http://messagesenttodeallocatedinstance.wordpress.com/2012/04/10/nsurlconnection-with-blocks/

Here is the corrections, it’s at the dictionary creation:
[NSDictionary dictionaryWithObjectsAndKeys:
request,@"request",
[[successBlock_ copy] autorelease],@”success”,
[[failureBlock_ copy] autorelease],@”failure”,
nil]];

Short explanation: blocks are allocated on the stack for optimization purpose, when the function returns the stack is cleaned up and the block dies out. The block has to be moved to the heap to exist beyond the scope, at that point it works like any other ObjC objects.

When the block is still on the stack, -retain doesn’t do anything to the object, -retain must always return the receiver, that’s part of the semantic, thus sending retain to a block (like what NSDictionary is doing) won’t copy it to the heap. It has to be copied explicitly, you can then autorelease it because the dictionary will retain the block, in this case it works properly because -retain on a heap block increase its retain count just like normal ObjC objects.

However, I’ve tidied it up a bit further. Instead of creating an NSThread to do the work just use Grand Central Dispatch. The method below is all you need.

+ (void)asyncRequest:(NSURLRequest *)request success:(void(^)(NSData *,NSURLResponse *))successBlock_ failure:(void(^)(NSData *,NSError *))failureBlock_
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSURLResponse *response = nil;
NSError *error = nil;
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

if (error) {
failureBlock_(data,error);
} else {
successBlock_(data,response);
}

[pool release];
});
}