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，在我从事项目的几个月里，我学到了关于 Objective-C 的所有知识.我还发现，在我使用过的任何其他语言中，Objective-C 似乎使简单的事情变得复杂，这让我感到非常沮丧.这个问题就是一个例子……

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...

在第一次运行时，我的应用程序下载了一组 JSON，用于填充 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，有一个字段包含一个数字 ("show_id":2)，如果有一个或 null ("show_id":null) 否则.在处理该字段时，我将其分配给 NSNumber 使用...

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"];

然后，我在尝试获取 & 之前尝试检查我的号码是否有效链接相关记录，以免在没有相关记录的情况下进行浪费处理.

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.

用...询问 shoIndex

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

给...

shoIndex: 19590600, <null>, <null>

JSON 值为 null &...

where the JSON value was null &...

shoIndex: 228300880, 51, 51

否则.

到目前为止，我所做的唯一成功的检查是...

So far the only successful check I've made is with...

if (![[shoIndex description] isEqualToString:@"<null>"]) {

谁能提出更好的方法?

更新...

以另一种方式看待它 shoIndex 被分配为一个 NSNumber 有时包含一个 NSString 值 @"<null>".Obj-C 是否有类似 isa 运算符之类的东西，我可以使用它来检查 shoIndex 内容的类型?

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，佩德罗.

推荐答案

使用[shoObject class]获取对象的类；因此，要测试 shoObject 的类，您可以使用

Use [shoObject class] to get the class of an object; so, to test shoObject's class, you would use

[shoObject isKindOfClass:[NSString class]];

一旦您整理出定义空字符串或 NSNumber 的标记，您就可以创建一个宏.为此，我将 IsEmpty 宏保存在名为 CommonMacros.h 的文件中.代码如下:

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);
}

然后，导入CommonMacros.h后，可以这样调用函数:

Then, after importing CommonMacros.h, you can call the function like this:

if (IsEmpty(shotIndex)) {
    //do stuff
}

这应该可以解决这个问题，并且也适用于字符串、数组等，正如您从代码中看到的那样.感谢威尔·希普利！

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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