Nonnull violation detection in Objective-C - objective-c

Question

I have some code like this where I declare an object with nonnull properties
#interface HelloObject : NSObject
#property (nonatomic,strong,nonnull) NSString *foo;
#property (nonatomic,strong,nonnull) NSString *bar;
-(instancetype _Nullable)initWithJson:(nonnull NSDictionary*)json;
#end
and then to initialize the object from the JSON I make use this code:
-(instancetype _Nullable)initWithJson:(nonnull NSDictionary*)json
{
if( (self = [super init]) ) {
_bar = json[#"bar"];
_foo = json[#"foo"];
}
return self;
}
The server may have sent me malformed JSON. For example, the "foo" field might be missing. It would be easy enough to check for nil and return nil, but I have a lot of this code and it would be inelegant and error prone.
Is there a easy and elegant way to check to see if an object violates its nonnull declarations at runtime? For example, I don't want to write code like this:
barX = json[#"bar"];
if (barX) {
_bar = barX;
} else {
return nil;
}
That's ugly and boilerplate (and therefore prone to error). I'd much rather have something like:
if (![self ValidForNonNulls]) {
return nil;
}
but I can't think of a way to write ValidForNonNulls for a general object.

Answer 1

I don't think that trying to work around the need to test for nil is practical in Objective-C.
If I were having to verify many such incoming terms, and I needed to check them for validity or return nil, then I would rewrite my initWithJson method to check, with a category to keep the code clean and readable.
- (instancetype _Nullable)initWithJSON:(nonnull NSDictionary *)json
{
if ( (self = [super init]) ) {
if ( ![json hasValuesForKeys:#[#"foo", #"bar"]] ) {
//Consider logging this as an error
return nil;
}
_bar = json[#"bar"];
_foo = json[#"foo"];
}
return self;
}
...
#interface NSDictionary (hasValuesForKeys)
- (BOOL)hasValuesForKeys:(NSArray *)keys;
#end
#implementation
- (BOOL)hasValuesForKeys:(NSArray *)keys
{
for (NSString *key in keys) {
if ( !self[key] || [self[key] isEqual:[NSNull null]] ) {
return NO;
}
}
return YES;
}
#end
You could make more specific tests for each value if you need to validate whether they are NSNumber for example.