NSURLConnection sendAsynchronousRequest with retry functionality - nsurlconnection

Question

I have my main UI thread that calls sendAsynchronousRequest method of NSURLConnection to fetch data.
[NSURLConnection sendAsynchronousRequest:[self request]
queue:[NSOperationQueue alloc] init
completionHandler:
^(NSURLResponse *response, NSData *data, NSError *error)
{
if (error)
{
//error handler
}
else
{
//dispatch_asych to main thread to process data.
}
}];
All this is fine and good.
My question here is, I need to implement retry functionality on error.
Can I do it in this block and call sendSynchronousRequest to retry as this is the background queue.
Or dispatch to main thread and let the main thread handle retry (by calling sendAsynchronousRequest and repeating the same cycle).

Answer 1

You're getting the request by calling [self request]. If request is an atomic #property, or is otherwise thread safe, I can't think of any reason you couldn't kick off a retry from a non-main thread.
Alternately, you could put a copy of the request into a local variable prior to your +sendAsynchronousRequest:queue: call. If you do that, and then reference it in your completion handler, then it will be retained implicitly and [self request] will only be called once.
Generally speaking, this probably isn't a great pattern. If the service is down, absent some other checks, it will just keep trying forever. You might try something like this:
NSURLRequest* req = [self request];
NSOperationQueue* queue = [[NSOperationQueue alloc] init];
__block NSUInteger tries = 0;
typedef void (^CompletionBlock)(NSURLResponse *, NSData *, NSError *);
__block CompletionBlock completionHandler = nil;
// Block to start the request
dispatch_block_t enqueueBlock = ^{
[NSURLConnection sendAsynchronousRequest:req queue:queue completionHandler:completionHandler];
};
completionHandler = ^(NSURLResponse *resp, NSData *data, NSError *error) {
tries++;
if (error)
{
if (tries < 3)
{
enqueueBlock();
}
else
{
// give up
}
}
else
{
//dispatch_asych to main thread to process data.
}
};
// Start the first request
enqueueBlock();