在包含@字符的键的NSDictionary中获取valueForKey时出错 [英] Error getting valueForKey in an NSDictionary for key containing @ character

问题描述

我试图在名为robotDesign的以下NSDictionary中获取@type键的值

  {
@id=2; 
@type=piggyDash; 
 turnAngle =-90; 
 width =90.0; 
} 
  

这是我使用的JSON对象的一部分

  [NSJSONSerialization JSONObjectWithData：options：error：]; 
  

我使用以下代码提取@type值

  NSString * robot_type =（NSString *）[robotDesign valueForKey：@@ type]; 
  

但回收以下错误：

由于未捕获异常而终止应用程序'NSUnknownKeyException'，
原因：'[< __ NSCFDictionary 0x71de9e0> valueForUndefinedKey：]：此
类不是键值编码符合键类型。

请注意，字典中的其他对象（如turnAngle和width）

解决方案

尝试

  NSString * robot_type = [robotDesign objectForKey：@@ type]; 
  

或移除领先的 @

valueForKey：

返回与给定键相关联的值。

- （id）valueForKey： （NSString *）键

参数键要为其返回相应值的键。注意，当使用键值编码时，
键必须是字符串（参见键值编码基础）。

返回值

与键相关联的值。

讨论如果键不以@开头，请调用objectForKey :.如果
键以@开头，则删除@并使用键的其余部分调用[super
valueForKey：]。

@ 导致调用超类的 valueForKey： NSDictionary。但NSObject不知道关键字类型的任何东西。

I am trying to get the value of "@type" key in the following NSDictionary named 'robotDesign'

robotDesign : {
    "@id" = "2";
    "@type" = "piggyDash";
    turnAngle = "-90";
    width = "90.0";
}

that is a part of a JSON object I got using

[NSJSONSerialization JSONObjectWithData: options: error:];

I am using following code to extract @type value

NSString * robot_type = (NSString*)[robotDesign valueForKey:@"@type"];

but recive following error:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSCFDictionary 0x71de9e0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key type.'

NOTE: Please note that other objects in dictionary like 'turnAngle' and 'width' are extracted easily using same code given above but with their respective keys.

解决方案

try

NSString * robot_type = [robotDesign objectForKey:@"@type"];

or remove the leading @

from the docs:

valueForKey:
Returns the value associated with a given key.

- (id)valueForKey:(NSString *)key
Parameters key The key for which to return the corresponding value. Note that when using key-value coding, the key must be a string (see "Key-Value Coding Fundamentals").

Return Value
The value associated with key.

Discussion If key does not start with "@", invokes objectForKey:. If key does start with "@", strips the "@" and invokes [super valueForKey:] with the rest of the key.

The @ leads to call the implementation of the valueForKey: of the superclass of NSDictionary. But NSObject does not know anything about the key type.

这篇关于在包含@字符的键的NSDictionary中获取valueForKey时出错的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！