Question

我正在尝试为我的WP7 Silverlight应用程序实现MVVM模式，我遇到了异步JSON Rest调用的问题。我将我的WP7应用页面中的以下两种方法移入了我的ViewModel类。

public void FetchGames()
{            

    ObservableCollection<Game> G = new ObservableCollection<Game>();
    //REST call in here
    var webClient = new WebClient();
    Uri uri = new Uri("http://www.somewebsite.com/get/games/league/" + league);
    webClient.OpenReadCompleted += new OpenReadCompletedEventHandler(OpenReadCompletedGames);   
    webClient.OpenReadAsync(uri);
}

private void OpenReadCompletedGames(object sender, OpenReadCompletedEventArgs e)
{
    DataContractJsonSerializer ser = null;
    ser = new DataContractJsonSerializer(typeof(ObservableCollection<Game>));
    Games = ser.ReadObject(e.Result) as ObservableCollection<Game>;
    this.IsDataLoaded = true;

}

现在的问题是，因为它是异步调用，所以以下代码不起作用。以下代码在我的应用页面上。

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    if (NavigationContext.QueryString.TryGetValue("league", out league))
    {              

 try
 {
     App.gViewModel.league = league;
     App.gViewModel.FetchGames();
     if(App.gViewModel.IsDataLoaded)
     {
  lbTeams.ItemsSource = App.gViewModel.Games;
     }     
 }
 catch ()
 {                
     //error logging in here              
 }
    }

}

单步执行代码显示调用FetchGames然后命中下一行（if（App.gViewModel.IsDataLoaded））在异步调用完成之前。所以IsDataLoaded总是假的，我不能绑定页面上的列表框。

做了很多googleing我有一些可能的解决方案，但我无法将它们转换为我的特定问题。一个是这样的，它与延续传递风格有关。我无法让它工作，并非常感谢一些帮助。

谢谢！

void DoSomethingAsync( Action<string> callback ) { 
    HttpWebRequest req; // TODO: build your request 

    req.BeginGetResponse( result => { 
        // This anonymous function is a closure and has access  
        // to the containing (or enclosing) function. 
        var response = req.EndGetResponse( result ); 

        // Get the result string and call the callback 
        string resultString = null; // TODO: read from the stream 

        callback(resultString); 
    }, null ); 
}

Answer 1

这可以通过移动来解决

lbTeams.ItemsSource = App.gViewModel.Games;

到OpenReadCompletedGames方法的末尾。您需要使用Dispatcher从此处更新UI。

Dispatcher.BeginInvoke( () => { lbTeams.ItemsSource = App.gViewModel.Games; } );

MVVM的ASync JSON REST调用问题

1 个答案: