CODEWITHSUNDEEP
objective-ccocoanserror
Stunner
Stunner

Reputation: 12194

How to Determine if Error Occured in this Case (Objective-C)

Hey guys, I have this function:

//retrieves checksum within core data store, returns 0 if a checksum has not been stored
-(double)getStoredChecksum {
    NSError *error;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription 
                                   entityForName:@"DataFeedManager" inManagedObjectContext:[self managedObjectContext]];
    [fetchRequest setEntity:entity];
    NSArray *fetchedObjects = [[self managedObjectContext] executeFetchRequest:fetchRequest error:&error];
    //+for (NSManagedObject* item in fetchedObjects) //REMOVE
        //+[[self managedObjectContext] deleteObject:item]; //REMOVE FOR CLEARING CHECKSUM CONTENTS

    if ([fetchedObjects count] == 0){ //array is empty
        NSLog(@"FETCHED OBJECTS IS EMPTY");
        checksumStored = NO;
        return 0;
    }
    if (fetchedObjects == nil) {
        NSLog(@"Error while fetching\n%@",
              ([error localizedDescription] != nil) ? [error localizedDescription] : @"Unknown Error");
    }
    if ([fetchedObjects count] > 1)
        NSLog(@"====BAD:Multiple Checksums Retrieved!!!!====");
    checksumStored = YES;
    double returnable = [[[fetchedObjects objectAtIndex:0] valueForKey:@"lastUpdateCheckSum"] doubleValue];
    return returnable;
}//getStoredChecksum()

As you can see, it executes a fetch request, however I am unsure of how to check if an error occurred. As you can see, I am checking to see if the array is empty via [fetchedObjects count] == 0 however, I heard somewhere that if the error occurs and the array is nil, 0 is returned for count as well, thus it is safe to test for array emptiness that way, but it masks my check for a null array. Thus the problem is that I have no definitive way of checking if an error occurred. Any thoughts on how I should go about doing this?

Thanks in advance!

EDIT:

How would I check for an error in this case?

NSUInteger numEntities = [[self managedObjectContext] countForFetchRequest:fetchRequest error:&error];

As NSUInteger is specified as an unsigned int, so I can't even assign it a value of -1 and check for a -1 later if I wanted to.

Upvotes: 1

Views: 698

Answers (2)

freespace
freespace

Reputation: 16701

Set error to nil before calling executeFetchRequest::. If error is not nil after then it was set and thus an error occured.

The reason the count check is masking the nil check is because any message sent to nil returns nil which in this case is also 0. This can be easily fixed by checking for nil first, or modify your if statement so it is as followed:

if (fetchedObjects && [fetchedObjects count] == 0){

For the case with countForFetchRequest:: NSNotFound is returned when an error occurs, so just check for that.

Upvotes: 2

Sherm Pendley
Sherm Pendley

Reputation: 13612

Copy & pasted from the docs for countForFetchRequest:error:

Return Value

The number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:, or NSNotFound if an error occurs.

So,

if (numEntities == NSNotFound) {
    // Handle the error
}

Upvotes: 2

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