NSDictionary 没有响应 objectForKey 和 valueForKey

Question

我有以下

// 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]];
}

self.answers 设置为

// 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];
}

我正在做同样的事情来加载问题，它工作得很好，但似乎 当我尝试做[answers valueForKey:@"answer_id"]时，答案失败。

我认为这不是 JSON 解析器的问题，因为它适用于 /questions 数据。

当调试器在异常上停止并且当我尝试右键单击 -> 在answers 上打印描述时，我得到

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 = +1326379080, type = kCFNumberSInt64Type}
    3 : <CFString 0x6ec44b0 [0x1474b38]>{contents = "community_owned"} = <CFBoolean 0x1474f68 [0x1474b38]>{value = false}
...

对我来说这似乎是一个常规哈希。我尝试了objectForKey 和valueForKey，但它们都不起作用，即

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

当我这样做时

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

Answer 1

你可以使用：--

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

或

 id answer = [self.answers objectAtIndex:indexPath.row];
if (answer isKindOfClass:[NSDictionary class])
{
  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]];
    }

Answer 2

: 是方法名的一部分，所以你需要这样做：

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

然后使用objectForKey:，而不是valueForKey:。第一个是访问字典中的对象，后者是所谓的Key-Value Coding。所以它是：

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]];
}

最后但并非最不重要的一点是，您从 answer 字典中得到的对象似乎是 NSNumber，而不是 NSString。因此，您可能希望将文本的设置更改为：

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

Answer 3

其实stackoverflow的api会发回一个数字。也就是说answer_id键存储的值是字典中的NSNumber而不是NSString，你应该相应地对待它。