从视图模型（MVVM）中的命令调用HttpWebRequest

Question

我第一次对我的应用程序使用了MVVM模式。我创建了一个视图类，一个ViewModel类，一个Command类。当我点击按钮上的视图时，我在viewModel中启动了绑定命令。在这个命令中，我创建了一个HttpWebRequest，从Web服务加载新闻作为json对象。在'GetNewsResponseCallback'方法中，我将我的数据作为json流。 seriallisation正常工作。当我在这个方法中调用'PropertyChanged（this，new PropertyChangedEventArgs（“News”））'时，我得到一个COMException。但我需要PropertyChange事件来更新我的视图中的Listview。我该如何解决这个问题？从我的命令开始服务是正确的吗？使用MVVM-Pattern开发我很新。在下面我的代码：

<Canvas> 
    <Button Content="Aktualisieren" 
      Canvas.Top="20" 
      Canvas.Left="600" 
      Command="{Binding SynchronizeNewsCommand}"/> 

    <ListView x:Name="viewBox" 
       ItemsSource="{Binding News}" 
       ItemTemplate="{StaticResource newsTemplate}"/> 

</Canvas> 

public class MyCommand<T> : ICommand 
    { 
     readonly Action<T> callback; 


     public MyCommand(Action<T> callback) 
     { 
      this.callback = callback; 
     } 


     public bool CanExecute(object parameter) 
     { 
      return true; 
     } 


     public event EventHandler CanExecuteChanged; 


     public void Execute(object parameter) 
     { 
      if (callback != null) { callback((T)parameter); } 
     } 

    } 



public class NewsViewModel : INotifyPropertyChanged 
    { 
     public event PropertyChangedEventHandler PropertyChanged; 

     public ObservableCollection<NewsSerializer> News { get; set; } 

     public MyCommand<object> SynchronizeNewsCommand { get; set; } 


     public NewsViewModel() 
     { 
      SynchronizeNewsCommand = new MyCommand<object>(SynchronizeNews); 
     } 


     private void SynchronizeNews(object obj) 
     { 
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Constans.uri); 
      request.ContentType = "application/json; charset=utf-8"; 
      request.Accept = "application/json"; 
      request.Method = "POST"; 
      request.BeginGetRequestStream(new AsyncCallback(GetNewsRequestStreamCallback), request); 
     } 


     private void SynchronizeFinished(object obj) 
     { 
      PropertyChanged(this, new PropertyChangedEventArgs("News")); 
     } 


     private void GetNewsRequestStreamCallback(IAsyncResult asynchronousResult) 
     { 
      HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState; 
      request.BeginGetResponse(new AsyncCallback(GetNewsResponseCallback), request); 
     } 


     private void GetNewsResponseCallback(IAsyncResult asynchronousResult) 
     { 
      HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState; 

      // End the operation 
      HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult); 
      Stream streamResponse = response.GetResponseStream(); 

      DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(ObservableCollection<NewsSerializer>)); 
      News = (ObservableCollection<NewsSerializer>)ser.ReadObject(streamResponse); 

      PropertyChanged(this, new PropertyChangedEventArgs("News")); 
     } 

    } 

Answer 1

通常情况下，使用一个ObservableCollection的时候 - 你不必每次都重新创建它。您应该调用Clear（）来代替（或删除已删除的项目），然后逐个添加新项目。你的看法会的变化自动通知你不会需要调用的PropertyChanged

private void GetNewsResponseCallback(IAsyncResult asynchronousResult) 
    { 
     HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState; 

     // End the operation 
     HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asynchronousResult); 
     Stream streamResponse = response.GetResponseStream(); 

     DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(ObservableCollection<NewsSerializer>)); 
     var res = (IList<NewsSerializer>)ser.ReadObject(streamResponse); 

     News.Clear(); 
     foreach(var item in res) 
     { 
      News.Add(item); 
     } 
    } 

在你的情况，你也应该考虑通过实施的AddRange方法扩展的ObservableCollection类。