登陆界面的用法
RAC(self.logInButton, enabled) = [RACSignal
     combineLatest:@[ 

         self.usernameTextField.rac_textSignal, 

         self.passwordTextField.rac_textSignal, 

         RACObserve(LoginManager.sharedManager, loggingIn), 

         RACObserve(self, loggedIn) 

     ] reduce:^(NSString *username, NSString *password, NSNumber *loggingIn, NSNumber *loggedIn) { 

         return @(username.length > 0 && password.length > 0 && !loggingIn.boolValue && !loggedIn.boolValue); 

     }]; 



 [[RACSignal 

    combineLatest:@[self.firstNameField.rac_textSignal, self.lastNameField.rac_textSignal] 

           reduce:^(NSString*firstName, NSString *lastName){ 

               return @(firstName.length> 0 && lastName.length> 0); 

           }] toProperty:@"enabled" onObject:self.button];