how do I catch a NSRangeException?

I want my app to continue gracefully when the online server messes up. I tried to wrap the dangerous line in a @try block. Yet it is still crashing like so:

the method:

+ (NSArray *)findAllFor:(NSObject *)ratable {
      NSString *ratingsPath = [NSString stringWithFormat:@"%@%@/%@/%@%@",
     [self getRemoteSite],
     [ratable getRemoteCollectionName],
     [ratable getRemoteId],
     [self getRemoteCollectionName],
     [self getRemoteProtocolExtension]];

     Response *res = [ORConnection get:ratingsPath withUser:[[self class] getRemoteUser] 
     andPassword:[[self class] getRemotePassword]];
     NSArray *ratings;
     @try {
          ratings = [self fromXMLData:res.body];
     }
     @catch (NSException *e) {
          ratings = [NSArray array];
     }
    retu开发者_开发知识库rn ratings;
}

the stack trace:

Program received signal:  “SIGABRT”.
2010-08-07 16:38:51.846 TalkToHer[68608:7003] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[NSArray objectAtIndex:]: index 1 beyond bounds [0 .. 0]'
*** Call stack at first throw:  
(
 0   CoreFoundation                      0x02932919 __exceptionPreprocess + 185  
 1   libobjc.A.dylib                     0x02a805de objc_exception_throw + 47  
 2   CoreFoundation                      0x0292858c -[__NSArrayI objectAtIndex:] + 236  
 3   TalkToHer                           0x00009fa7 -[FromXMLElementDelegate parser:didEndElement:namespaceURI:qualifiedName:] + 425  
 4   Foundation                          0x0017bcc1 _endElementNs + 453  
 5   libxml2.2.dylib                     0x02d9deb6 xmlParseXMLDecl + 1353  
 6   libxml2.2.dylib                     0x02da8bc1 xmlParseChunk + 3985  
 7   Foundation                          0x0017b4c2 -[NSXMLParser parse] + 321  
 8   TalkToHer                           0x0000b14d +[NSObject(XMLSerializableSupport) fromXMLData:] + 201  
 9   TalkToHer                           0x00031a6c +[Rating findAllFor:] + 320  
 10  TalkToHer                           0x00032d67 -[FirstClassContentPiece(Ratable) updateRatings] + 96  
 11  TalkToHer                           0x00004d5f __-[InspirationController tableView:didSelectRowAtIndexPath:]_block_invoke_3 + 33  
 12  libSystem.B.dylib                   0x9792efe4 _dispatch_call_block_and_release + 16  
 13  libSystem.B.dylib                   0x97921a4c _dispatch_queue_drain + 249  
 14  libSystem.B.dylib                   0x979214a8 _dispatch_queue_invoke + 50  
 15  libSystem.B.dylib                   0x979212be _dispatch_worker_thread2 + 240  
 16  libSystem.B.dylib                   0x97920d41 _pthread_wqthread + 390  
 17  libSystem.B.dylib                   0x97920b86 start_wqthread + 30  
)
terminate called after throwing an instance of 'NSException'

Is my syntax for @try @catch wrong? I attempted to add a @catch block for NSRangeException but it seems that's not the right approach (it's not a class).

Also, the server error is caused by [ratable getRemoteId] sometimes returning (null) instead of an integer. This behavior seems pretty unpredictable; if anyone has a clue why ObjectiveResource might be doing that it would be helpful. But I still would like to know how to use @try @catch.

---

As I now understand it, throwing exceptions should only be done to alert users of your library that they have made a programming error. I am still curious why the syntax I used did not prevent the crash. I know the error was occurring several levels down; but the @try {} @catch {} block should handle all methods called by the methods I call...

At any rate, here is the fixed code, for anyone who wants to fetch scoped objects from a Rails-style restful resource.

+ (NSArray *)findAllFor:(NSObject *)ratable {
    NSString *ratingsPath = [NSString stringWithFormat:@"%@%@/%@/%@%@",
                          [self getRemoteSite],
                          [ratable getRemoteCollectionName],
                          [ratable getRemoteId],
                          [self getRemoteCollectionName],
                          [self getRemoteProtocolExtension]];

    Response *res = [ORConnection get:ratingsPath withUser:[[self class] getRemoteUser] 
                      andPassword:[[self class] getRemotePassword]];
    NSError **aError;
    if([res isError]) {
        *aError = res.error;
        return nil;
    }
    else {
        return [self performSelector:[self getRemoteParseDataMethod] withObject:res.body];
    }
}

---

You don't. You fix your code to not throw an exception under these circumstances.