为什么NSDictionary在它的键中有@符号时会抓狂呢?(复制)

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Error getting valueForKey in an NSDictionary for key containing @ character 2 answers
在NSDictionary中获取包含@字符2答案的键的valueForKey时出错

I am having trouble with RestKit when I had a JSON response with @ symbols in the keys. After some debugging it seems the issue is happening in __NSCFDictionary

当我有一个带有@符号的JSON响应时，我在RestKit上遇到了麻烦。经过一些调试之后，问题似乎正在__NSCFDictionary中发生

So I tried the following simple code:

所以我尝试了以下简单的代码:

NSArray *keys = [NSArray arrayWithObjects:@"@key1", @"@key2", nil];
NSArray *objects = [NSArray arrayWithObjects:@"value1", @"value2", nil];
NSDictionary *dictionary = [NSDictionary dictionaryWithObjects:objects
                                                   forKeys:keys];
for (id key in dictionary) {
    NSLog(@"key: %@, value: %@", key, [dictionary valueForKey:key]);
}

And I am getting the following error:

我得到的错误如下:

[<__NSDictionaryI 0x618000268ac0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key key1.

[<__NSDictionaryI 0x618000268ac0> valueForUndefinedKey:]:这个类不支持键key1的键值编码。

Can someone please explain why I am getting this error and if there is any workaround?

有人能解释一下我为什么会犯这个错误吗?

2 个解决方案

#1

You can't use @ in the keys in conjunction with valueForKey:. NSDictionary has some documented but perhaps unexpected behavior in that case: it strips the @ and invokes [super valueForKey:] with the new key. That looks for the key on the object, not in the dictionary's contents. No such key exists on instances of NSDictionary, so an exception is raised.

你不能在键中使用@与valueForKey:。在这种情况下，NSDictionary有一些记录在案但可能是意外的行为:它去掉@并使用新键调用[super valueForKey:]。查找对象上的键，而不是字典的内容。在NSDictionary的实例中不存在这样的键，因此会引发异常。

You should in general use objectForKey: to retrieve values from an NSDictionary.

您通常应该使用objectForKey:从NSDictionary中检索值。

Credit must go to Ken Thomases for his comments below, correcting earlier versions of this answer.

下面的评论必须归功于肯·托马斯，他纠正了这个答案的早期版本。

#2

Because you copied the code of someone who uses valueForKey: instead of objectForKey: without understanding what each is good for.

因为您复制了使用valueForKey:而不是objectForKey的人的代码:而不理解它们的好处。

objectForKey: is what you should use. It takes the key that you pass in, and looks up the value for that key.

objectForKey:是您应该使用的。它使用你传入的键，并查找该键的值。

valueForKey: is a highly complicated method that looks at the text in the key, and does all kind of complicated things, for example a key of @"@sum" will add up some values. If that is what you want, fine. If you want to look up a value, use objectForKey. It does what you think it should do, and is usually several times faster than valueForKey: anyway.

valueForKey:是一个非常复杂的方法，它查看键中的文本，并执行各种复杂的操作，例如@"@ "@sum"的键将加一些值。如果这是你想要的，没问题。如果您想查找一个值，请使用objectForKey。它做你认为它应该做的事，而且通常比valueForKey快几倍。

As a rule of thumb: If I asked you whether you should use objectForKey: or valueForKey:, and you don't know what the difference is, use objectForKey:

作为一个经验法则:如果我问你是否应该使用objectForKey:或valueForKey:，而你不知道两者之间的区别是什么，使用objectForKey:

#1