Can't get ssid

[iThanh]

url = [[NSURL alloc] initWithString:@"http://localhost:3000"];

socket = [[VPSocketIOClient alloc] init:url withConfig:@{@"log": @false}];

[socket on:@"connect" callback:^(NSArray* data, VPSocketAckEmitter* ack) {
NSLog(@"socket connected: %@", [self->socket ssid]);
}];

But Log window show: socket connected: (null)
And urlRequest show: "<CFHTTPMessageRef 0x28365d5c0(0x28365d5d0)> { GET request, url ws://localhost:3000/socket.io/?transport=websocket&sid=vEquHExR-HYULOyPAAAE }"