Parsing JSON with NSJSONSerialization - NSException thrown

Question

I'm using NSJSONSerialization to parse JSON pulled down from a PHP page. The following code is supposed to parse the JSON and show the value of the data field (JSON sample included first):

{"status":200,"status_message":"Book Found","data":348}

code:

- (IBAction)btnClick:(id)sender
{
    if(![_txtBox.text isEqual:@""])
    {
        NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"http://www.mysite.comm/project/ws/name/%@",@"c"]]; //url for JSON

        NSError *jsonError=nil;

        NSString *jsonString = [self jsonStringWithUrl:url]; //get data as JSON

        NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding]; //convert to data

        NSDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:jsonData options:kNilOptions error: &jsonError];

        _txtView.text = (NSString *)[jsonObject objectForKey:@"data"];
    }
}

However, the code crashes and I get the following error messages. I suspect it has something to do with this line: _txtView.text = (NSString *)[jsonObject objectForKey:@"data"]; but I'm not really sure what to look for. Am I trying to do get the data from the NSDictionary the wrong way?

2014-03-23 22:04:35.292 WebServiceTest[38975:60b] -[__NSCFNumber length]: unrecognized selector sent to instance 0x8d7f140
2014-03-23 22:04:35.296 WebServiceTest[38975:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber length]: unrecognized selector sent to instance 0x8d7f140'
*** First throw call stack:
(
    0   CoreFoundation                      0x017ec1e4 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x0156b8e5 objc_exception_throw + 44
    2   CoreFoundation                      0x01889243 -[NSObject(NSObject) doesNotRecognizeSelector:] + 275
    3   CoreFoundation                      0x017dc50b ___forwarding___ + 1019
    4   CoreFoundation                      0x017dc0ee _CF_forwarding_prep_0 + 14
    5   Foundation                          0x011894e4 -[NSConcreteAttributedString length] + 42
    6   Foundation                          0x01188a6c -[NSConcreteAttributedString initWithString:attributes:] + 182
    7   UIKit                               0x00939e9d -[UITextView setText:] + 125
    8   WebServiceTest                      0x00002922 -[WebServiceTestViewController btnClick:] + 610
    9   libobjc.A.dylib                     0x0157d880 -[NSObject performSelector:withObject:withObject:] + 77
    10  UIKit                               0x0022d3b9 -[UIApplication sendAction:to:from:forEvent:] + 108
    11  UIKit                               0x0022d345 -[UIApplication sendAction:toTarget:fromSender:forEvent:] + 61
    12  UIKit                               0x0032ebd1 -[UIControl sendAction:to:forEvent:] + 66
    13  UIKit                               0x0032efc6 -[UIControl _sendActionsForEvents:withEvent:] + 577
    14  UIKit                               0x0032e243 -[UIControl touchesEnded:withEvent:] + 641
    15  UIKit                               0x0026cddd -[UIWindow _sendTouchesForEvent:] + 852
    16  UIKit                               0x0026d9d1 -[UIWindow sendEvent:] + 1117
    17  UIKit                               0x0023f5f2 -[UIApplication sendEvent:] + 242
    18  UIKit                               0x00229353 _UIApplicationHandleEventQueue + 11455
    19  CoreFoundation                      0x0177577f __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
    20  CoreFoundation                      0x0177510b __CFRunLoopDoSources0 + 235
    21  CoreFoundation                      0x017921ae __CFRunLoopRun + 910
    22  CoreFoundation                      0x017919d3 CFRunLoopRunSpecific + 467
    23  CoreFoundation                      0x017917eb CFRunLoopRunInMode + 123
    24  GraphicsServices                    0x037e05ee GSEventRunModal + 192
    25  GraphicsServices                    0x037e042b GSEventRun + 104
    26  UIKit                               0x0022bf9b UIApplicationMain + 1225
    27  WebServiceTest                      0x000022fd main + 141
    28  libdyld.dylib                       0x01e33701 start + 1
    29  ???                                 0x00000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)

`[__NSCFNumber length]: unrecognized selector sent to instance` -- You've got a NSNumber you're trying to treat like an NSString. It appears that your "data" attribute is an NSNumber. — Hot Licks, Mar 24 '14 at 02:23
Actually I did include the JSON at the top of the code as a comment. Sorry for the confusion! — muttley91, Mar 24 '14 at 02:24

score 1 · Accepted Answer · answered Mar 24 '14 at 02:20

1

The JSON object you've shown has 348 listed as an integer value. So when NSJSONSerialization gets its hands on it, it converts it to an NSNumber. You'll need to call -[NSNumber stringValue] on [jsonObject objectForKey:@"data"] to convert the NSNumber to an NSString.

answered Mar 24 '14 at 02:20

Eagerod

811
9
11

This fixed my problem. However, the following JSON: `{"status":200,"status_message":"Book Found","data":"729"}` gives me an error of: `'-[__NSCFString stringValue]: unrecognized selector sent to instance 0x1702286a0'` Why is it using `__NSCFString` ? – muttley91 Mar 24 '14 at 03:09
In that case, the "data" element's value is wrapped in quotes. It's a string. If your JSON objects aren't behaving well, and you can sometimes get a number, and sometimes a string, you'll probably want to wrap your label text assignment around an `if ( [[jsonObject objectForKey:@"data"] isKindOfClass:[NSString class]] ) {} else if ( [[jsonObject objectForKey:@"data"] isKindOfClass:[NSNumber class]] ) {}` kind of construct to make sure that you're always calling the right methods on the right objects, depending on what the server/datasource is giving you. – Eagerod Mar 24 '14 at 03:13
That's what I ended up doing. It's a bit strange, but it gets the job done. Thanks! – muttley91 Mar 24 '14 at 03:14

Parsing JSON with NSJSONSerialization - NSException thrown

1 Answers1