如何检查NSNumber中的空值 [英] How to check for null value in NSNumber

问题描述

首先我承认我的无知，我已经了解了我在我的项目工作的几个月内我知道的关于Objective-C的一切。我也发现它完全令人沮丧的是如何Objective-C似乎使任何其他语言，我曾经与简单的事情复杂。这个问题是一个例子。

在第一次运行我的应用程序下载一堆JSON，它用来填充CoreData存储。我使用Obj-C / JSON库（CJSONDeserializer）将JSON转换为NSArrays。在一个CoreData实体的JSON下载中，有一个字段包含标识另一个实体中的相关字段的数字（show_id：2 ），如果有一个或空c $ c>show_id：null ）。在处理该字段时，使用...将其分配给NSNumber。

  NSNumber * shoIndex = [[object objectForKey：typeKey] objectForKey ：@show_id]; 
  

然后我尝试检查我有一个有效的数字，链接相关记录，以便在没有相关记录的情况下不进行浪费处理。

与 shoIndex 一起询问。 ..

  NSLog（@shoIndex：％i，％@，％@，shoIndex，shoIndex，[shoIndex description] ）; 
  

给予...

  shoIndex：19590600，< null>，< null> 
  

其中JSON值为 null ...

  shoIndex：228300880，51，51 
  

否则。

到目前为止，我唯一成功的检查是...

  if（！[[shoIndex description] isEqualToString：@< null>]）{
  
 
 
 
任何人都可以提出更好的方法？
 
 
 
更新... 
 
 
 
以另一种方式看待 shoIndex 被指定为 NSNumber ，有时包含 NSString  value  @< null>。 Obj-C有类似 isa 运算符，我可以用来检查 shoIndex 的内容类型？ 
 
 
 
 TIA，Pedro。
解决方案
使用 [shoObject class] 获取对象的类;因此，要测试 shoObject 的类，您将使用
  shoObject isKindOfClass：[NSString class]]; 
  
一旦你整理出了什么标记定义一个空字符串或NSNumber，你可以创建一个宏。我通过在名为CommonMacros.h的文件中保留一个IsEmpty宏来做这件事。以下是代码：
  //感谢Wil 
 //hilp://wilshipley.com/blog/2005/ 10 / pimp-my-code-interlude-free-code.html 
 
 static inline BOOL IsEmpty（id thing）{
 return thing == nil 
 || （[thing isEqual：[NSNull null]]）// JS添加为coredata 
 || （[something responsesToSelector：@selector（length）] 
&& [（NSData *）thing length] == 0）
 || （[something respondingToSelector：@selector（count）] 
&& [（NSArray *）thing count] == 0）; 
} 
  
然后，导入CommonMacros.h后，可以像下面这样调用函数： 
  if（IsEmpty（shotIndex））{
 // do stuff 
} 
  
这应该解决这个问题，并且也可以处理字符串，数组等， 。感谢Wil Shipley！
 
First off I confess my ignorance, I've learned everything I know about Objective-C in the few months I've been working on my project. I also find it utterly frustrating how Objective-C seems to complicate what would be simple matters in any other language I've worked with. This question is a case in point...

In the first run my app downloads a bunch of JSON which it uses to populate a CoreData store. I use an Obj-C/JSON library (CJSONDeserializer) to convert the JSON to NSArrays. In the JSON download for one CoreData entity there's a field containing a number ("show_id":2) identifying the related field in another entity if there is one or null ("show_id":null) otherwise. In processing that field I assign it to an NSNumber using...
NSNumber *shoIndex = [[item objectForKey:typeKey] objectForKey:@"show_id"];
I then try to check that I have a valid number before attempting to fetch & link the related record so as to not do wasteful processing where there is no related record.

Interrogating shoIndex with...
NSLog(@"shoIndex: %i, %@, %@", shoIndex, shoIndex, [shoIndex description]);
Gives...
shoIndex: 19590600, <null>, <null>
where the JSON value was null &...
shoIndex: 228300880, 51, 51
otherwise.

So far the only successful check I've made is with...
if (![[shoIndex description] isEqualToString:@"<null>"]) {
Can anyone suggest a better way?

Update...

Looking at it another way shoIndex is assigned as a NSNumber that sometimes contains a NSString value @"<null>". Does Obj-C have something like an isa operator that I could use to check the type of the contents of shoIndex?

TIA, Pedro.
解决方案
Use [shoObject class] to get the class of an object; so, to test shoObject's class, you would use
[shoObject isKindOfClass:[NSString class]];
Once you've sorted out what markers define an empty string or NSNumber, you can create a macro. I do with this by keeping an IsEmpty macro in a file called CommonMacros.h. Here's the code:
//Thanks Wil
//http://wilshipley.com/blog/2005/10/pimp-my-code-interlude-free-code.html

static inline BOOL IsEmpty(id thing) {
    return thing == nil
    || ([thing isEqual:[NSNull null]]) //JS addition for coredata
    || ([thing respondsToSelector:@selector(length)]
        && [(NSData *)thing length] == 0)
    || ([thing respondsToSelector:@selector(count)]
        && [(NSArray *)thing count] == 0);
}
Then, after importing CommonMacros.h, you can call the function like this:
if (IsEmpty(shotIndex)) {
    //do stuff
}
This should take care of this problem, and will also work on strings, arrays, etc, as you can see from the code. Thanks to Wil Shipley!

                        
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