Question

I have the following

// this code is inside cellForRowAtIndexPath for a TableViewController

id answer = [self.answers objectAtIndex:indexPath.row];
if ([answer respondsToSelector:@selector(objectForKey)]) {
    cell.textLabel.text = [answer valueForKey:@"answer_id"];
} else {
    // I'm ending up here, instead of the cell.textLabel being set
    [NSException raise:@"Answer is of invalid class" format:@"It should be able to respond to valueForKey, class: %@", [answer class]];
}

where self.answers is set to

// the question that gets passed here is a parsed single object 
// from the `/questions` path
- (NSArray *)answersForQuestion:(NSDictionary *)question {
    NSString *contents = [self loadContentsForPath:[question valueForKey:@"question_answers_url"]];
valueForKey:@"question_answers_url"]];
    NSDictionary *data = [contents JSONValue];
    NSArray *answers = [data valueForKey:@"answers"];
    return answers;
}

- (NSString *)loadContentsForPath:(NSString *)path {
    NSString *wholeURL = [@"http://api.stackoverflow.com/1.1" stringByAppendingString:path];    
    return [NSString stringWithContentsOfURL:[NSURL URLWithString:wholeURL] encoding:NSUTF8StringEncoding error:nil];
}

I’m doing exactly the same thing for loading questions which works just fine, but it seems
to fail on answers when I try to do [answers valueForKey:@"answer_id"].

I don’t think this is a problem with the JSON parser, because it works fine for the /questions data.

When the debugger stops on the exception and when I try to right click -> Print Description on answers, I get

Printing description of answer:
<CFBasicHash 0x6ec1ec0 [0x1474b38]>{type = mutable dict, count = 13,
entries =>
    1 : <CFString 0x6ec57d0 [0x1474b38]>{contents = "down_vote_count"} = <CFNumber 0x6e1dc00 [0x1474b38]>{value = +0, type = kCFNumberSInt32Type}
    2 : <CFString 0x6ec4ee0 [0x1474b38]>{contents = "last_activity_date"} = <CFNumber 0x6ec5780 [0x1474b38]>{value = [phone], type = kCFNumberSInt64Type}
    3 : <CFString 0x6ec44b0 [0x1474b38]>{contents = "community_owned"} = <CFBoolean 0x1474f68 [0x1474b38]>{value = false}
...

which to me seems like a regular hash. I tried both objectForKey and valueForKey and neither of them work, i.e.

exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber isEqualToString:]: unrecognized selector sent to instance 0x6b2a330'

when I do just

cell.textLabel.text = [answer objectForKey:@"answer_id"];

Answer 1

The : is a part of the method name, so you need to do:

if ([answer respondsToSelector:@selector(objectForKey:)]) {

And then use objectForKey:, not valueForKey:. The first is to access objects in the dictionary, the later is for the so-called Key-Value Coding. So it’s:

id answer = [self.answers objectAtIndex:indexPath.row];
if ([answer respondsToSelector:@selector(objectForKey:)]) {
    cell.textLabel.text = [answer objectForKey:@"answer_id"];
} else {
    [NSException raise:@"Answer is of invalid class" format:@"It should be able to respond to objectForKey:, class: %@", [answer class]];
}

Last but not least, it looks like the object you get out of the answer dictionary is a NSNumber, not an NSString. So you might want to change the setting of the text to:

cell.textLabel.text = [[answer objectForKey:@"answer_id"] description];