dataWithContentsOfURL return value not nil when URL empty

Question

Hi everybody,

i have a problem using dataWithContentsOfURL. I'm loading some images from the internet in a loop. The Problem is: if the image at the URL doesn't exist, dataWithContentsOfURL does NOT return nil as expected. The apple reference says it returns nil if NSData could not be created. Here's the Code:

NSString *TermineImgFileName = nil;
NSString *TermineImgPath = nil;
NSURL *TermineImgURL = nil;
NSData *TermineImg = nil;   
for (deviceTermineHighInt; deviceTermineHighInt <= serverTermineHighInt; deviceTermineHighInt++) {
    TermineImgFileName = [NSString stringWithFormat:@"Termine%i.png", deviceTermineHighInt];
    TermineImgURL = [rootURL URLByAppendingPathComponent:TermineImgFileName];
    TermineImg = [NSData dataWithContentsOfURL:TermineImgURL];
    if (TermineImg != nil) {
        TermineImgPath = [[paths objectAtIndex:0] stringByAppendingPathComponent:TermineImgFileName];
        [TermineImg writeToFile:TermineImgPath atomically:YES];
        updateCount += 1;
        NSLog(@"File %@ saved", TermineImgFileName);
    }
    else {
        NSLog(@"Write Error");
    }               
    TermineImg = nil;
}

Do you know why the method doesn't return nil if the file at the URL doesn't exist?

And a second question: Does it make sense to use the Strings, NSURL and NSData as i did? I thought for memory reasons it would be the best way.

Thank you in advance, Nikos

Edit: The variables for the loop are defined before the code, the loop works fine. Also the variable rootURL is a constant defined in the header. The URL is built fine and it works.

Answer 1

Testing the code (on a single run ). If the image does not exist I get the Write Error as expected. Whats returned is (NULL). Which I think acts the same in this case, and points to a zero value.

There is a stackoverflow post which tries to explain it Here. are-null-and-nil-equivalent

Answer 2

I'd say the question is what the server actually does if you request a non-existing image.
If it gives you a 404, -dataWithContentsOfURL: should return nil - but if it doesn't, a NSData instance with the results will be created which just happens to not contain any useful image data.