有的信号不需要返回值, 只是当一个信号触发后立即执行:

[[[[self.signInButton
   rac_signalForControlEvents:UIControlEventTouchUpInside]
   //立即执行(不需要经历next)
   doNext:^(id x) {
     self.signInButton.enabled = NO;
     self.signInFailureText.hidden = YES;
   }]
   flattenMap:^id(id x) {
     return [self signInSignal];
   }]
   subscribeNext:^(NSNumber *signedIn) {
     self.signInButton.enabled = YES;
     BOOL success = [signedIn boolValue];
     self.signInFailureText.hidden = success;
     if (success) {
       [self performSegueWithIdentifier:@"signInSuccess" sender:self];
     }
   }];