如何将 WP7 接口绑定到我异步检索的数据？

Question

我正在开发一个 Windows Phone 7 应用程序，该应用程序允许用户查看其 Chargify.com 帐户下每个“站点”的统计信息。

我一直在关注复数视觉培训视频，该视频让我大部分时间都到了那里，但是我的数据来自复杂的来源，并且他们硬编码了一个列表。

因此，设置如下：

模型：

 public SiteStats
 {
     public string seller_name { get; set;}
     public static GetSiteStatistics(string subdomain, string apiKey)
     {
        SiteStats retVal = null;
        HttpWebRequest request = WebRequest.Create(string.Format("https://{0}.chargify.com/stats.json", subdomain)) as HttpWebRequest;
        NetworkCredential credentials = new NetworkCredential(apiKey, "X");
        request.Credentials = credentials;
        request.Method = "GET";
        request.Accept = "application/json";
        request.BeginGetResponse(result =>
        {
            using (var response = request.EndGetResponse(result))
            {
                using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                {
                    string stats = reader.ReadToEnd();
                    retVal = Json.Deserialize<SiteStats>(stats);
                }
            }
        }, request);
        return retVal;
     }
 }

ViewModel：

 public class SiteDetailViewModel : ViewModelBase
 {
    private SiteStats _siteStats;
    public SiteDetailViewModel(string subdomain) : this()
    {
       this._siteStats = SiteStats.GetSiteStatistics(subdomain, "apiKeyHere");
    }
    public SiteDetailViewModel : base() { ViewName = "site details"; }
    public SiteStats SiteStats
    {
      get { return _siteStats; }
      set {
        if (_siteStats != value) {
          _siteStats = value;
          OnPropertyChanged("SiteStats");
        }
     }
 }

视图：

 public partial class SiteDetailView : PhoneApplicationPage
 {
   private SiteDetailViewModel _viewModel;

   public SiteDetailView()
   {
      InitializeComponent();
      Loaded += new RoutedEventHandler(SiteDetailView_Loaded);
   }
   void SiteDetailView_Loaded(object sender, RoutedEventArgs e)
   {
      string subdomain = NavigationContext.QueryString["subdomain"];
      _viewModel = new SiteDetailViewModel(subdomain);
      this.DataContext = _viewModel;
   }
 }

问题是，当我调用 this.DataContext 时 - _viewModel 成员还没有数据。因此，视图数据绑定 - 但该值为空。

有什么建议吗？一切工作正常，除了视图没有填充数据的绑定控件..

-- Kori

Answer 1

_viewModel 应该有一个 ObservableCollection<> 用于统计信息，并且您将 UI 绑定到该集合。每当向集合中添加或删除项目时，UI 都会自动更新（因为它发出 OnPropertyChanged 事件）

Answer 2

您的问题不是 WPF，而是 GetSiteStatistics。由于您以异步方式获取结果，因此您的方法几乎总是返回 null，除非碰巧在 GetSiteStatistics 方法返回之前执行 BeginGetResponse。它在任何应用程序中都会失败。

您可以让 GetSiteStatistics 始终创建并返回一个对象，并且仅将其填充到 BeginGetResponse 中。但是你应该确保整个事情是线程安全的。

Answer 3

好吧，我想我已经弄清楚了。

它一直在那里，我只是错过了它。

1）您正在异步获取数据。 “request.BeginGetResponse(result=> .... );”将来的某个时候发生，但您在发生之前返回结果。代码不断移动，它不会等待您的结果。这就是您想要做的：

public class SiteStats
{
  public string seller_name { get; set;}
  public static void GetSiteStatistics(string subdomain, string apiKey, Action<SiteStats> callback)
  {
    SiteStats retVal = null;
    HttpWebRequest request = WebRequest.Create(string.Format("https://{0}.chargify.com/stats.json", subdomain)) as HttpWebRequest;
    NetworkCredential credentials = new NetworkCredential(apiKey, "X");
    request.Credentials = credentials;
    request.Method = "GET";
    request.Accept = "application/json";
    request.BeginGetResponse(result =>
    {
        using (var response = request.EndGetResponse(result))
        {
            using (StreamReader reader = new StreamReader(response.GetResponseStream()))
            {
                string stats = reader.ReadToEnd();
                retVal = Json.Deserialize<SiteStats>(stats);
                callback(retVal);
            }
        }
    }, request);
    //return retVal; // you can't return here
  }
}

相关的 ViewModel 代码将如下所示：

public SiteDetailViewModel(string subdomain) : this()
{
   SiteStats.GetSiteStatistics(subdomain, "apiKeyHere", (result)=> {
     // Note you may need to wrap this in a Dispatcher call 
     // as you may be on the wrong thread to update the UI 
     // if that happens you'll get a cross thread access 
     // you will have to expose the dispatcher through some 
     // other mechanism. One way to do that would be a static
     // on your application class which we'll emulate and 
     // I'll give you the code in a sec
     myRootNamespace.App.Dispatcher.BeginInvoke(()=>this._siteStats = results);
   });
}

这是您需要对 Application 类进行的更改（我不确定这有多线程安全，我真的建议您使用类似 MVVMLight 的东西调度助手。

public partial class App : Application
{

    public static Dispatcher Dispatcher { get; private set; } // Add this line!!

    // More code follows we're skipping it

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        this.RootVisual = new MainPage(); 
        Dispatcher = this.RootVisual.Dispatcher; // only add this line!!
    }

    private void Application_Exit(object sender, EventArgs e)
    {
        // Do this to clean up
        Dispatcher = null;
    }
}