class API { class func rac_login() -> RACSignal { return RACSignal.createSignal({ (subscriber) in // API call if error != nil { subscriber.sendError(error) } else { subscriber.sendCompleted() } return nil }) } } let command = RACCommand(signalBlock: { _ in API.rac_login() }) command.executionSignals.subscribeNext({ let signal = $0 as RACSignal signal.subscribeCompleted({ println("Error時もCompleted時