Question

我们目前正在开展一个项目，并且我正在开发某种警报/通知系统。基本理念：

有几个问题，每个问题都包含一个Observable Answer对象集合。所有这些项目都保存到SQL Server数据库（我们使用Entity Framework Code First方法）。问题类的重要部分：

public class Question
{
    public Question()
    {
      ...
      Answers = new ObservableCollection<Answer>();
      ...
    }

    public virtual ObservableCollection<Answer> Answers { get; set; }

    ...Extra stuff omitted
}

我想将一个CollectionChanged Handler添加到Answer实例的ObservableCollection中。我在为该问题创建订阅对象时执行此操作。此Subscription对象被创建并保存在我的manager类中，该类与Database上下文进行通信。执行此操作的方法：

public void CreateQuestionSubscription(int userId, int questionId, NotificationFrequency notificationFrequency)
{
  Regular subscriber = _politiciOnlineDbContext.Regulars.SingleOrDefault(r => r.UserId == userId);
  Question question = _politiciOnlineDbContext.Questions.SingleOrDefault(q => q.QuestionId == questionId);
  if (question == null || subscriber == null) return;

  question.Answers.CollectionChanged += new NotifyCollectionChangedEventHandler(CollectionChangedMethod);

  QuestionSubscription questionSubscription = new QuestionSubscription()
  {
    NotificationFrequency = notificationFrequency,
    Question = question,
    Subscriber = subscriber,
    WantsToReceiveEmail = true
  };
  _politiciOnlineDbContext.QuestionSubscriptions.AddOrUpdate(questionSubscription);
  _politiciOnlineDbContext.SaveChanges();
}

private void CollectionChangedMethod(object sender, NotifyCollectionChangedEventArgs e)
{
  if (e.Action == NotifyCollectionChangedAction.Add)
  {
    Debug.WriteLine("An answer has been added! (sub " + ((Subscription)sender).Subscriber.UserId + "-" + ((QuestionSubscription)sender).Question.QuestionId + ")");

    ((Subscription)sender).Alerts.Add(new Alert((Subscription)sender, AlertType.QuestionAnswerAdded));
  }
}

当我这样做时，Handler确实连接到ObservableCollection的CollectionChanged事件。但它很快就消失了，当我从数据库中获取Question对象时，CollectionChanged事件不包含任何处理程序。

这是相当合乎逻辑的，因为我不知道他们将如何以及在何处得到保存，但这不是我想要实现的目标。我需要处理程序保持连接，当然也可以在数据库中放入和提取对象时。我怎么能这样做？

从数据库获取项目时，ObservableCollection的CollectionChanged处理程序为null

0 个答案: