ios -[NSNull 长度]：无法识别的选择器发送到 JSON 对象
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-[NSNull length]: unrecognized selector sent to JSON objects
提问by VansFannel
I'm developing an iOS 5.0+ app with latest SDK.
我正在使用最新的 SDK 开发 iOS 5.0+ 应用程序。
I get a very strange error with this code:
这段代码出现了一个非常奇怪的错误：
- (NSMutableURLRequest*)setupRequestWithService:(NSString*)service andMethod:(NSString*)method
{
NSString* url = [NSString stringWithFormat:@"%@%@.svc/%@", serverUrl, service, method];
NSMutableURLRequest* request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
// Set authentication token.
NSLog(@"???????????? %@", authenticationToken);
if (authenticationToken == nil)
NSLog(@"NULL AUTHTOKEN");
if ([authenticationToken isEqual:[NSNull null]])
NSLog(@"NSNULL AUTHTOKEN");
if (request == nil)
NSLog(@"NULL REQUEST");
[request addValue:authenticationToken forHTTPHeaderField:REQUEST_HEADER_AUTH_TOKEN];
return request;
}
This is my log:
这是我的日志：
???????????? <null>
NSNULL AUTHTOKEN
-[NSNull length]: unrecognized selector sent to instance 0x3b5a5090
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance 0x3b5a5090'
It seems that authenticationTokenis NULL. But I don't understand that, if authenticationTokenis NULLwhy I don't see NULL AUTHTOKENon the log.
看来authenticationToken是NULL。但我不明白，如果authenticationToken这NULL就是我没有NULL AUTHTOKEN在日志上看到的原因。
I get this error the second time I run that method, the first time, I don't get any error. This is my log:
我第二次运行该方法时出现此错误，第一次没有出现任何错误。这是我的日志：
???????????? (null)
NULL AUTHTOKEN
By the way:
顺便一提：
NSString* authenticationToken;
Any advice?
有什么建议吗？
Maybe there is a Memory Leaksomewhere...
也许Memory Leak某处...
回答by David H
My solution to this maddening use of NSNull by JSON interpreters is to create a category on NSNull, where I define integerValue, floatValue, length, etc - return 0 for all. Everytime you get another crash add a new category. I think I had 6 or 7 when I had this issue.
我对 JSON 解释器对 NSNull 的这种疯狂使用的解决方案是在 NSNull 上创建一个类别，我在其中定义 integerValue、floatValue、length 等 - 全部返回 0。每次遇到另一次崩溃时，请添加一个新类别。当我遇到这个问题时，我想我有 6 或 7 个。
The problem with NOT doing this is you have to look for the NULL everywhere in your converted objects - a PITA in my opinion.
不这样做的问题是您必须在转换后的对象中到处寻找 NULL - 在我看来是 PITA。
EDIT: the code I'm using, all in a NSNull+JSON.m file:
编辑：我正在使用的代码，都在一个 NSNull+JSON.m 文件中：
@interface NSNull (JSON)
@end
@implementation NSNull (JSON)
- (NSUInteger)length { return 0; }
- (NSInteger)integerValue { return 0; };
- (float)floatValue { return 0; };
- (NSString *)description { return @"0(NSNull)"; }
- (NSArray *)componentsSeparatedByString:(NSString *)separator { return @[]; }
- (id)objectForKey:(id)key { return nil; }
- (BOOL)boolValue { return NO; }
@end
EDIT2: Now in Swift 3:
EDIT2：现在在 Swift 3 中：
extension NSNull {
func length() -> Int { return 0 }
func integerValue() -> Int { return 0 }
func floatValue() -> Float { return 0 };
open override var description: String { return "0(NSNull)" }
func componentsSeparatedByString(separator: String) -> [AnyObject] { return [AnyObject]() }
func objectForKey(key: AnyObject) -> AnyObject? { return nil }
func boolValue() -> Bool { return false }
}
回答by Mar0ux
The error message is pretty clear. NSNulland nilare different things:
错误信息非常清楚。NSNull并且nil是不同的事情：
The NSNull class defines a singleton object used to represent null values in
collection objects (which don't allow nil values).
If you want to check if authenticationTokenis NSNulltry: [authenticationToken isEqual: [NSNull null]]
如果你想检查是否authenticationToken是NSNull尝试：[authenticationToken isEqual: [NSNull null]]
回答by Eric Baker
In line with David H's answer, how about a category on NSNullthat just uses ObjC's message forwarding to "do nothing", to emulate the runtime's behavior when sending messages to nil?
与 David H 的回答一致，NSNull仅使用 ObjC 的消息转发“什么都不做”的类别如何在向 发送消息时模拟运行时的行为nil？
Like this:
像这样：
@interface NSNull (ForwardInvocation)
@end
@implementation NSNull (ForwardInvocation)
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
return [NSNull methodSignatureForSelector:@selector(description)];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
// do nothing; prevent 'unrecognized selector' crashes
}
@end
The [NSNull methodSignatureForSelector:@selector(description)];takes advantage of the fact that NSNullinherits from NSObject, which provides the descriptionmethod. This satisfies the forwarding mechanism requirement for implementing -methodSignatureForSelector:.
的[NSNull methodSignatureForSelector:@selector(description)];利用NSNull继承自的事实NSObject，它提供了description方法。这满足了实现的转发机制要求-methodSignatureForSelector:。
回答by Daniel C.
The problem comes because your method return an NSNull object.
You can't check [authenticationToken isEqual:[NSNull null]])because [NSNull null]give an instance of an object. So it's different from your object itself.
If you want to check if you received an NSNull object you need to check like this: [authenticationToken isKindOfClass:[NSNull class]]instead.
问题出现是因为您的方法返回一个 NSNull 对象。你不能检查，[authenticationToken isEqual:[NSNull null]])因为[NSNull null]给了一个对象的实例。所以它与你的对象本身不同。如果你想检查你是否收到了一个 NSNull 对象，你需要像这样检查：[authenticationToken isKindOfClass:[NSNull class]]相反。
