如何从异步方法获取复杂类型

Question

 public ObservableCollection<Offer> ModelPersons
    {
        get
        {

            return _persons;
        }


        set
        {
            _persons = value;

            RaisePropertyChanged(() => ModelPersons);
        }
    }

//如何从Task<ObservableCollection<Person>>获取ObservableCollection<Person> ？ 现在它抱怨无法将源类型Task<Observable<Person>> to Observable<Person>

  private void GetPersonOrders()
    {
        ModelPersons = _service.GetPersonOrders();
    }

在服务类：

  private ObservableCollection<Person> _modelPersons;


    public async Task<ObservableCollection<Person>> GetPersonOrders()
    {

        var req =
           WebRequest.Create(
              "http....")
               as HttpWebRequest;

        if (req == null) return null;
        req.KeepAlive = false;
        req.Method = "GET";
        req.ContentType = "text/xml; encoding='utf-8'";

        WebResponse response = req.GetResponse();
        Stream receiveStream = response.GetResponseStream();

        Encoding encode = Encoding.GetEncoding("utf-8");
        string message = null;
        if (receiveStream != null)
            using (var readStream = new StreamReader(receiveStream, encode))
            {
                var result = new char[256];
                int count = await readStream.ReadAsync(result, 0, 256);
                while (count > 0)
                {
                    var value = new String(result, 0, count);
                    message += value;
                    count = await readStream.ReadAsync(result, 0, 256);
                }


                var stringReader = new StringReader(message);
                _modelPersons= new ObservableCollection<Person>();
                var xmlTextReader = new XmlTextReader(stringReader);
                xmlTextReader.WhitespaceHandling = WhitespaceHandling.None;
                Person person = null;
                string element = "";
                while (xmlTextReader.Read())
                {
                    if (xmlTextReader.NodeType == XmlNodeType.Element)
                    {
                        element = xmlTextReader.Name;
                    }

                    else if (xmlTextReader.NodeType == XmlNodeType.Text)
                    {
                        switch (element)
                        {
                            case "name":
                                person = new Person { Name = xmlTextReader.Value };
                                _modelPersons.Add(person);
                                break;



                        }
                    }
                }
            }
        return _modelPersons;
    }

Answer 1

我建议你使用我的AsyncEx库中的NotifyTaskCompletion类型 。

然后你的代码将如下所示：

private INotifyCompletion<ObservableCollection<Offer>> _persons;
public INotifyCompletion<ObservableCollection<Offer>> ModelPersons
{
  get { return _persons; }
  set
  {
    _persons = value;
    RaisePropertyChanged(() => ModelPersons);
  }
}

private void GetPersonOrders()
{
  ModelPersons = NotifyCompletion.Create(_service.GetPersonOrdersAsync());
}

您的数据绑定将从ModelPersons更改为ModelPersons.Result 。

进一步推荐阅读：

• 我的async / await介绍及其后续资源，特别是TAP 。
• 我关于async属性的博客文章。

Answer 2

由于您的方法返回Task<ObservableCollection<Person>>您可以通过Result属性获取任务的Result 。

private void GetPersonOrders()
{
    ModelPersons = _service.GetPersonOrders().Result;
}

但请记住，访问Result属性将阻止调用线程，直到任务完成，即直到您将收到WebResponse 。

另一种方法是使用ContinueWith将延续任务附加到方法返回的任务。 continuation将属性设置为从task返回的值。

private void GetPersonOrders()
{
    _service.GetPersonOrders()
        .ContinueWith(t => ModelPersons = t.Result);
}