NSDictionary extend valueForKey method - ios

Question

Can I create category that will extend - (id)valueForKey:(NSString *)key method and if the value of this key will be equal [NSNull null] it will return me empty string #"".
It is connected to nil values for some keys that I get from the backend server in JSON.
So if I need to save some value to the property it can be nil.
Suppose I use this code with value for key nil
artist.name = ad[#"artist"][#"name"]; // nil value on the right.
so if I will have extension it will check value for key and return me #"" intend of nil. Does it make sense.
Or do I need extend NSManagedObject for checking attribute type and then return corresponded value #"", 0 or nil, or 0.0f if the attribute has another not string type.

Answer 1

Rather than defining a category on NSDictionary, you could define a category on NSObject, and do the conversion there:
#interface NSObject (DBNullHandling)
-(NSString*)nullToEmpty;
#end
#implementation NSObject (DBNullHandling)
-(id)nullToEmpty {
return (self == [NSNull null]) ? #"" : self;
}
#end
Now you can do this:
artist.name = [ad[#"artist"][#"name"] nullToEmpty];
If ad[#"artist"][#"name"] returns [NSNull null], artist.name gets assigned #""; otherwise, it gets assigned the value returned by ad[#"artist"][#"name"].

Answer 2

What I do is check before performing the assignment. The tripartite operator is a good shorthand way to do this. You have:
artist.name = ad[#"artist"][#"name"];
Instead, I would write
id val = ad[#"artist"][#"name"];
artist.name = val ? val : #""; // or val ?: #""
(I have not completely understood the question, so it might be necessary to break that check down into more stages or modify it in some way to check for [NSNull null] instead of nil. But you get the idea.)

Answer 3

Instead of subclassing NSDictionary to change the implementation of valueForKey:, I suggest you check if the key exists. So you would instead do this:
BOOL keyExistsInDictionary = [[myDict allKeys] containsObject:key];
id objectForKey = [myDict objectForKey:key];
if (nil == objectForKey && keyExistsInDictionary) objectForKey = #"";
If objectForKey: returns nil, you can check keyExistsInDictionary to see if the key is there and simply contains [NSNull null]. This lets you differentiate between a key that exists and a key that doesn't without modifying NSDictionary.

Answer 4

Can I create category that will extend - (id)valueForKey:(NSString *)key method
The short answer here is no, don't do that. Categories let you extend classes by adding method, but you shouldn't use them to override existing methods. The main reason for this is that it's unreliable -- the order in which the methods in categories are added to a class isn't defined, which means that if you have two categories that override the same method, you'd have no idea which version of the method the class would ultimately end up with.
Another reason is that you have no way to refer to the original method from the overriding method, so you'd have to re-implement the behavior that you want to keep.
Finally, a category changes the class, so every instance of that class will have the new behavior. That's not usually a problem when you're adding methods -- code that doesn't use the new methods remains unaffected by the new functionality. But when you change an existing method, particularly for a class that's as widely used as NSDictionary, it's very likely that you'll break a lot of existing code.
if the value of this key will be equal [NSNull null] it will return me empty string #""
You've gotten some good answers already, but since you asked about categories, here's an approach that solves the problem using a category appropriately. Instead of trying to change -valueForKey:, use a category to add a new method called -stringForKey: that returns a string. It sounds like you're working exclusively with strings, so this seems like a reasonable solution. The implementation can call -valueForKey: and transform non-string objects into strings:
#interface NSDictionary (StringValue)
- (NSString *)stringForKey:(NSString*)key;
#end
#implementation NSDictionary (StringValue)
- (NSString *)stringForKey:(NSString*)key
{
id value = [self valueForKey:key];
NSString *string;
if ([value isKindOfClass:[NSString class]]) {
string = (NSString*)value;
}
else if ([value isKindOfClass:[NSNull class]]) {
string = #"";
}
else {
string = [value description];
}
return string;
}
#end