Question

Why the code if (dispatch_get_current_queue() == socketQueue) is needed? why can’t we just use dispatch_sync(socketQueue, block) directly???

Thanks in advance!

- (BOOL)isConnected
{
__block BOOL result = NO;

dispatch_block_t block = ^{
    result = (flags & kConnected) ? YES : NO;
};

if (dispatch_get_current_queue() == socketQueue)
    block();
else
    dispatch_sync(socketQueue, block);

return result;
}

BTW, the code is from XMPPFramework

Answer 1

You cannot call dispatch_sync to schedule blocks on the current serial queue since this will deadlock. Dispatch_sync waits until the block finished executing, but it will never start to run before the current block finished running.