Parsing JSON using Grand Central Dispatch in connectionDidFinishLoading:?

Question

I'm am performing JSON deserialization with JSONTouch, but it's taking too long and it's blocking the UI, so I've tried to make a GCD background queue to put the serialization in a background thread. Sometimes it runs fine, but sometimes I get a EXC_BAD_ACCESS on deserializeAsDictionary:weakSelf.mutableData error:&theError];.

I'm at a loss as to why. I've made mutableData an atomic property. I'm making 3 requests at once, so I guess something it has to do with threads trying to access the mutableData? or perhaps mutableData is not in a good state when the block runs?

Thanks a lot!

-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
     __weak myViewController *weakSelf = self;

    dispatch_queue_t updateQueue = dispatch_queue_create("parse json", NULL);
    dispatch_async(updateQueue, ^{
        NSError *theError = nil;

    // This is the call that gives me EXC_BAD_ACCESS
    NSDictionary *dict = [[CJSONDeserializer deserializer] 
       deserializeAsDictionary:weakSelf.mutableData error:&theError];

   dispatch_async(dispatch_get_main_queue(), ^{
            [weakSelf setMutableData: nil];
      });
   });
   dispatch_release(updateQueue);
}

score 1 · Accepted Answer · answered Mar 05 '12 at 08:09

The problem was that I was using one mutableData object to store the response of the three requests, and when one connection finished and the connectionDidFinishLoading: handler was reading the data, another connection in another thread was appending data to it.

I created a separate NSMutableData object for every request, and added code to connectionDidFinishLoading: so that it reads from the correct NSMutableData object.

My application is very responsive now that the handler is running in another GCD queue!

Parsing JSON using Grand Central Dispatch in connectionDidFinishLoading:?

1 Answers1