how do I catch a NSRangeException?

Question

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];
 }
    return 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.

Answer 1

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

Answer 2

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];
    }
}