gistfile1.m

RACSignal *formValid = [RACSignal
  combineLatest:@[
    self.firstNameField.rac_textSignal,
    self.lastNameField.rac_textSignal,
    self.emailField.rac_textSignal,
    self.reEmailField.rac_textSignal
  ]
  reduce:^(NSString *firstName, NSString *lastName, NSString *email, NSString *reEmail) {
    return @(firstName.length > 0 && lastName.length > 0 && email.length > 0 && reEmail.length > 0 && [email isEqual:reEmail]);
  }];

RAC(self.createButton.enabled) = formValid;