
回答4:
Another method is
-[NSObject setValuesForKeysWithDictionary:]
In this scenario you could do
[fight setValuesForKeysWithDictionary:dict];
In the header NSKeyValueCoding.h it defines that "Dictionary entries whose values are NSNull result in -setValue:nil forKey:key messages being sent to the receiver.
The only downside is you will have to transform any keys in the dictionary to keys that are in the receiver. i.e.
dict[@"winnerID"] = dict[@"winner"];
[dict removeObjectForKey:@"winner"];
回答5:
I was stuck with the same problem, found this post, did it in a slightly different way.Using category only though -
Make a new category file for "NSDictionary" and add this one method -
@implementation NSDictionary (SuperExtras)
- (id)objectForKey_NoNSNULL:(id)aKey
{
id result = [self objectForKey:aKey];
if(result==[NSNull null])
{
return nil;
}
return result;
}
@end
Later on to use it in code, for properties that can have NSNULL in them just use it this way -
newUser.email = [loopdict objectForKey_NoNSNULL:@"email"];
Thats it
来源：https://stackoverflow.com/questions/9137920/nsnull-handling-for-nsmanagedobject-properties-values