blocks|key|285807|text|<null>是NSNull单例日志的方式。所以：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|285808|if+(tel+==+(id)[NSNull+null])+{
++++//+tel+is+null
}|code-block|syntax|javascript|285809|(单例的存在是因为您不能将nil添加到集合类。)|285810|entityMap^0|0|6|0|0|D|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

<code>&lt;null&gt;</code> is how the NSNull singleton logs. So:

<pre><code>if (tel == (id)[NSNull null]) {
 // tel is null
}
</code></pre>

(The singleton exists because you can't add <code>nil</code> to collection classes.)

blocks|key|2993947|text|以下是强制转换的示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2993948|if+(tel+==+(NSString+*)[NSNull+null])
{
+++//+do+logic+here
}|code-block|syntax|javascript|2993949|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here is the example with the cast:

<pre><code>if (tel == (NSString *)[NSNull null])
{
 // do logic here
}
</code></pre>

blocks|key|283762|text|在这里，你也可以通过检查字符串的长度来做到这一点。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|283763|if(tel.length==0)
{
++++//do+some+logic+here
}|code-block|syntax|javascript|283764|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Here you can also do that by checking the length of the string i.e.

<pre><code>if(tel.length==0)
{
 //do some logic here
}
</code></pre>

blocks|key|283771|text|你也可以像这样检查传入的字符串：-|type|unstyled|depth|inlineStyleRanges|entityRanges|data|283772|if(tel==(id)+[NSNull+null]+%7C%7C+[tel+length]==0+%7C%7C+[tel+isEqualToString:@""])
{
++++NSlog(@"Print+check+log");
}
else
{++

++++NSlog(@Printcheck+log+%25@",tel);++

}|code-block|syntax|javascript|283773|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

you can also check this Incoming String like this way also:-

<pre><code>if(tel==(id) [NSNull null] || [tel length]==0 || [tel isEqualToString:@""])
{
 NSlog(@"Print check log");
}
else
{ 

 NSlog(@Printcheck log %@",tel); 

}
</code></pre>

blocks|key|280173|text|最好是坚持最佳实践--即使用真实的数据模型来读取JSON数据。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|280174|看看JSONModel+-它很容易使用，它会自动将NSNUll+null转换为*+nil+*值，所以你可以像往常一样在Obj-c中执行检查，如下所示：|offset|length|style|BOLD|280175|if+(mymodel.Telephone==nil)+{
++//telephone+number+was+not+provided,+do+something+here+
}|code-block|syntax|javascript|280176|看看JSONModel的页面：http://www.jsonmodel.com|280177|下面是创建基于JSON的应用程序的简单演练：http://www.touch-code-magazine.com/how-to-make-a-youtube-app-using-mgbox-and-jsonmodel/|280178|entityMap|0|LINK|mutability|MUTABLE|url|http://www.jsonmodel.com/|1|http://www.touch-code-magazine.com/how-to-make-a-youtube-app-using-mgbox-and-jsonmodel/^0|0|15|3|0|0|F|O|0|0|M|2F|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@$D|12|E|13|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|14|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|15|8|@]|9|@$D|16|E|17|1|18]]|A|$]]|$1|O|3|P|5|6|7|19|8|@]|9|@$D|1A|E|1B|1|1C]]|A|$]]|$1|Q|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]|Y|$5|T|U|V|A|$W|Z]]]]

Best would be if you stick to best practices - i.e. use a real data model to read JSON data. 

Have a look at JSONModel - it's easy to use and it will convert [NSNUll null] to * nil * values for you automatically, so you could do your checks as usual in Obj-c like:

<pre><code>if (mymodel.Telephone==nil) {
 //telephone number was not provided, do something here 
}
</code></pre>

Have a look at JSONModel's page: <a href="http://www.jsonmodel.com" rel="nofollow">http://www.jsonmodel.com</a>

Here's also a simple walk-through for creating a JSON based app: <a href="http://www.touch-code-magazine.com/how-to-make-a-youtube-app-using-mgbox-and-jsonmodel/" rel="nofollow">http://www.touch-code-magazine.com/how-to-make-a-youtube-app-using-mgbox-and-jsonmodel/</a>

blocks|key|283784|text|如果您正在处理一个“不稳定”的API，您可能希望遍历所有键以检查是否为空。我创建了一个类别来处理这个问题：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|283785|@interface+NSDictionary+(Safe)
-(NSDictionary+*)removeNullValues;
@end

@implementation+NSDictionary+(Safe)

-(NSDictionary+*)removeNullValues
{
++++NSMutableDictionary+*mutDictionary+=+[self+mutableCopy];
++++NSMutableArray+*keysToDelete+=+[NSMutableArray+array];
++++[mutDictionary+enumerateKeysAndObjectsUsingBlock:%5E(id+key,+id+obj,+BOOL+*stop)+{
++++++++if+(obj+==+[NSNull+null])+
++++++++{
++++++++++++[keysToDelete+addObject:key];
++++++++}
++++}];
++++[mutDictinary+removeObjectsForKeys:keysToDelete];
++++return+[mutDictinary+copy];
}
@end|code-block|syntax|javascript|283786|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you are dealing with an "unstable" API, you may want to iterate through all the keys to check for null. I created a category to deal with this:

<pre><code>@interface NSDictionary (Safe)
-(NSDictionary *)removeNullValues;
@end

@implementation NSDictionary (Safe)

-(NSDictionary *)removeNullValues
{
 NSMutableDictionary *mutDictionary = [self mutableCopy];
 NSMutableArray *keysToDelete = [NSMutableArray array];
 [mutDictionary enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
 if (obj == [NSNull null]) 
 {
 [keysToDelete addObject:key];
 }
 }];
 [mutDictinary removeObjectsForKeys:keysToDelete];
 return [mutDictinary copy];
}
@end
</code></pre>

blocks|key|283801|text|如果您在json中有许多属性，使用if语句逐个检查它们是很麻烦的。更糟糕的是，代码会很难看，很难维护。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|283802|我认为更好的方法是创建一个NSDictionary类别|283803|//+NSDictionary%2BAwesomeDictionary.h

#import+<Foundation/Foundation.h>

@interface+NSDictionary+(AwesomeDictionary)
-+(id)validatedValueForKey:(NSString+*)key;
@end

//+NSDictionary%2BAwesomeDictionary.m

#import+"NSDictionary%2BAwesomeDictionary.h"

@implementation+NSDictionary+(AwesomeDictionary)
-+(id)validatedValueForKey:(NSString+*)key+{
++++id+value+=+[self+valueForKey:key];
++++if+(value+==+[NSNull+null])+{
++++++++value+=+nil;
++++}
++++return+value;
}
@end|code-block|syntax|javascript|283804|导入此类别后，您可以：|283805|[json+validatedValueForKey:key];|283806|entityMap^0|H|2|0|D|C|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|R|$]]

if you have many attributes in json, using <code>if</code> statement to check them one by one is troublesome. What even worse is that the code would be ugly and hard to maintain.

I think the better approach is creating a category of <code>NSDictionary</code>:

<pre><code>// NSDictionary+AwesomeDictionary.h

#import &lt;Foundation/Foundation.h&gt;

@interface NSDictionary (AwesomeDictionary)
- (id)validatedValueForKey:(NSString *)key;
@end

// NSDictionary+AwesomeDictionary.m

#import "NSDictionary+AwesomeDictionary.h"

@implementation NSDictionary (AwesomeDictionary)
- (id)validatedValueForKey:(NSString *)key {
 id value = [self valueForKey:key];
 if (value == [NSNull null]) {
 value = nil;
 }
 return value;
}
@end
</code></pre>

after importing this category, you can:

<pre><code>[json validatedValueForKey:key];
</code></pre>

blocks|key|282287|text|最好的答案是Aaron+Hayman在公认的答案下面所评论的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|282288|if+([tel+isKindOfClass:[NSNull+class]])|code-block|syntax|javascript|282289|它不会产生警告:)|282290|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The best answer is what Aaron Hayman has commented below the accepted answer:

<pre><code>if ([tel isKindOfClass:[NSNull class]])
</code></pre>

It doesn't produce a warning :)

blocks|key|2994060|text|我通常这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2994061|Assuma+I有一个用于用户的数据模型，它有一个名为email的NSString属性，该属性取自JSON+dict。如果在应用程序内部使用电子邮件字段，则将其转换为空字符串可防止可能的崩溃：|2994062|-+(id)initWithJSONDictionary:(NSDictionary+*)dictionary{

++++//Initializer,+other+properties+etc...

++++id+usersmail+=+[[dictionary+objectForKey:@"email"]+copy];
++++_email+=+(+usersmail+&&+usersmail+!=+(id)[NSNull+null]+)?+[usersmail+copy]+:+[[NSString++++++alloc]initWithString:@""];
}|code-block|syntax|javascript|2994063|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I usually do it like this:

Assuma I have a data model for the user, and it has an NSString property called email, fetched from a JSON dict. If the email field is used inside the application, converting it to empty string prevents possible crashes:

<pre><code>- (id)initWithJSONDictionary:(NSDictionary *)dictionary{

 //Initializer, other properties etc...

 id usersmail = [[dictionary objectForKey:@"email"] copy];
 _email = ( usersmail &amp;&amp; usersmail != (id)[NSNull null] )? [usersmail copy] : [[NSString alloc]initWithString:@""];
}
</code></pre>

blocks|key|282298|text|试试这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|282299|if+(tel+==+(NSString+*)[NSNull+null]+%7C%7C+tel.length==0)
{
++++//+do+logic+here
}|code-block|syntax|javascript|282300|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Try this:

<pre><code>if (tel == (NSString *)[NSNull null] || tel.length==0)
{
 // do logic here
}
</code></pre>

blocks|key|2994069|text|在Swift中，您可以执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2994070|let+value:+AnyObject?+=+xyz.objectForKey("xyz")++++
if+value+as+NSObject+==+NSNull()+{
++++//+value+is+null
++++}|code-block|syntax|javascript|2994071|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

In Swift you can do:

<pre><code>let value: AnyObject? = xyz.objectForKey("xyz") 
if value as NSObject == NSNull() {
 // value is null
 }
</code></pre>

blocks|key|282312|text|我尝试了很多方法，但都不管用。最后，这对我来说是有效的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|282313|NSString+*usernameValue+=+[NSString+stringWithFormat:@"%25@",[[NSUserDefaults+standardUserDefaults]+valueForKey:@"usernameKey"]];

if+([usernameValue+isEqual:@"(null)"])
{
+++++//+str+is+null
}|code-block|syntax|javascript|282314|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I tried a lot method, but nothing worked.
Finally this worked for me.

<pre><code>NSString *usernameValue = [NSString stringWithFormat:@"%@",[[NSUserDefaults standardUserDefaults] valueForKey:@"usernameKey"]];

if ([usernameValue isEqual:@"(null)"])
{
 // str is null
}
</code></pre>

blocks|key|3207615|text|if([tel+isEqual:[NSNull+null]])
{
+++//do+something+when+value+is+null
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3207616|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>if([tel isEqual:[NSNull null]])
{
 //do something when value is null
}
</code></pre>

blocks|key|285910|text|我使用的是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|285911|#define+NULL_TO_NIL(obj)+({+__typeof__+(obj)+__obj+=+(obj);+__obj+==+[NSNull+null]+?+nil+:+obj;+})+|code-block|syntax|javascript|285912|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I use this:

<pre><code>#define NULL_TO_NIL(obj) ({ __typeof__ (obj) __obj = (obj); __obj == [NSNull null] ? nil : obj; }) 
</code></pre>

blocks|key|282373|text|如果我们像这样得到空值，那么我们可以用下面的代码片段来检查它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|282374|+if(![[dictTripData+objectForKey:@"mob_no"]+isKindOfClass:[NSNull+class]])
++++++strPsngrMobileNo+=+[dictTripData+objectForKey:@"mobile_number"];
++else
+++++++++++strPsngrMobileNo+=+@"";|code-block|syntax|javascript|282375|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

if we are getting null value like then we can check it with below code snippet.

<pre><code> if(![[dictTripData objectForKey:@"mob_no"] isKindOfClass:[NSNull class]])
 strPsngrMobileNo = [dictTripData objectForKey:@"mobile_number"];
 else
 strPsngrMobileNo = @"";
</code></pre>

I have a JSON object that is coming from a webserver.

The log is something like this:

<pre><code>{ 
 "status":"success",
 "UserID":15,
 "Name":"John",
 "DisplayName":"John",
 "Surname":"Smith",
 "Email":"email",
 "Telephone":null,
 "FullAccount":"true"
}
</code></pre>

Note the Telephone is coming in as null if the user doesn't enter one.

When assigning this value to a <code>NSString</code>, in the <code>NSLog</code> it's coming out as <code>&lt;null&gt;</code>

I am assigning the string like this:

<pre><code>NSString *tel = [jsonDictionary valueForKey:@"Telephone"];
</code></pre>

What is the correct way to check this <code>&lt;null&gt;</code> value? It's preventing me from saving a <code>NSDictionary</code>.

I have tried using the conditions <code>[myString length]</code> and <code>myString == nil</code> and <code>myString == NULL</code>

Additionally where is the best place in the iOS documentation to read up on this?

Checking a null value in Objective-C that has been returned from a JSON string

服务器

Swift

JSON

我有一个来自from服务器的JSON对象。日志如下所示：{             "status":"success",   "UserID":15,   "Name":"John",   "DisplayName":"John",   "Surname":"Smith",   "Email":"email",   ...

问在Objective-C中检查从JSON字符串返回的空值
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