是否可以安全地使用切换FlowType联合类型（String Enums）？(Is it possible to safely use Switch over FlowType union types (String Enums)?)

 您必须传递对NSError对象的引用NSError将执行该过程，如果有任何错误，它将更新错误对象并出现相应的错误。 
 
 你的新代码应该是这样的 
 
-(BOOL)registerUser{
    NSError *error = nil;
    PFUser *newUser = [PFUser user];
    newUser.username = self.username;
    newUser.password = self.password;
    [newUser signUp:&error]; // error

    if( error != nil)
    {
      //log the error or show alert
      return NO;
    }

    return YES;


}

You have to pass a reference to a NSError object.. Parse will do the process and if any error is there it will update the error object with appropriate error.
 
You new code should be like
 
-(BOOL)registerUser{
    NSError *error = nil;
    PFUser *newUser = [PFUser user];
    newUser.username = self.username;
    newUser.password = self.password;
    [newUser signUp:&error]; // error

    if( error != nil)
    {
      //log the error or show alert
      return NO;
    }

    return YES;


}