GCD-created thread throws abort after completing block message

Question

I developed a simple RSS fetcher application that parses an XML RSS feed from a programmer-provided website and displays the articles in the Master View Controller and the UIWebView in a Detail View Controller. I custom set-up a personal server to render the RSS article selected in the master table view (for certain reasons) as a PDF. However, this obviously takes some time on the server end, unless the selected table view cell has already been rendered as a PDF on the server end. My server and blog don't talk to one another (and can't, for other reasons), so I can't pre-render the PDFs as I create the blog posts. The PDF render must be done from the application itself.

I decided to use Grand Central Dispatch to create a separate thread to talk to the server and render the PDFs before the user can select an arbitrary cell to see the post. Here is the code I used to create the queue.

dispatch_queue_t networkQueue = dispatch_queue_create("com.company.networkQueue", NULL);

...and the code I used to create the new thread...

dispatch_async(networkQueue, ^{ [self cachePDFRequests]; });

...here is my cachePDFRequests method that is called in the block request...

- (void) cachePDFRequests {
    NSURL *myURL;
    NSString *cacheUrl;
    NSURLRequest *request;

    for (int i = 0; i <= feeds.count; i++) {

        cacheUrl = [feeds[i] objectForKey:@"link"];
        cacheUrl = [cacheUrl stringByReplacingOccurrencesOfString:@" " withString:@""];
        cacheUrl = [cacheUrl stringByReplacingOccurrencesOfString:@"\n" withString:@""];

        NSString *fullUrl = [NSString stringWithFormat:@"http://myserver.com/render.php?url=%@", cacheUrl];

        myURL = [NSURL URLWithString:fullUrl];
        request = [NSURLRequest requestWithURL:myURL];

        [cacheView loadRequest:request];
    }
}

Note: cacheView is a UIWebView that is not on any UI...it is just an ivar of my Master VC class.

So, when I run the dispatch_async() function in -[viewDidLoad], it runs the -[cachePDFRequests] method and the for() loop inside of it, then throws SIGABRT on my newly-created thread. Please ask any questions as necessary. Let me know if I need to include any code that I haven't already provided.

Here is a picture of the SIGABRT error that appears whenever I run the GCD thread:

Thanks in advance!

P.S. I used to run cacheView = [[UIWebView alloc] init]; in an if() loop if the cacheView was set to nil...that used to throw the SIGABRT error. Now, it has no references to code in my Master View Controller after I removed that line.

EDIT: Here is the code for what the feeds mutable array is containing:

- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName {

if ([elementName isEqualToString:@"item"]) {

    [item setObject:title forKey:@"title"];
    [item setObject:link forKey:@"link"];

    [feeds addObject:[item copy]];

}

Answer 1

It looks like you are accessing feeds out of bounds. NSArray will throw an exception if you try to access it with an index that is outside of its bounds, and that is exactly what the backtrace indicates. This is the correct way to loop over an array with indices starting with 0:

for (int i = 0; i < feeds.count; i++) // Notice that it uses < instead of <= as comparator

As a side note, there is no such thing as an if() loop.