runloop创建

 NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:req delegate:self];
     if (conn) {
         NSPort* port = [NSPort port];
         NSRunLoop* rl = [NSRunLoop currentRunLoop]; // Get the runloop
         [rl addPort:port forMode:NSDefaultRunLoopMode];
         [conn scheduleInRunLoop:rl forMode:NSDefaultRunLoopMode];
     }

 while(!isFinished) { [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]; }