[__NSArrayM objectForKeyedSubscript:]: unrecognized selector sent to instance - source code and screenshot attached

Question

At GitHub I have a simple iPhone app, which fetches user information (by using OAuth) from the social network Mail.ru:

It does fetch and print the information, but then crashes.

As an iOS programming newbie I'm confused by the output below (also please see the full output at PasteBin):

2014-01-21 21:21:10.873 oauthMailru[8228:3307] -[__NSArrayM objectForKeyedSubscript:]: unrecognized selector sent to instance 0x8a97290
2014-01-21 21:21:10.875 oauthMailru[8228:3307] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayM objectForKeyedSubscript:]: unrecognized selector sent to instance 0x8a97290'
*** First throw call stack:
(
    0   CoreFoundation                      0x01aa65e4 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x018298b6 objc_exception_throw + 44
    2   CoreFoundation                      0x01b43903 -[NSObject(NSObject) doesNotRecognizeSelector:] + 275
    3   CoreFoundation                      0x01a9690b ___forwarding___ + 1019
    4   CoreFoundation                      0x01a964ee _CF_forwarding_prep_0 + 14
    5   oauthMailru                         0x00003a62 __47-[ViewController fetchMailruWithToken:ForUser:]_block_invoke + 402
    6   Foundation                          0x01545695 __67+[NSURLConnection sendAsynchronousRequest:queue:completionHandler:]_block_invoke_2 + 151
    7   Foundation                          0x014a5945 -[NSBlockOperation main] + 88
    8   Foundation                          0x014fe829 -[__NSOperationInternal _start:] + 671
    9   Foundation                          0x0147b558 -[NSOperation start] + 83
    10  Foundation                          0x01500af4 __NSOQSchedule_f + 62
    11  libdispatch.dylib                   0x021344b0 _dispatch_client_callout + 14
    12  libdispatch.dylib                   0x02121018 _dispatch_async_redirect_invoke + 202
    13  libdispatch.dylib                   0x021344b0 _dispatch_client_callout + 14
    14  libdispatch.dylib                   0x02122eeb _dispatch_root_queue_drain + 287
    15  libdispatch.dylib                   0x02123137 _dispatch_worker_thread2 + 39
    16  libsystem_pthread.dylib             0x024c0dab _pthread_wqthread + 336
    17  libsystem_pthread.dylib             0x024c4cce start_wqthread + 30
)
libc++abi.dylib: terminating with uncaught exception of type NSException

Does anybody please know what's happening and how to read such crashes?

My source files: ViewController.m, which displays UIWebView and then segues to DetailViewController.m

Answer 1

Your code thinks the JSON deserializes to an object (dictionary), but in fact it deserializes to an array containing one object. Try this:

 NSMutableArray *topLevelArray = [NSJSONSerialization JSONObjectWithData:data
     options:NSJSONReadingMutableContainers error:nil];
 NSDictionary *dict = topLevelArray[0];

If you want to check what you're getting, you can use isKindOfClass: like this:

id jso = [NSJSONSerialization JSONObjectWithData:data
    options:NSJSONReadingMutableContainers error:nil];
if (jso == nil) {
    // Error.  You should probably have passed an NSError ** as the error
    // argument so you could log it.
} else if ([jso isKindOfClass:[NSArray class]]) {
    NSArray *array = jso;
    // process array elements
} else if ([jso isKindOfClass:[NSDictionary class]]) {
    NSDictionary *dict = jso;
    // process dictionary elements
} else {
    // Shouldn't happen unless you use the NSJSONReadingAllowFragments flag.
}