如何在Silverlight的foreach循環中使用嵌套異步（WCF）調用？

Question

以下代碼在一些foreach循環中包含一些嵌套的異步調用。我知道silverlight/wcf調用被稱爲asyncrously - 但我怎麼能確保我的wcfPhotographers，wcfCategories和wcfCategories對象在foreach循環開始之前就準備好了？我敢肯定，我正在以這種錯誤的方式去做 - 並希望能給你一個幫助。

private void PopulateControl() 
    { 

     List<CustomPhotographer> PhotographerList = new List<CustomPhotographer>(); 

     proxy.GetPhotographerNamesCompleted += proxy_GetPhotographerNamesCompleted; 
     proxy.GetPhotographerNamesAsync(); 


     //for each photographer 
     foreach (var eachPhotographer in wcfPhotographers) 
     { 

      CustomPhotographer thisPhotographer = new CustomPhotographer(); 

      thisPhotographer.PhotographerName = eachPhotographer.ContactName; 
      thisPhotographer.PhotographerId = eachPhotographer.PhotographerID; 

      thisPhotographer.Categories = new List<CustomCategory>(); 

      proxy.GetCategoryNamesFilteredByPhotographerCompleted += proxy_GetCategoryNamesFilteredByPhotographerCompleted; 
      proxy.GetCategoryNamesFilteredByPhotographerAsync(thisPhotographer.PhotographerId); 


      // for each category 
      foreach (var eachCatergory in wcfCategories) 
      { 

       CustomCategory thisCategory = new CustomCategory(); 

       thisCategory.CategoryName = eachCatergory.CategoryName; 
       thisCategory.CategoryId = eachCatergory.CategoryID; 

       thisCategory.SubCategories = new List<CustomSubCategory>(); 

       proxy.GetSubCategoryNamesFilteredByCategoryCompleted += proxy_GetSubCategoryNamesFilteredByCategoryCompleted; 
       proxy.GetSubCategoryNamesFilteredByCategoryAsync(thisPhotographer.PhotographerId,thisCategory.CategoryId); 

       // for each subcategory 
       foreach(var eachSubCatergory in wcfSubCategories) 
       { 
        CustomSubCategory thisSubCatergory = new CustomSubCategory(); 

        thisSubCatergory.SubCategoryName = eachSubCatergory.SubCategoryName; 
        thisSubCatergory.SubCategoryId = eachSubCatergory.SubCategoryID; 
       } 


       thisPhotographer.Categories.Add(thisCategory); 
      } 

      PhotographerList.Add(thisPhotographer); 
     } 

     PhotographerNames.ItemsSource = PhotographerList; 
    } 




    void proxy_GetPhotographerNamesCompleted(object sender, GetPhotographerNamesCompletedEventArgs e) 
    { 
     wcfPhotographers = e.Result.ToList(); 
    } 


    void proxy_GetCategoryNamesFilteredByPhotographerCompleted(object sender, GetCategoryNamesFilteredByPhotographerCompletedEventArgs e) 
    { 
     wcfCategories = e.Result.ToList(); 
    } 

    void proxy_GetSubCategoryNamesFilteredByCategoryCompleted(object sender, GetSubCategoryNamesFilteredByCategoryCompletedEventArgs e) 
    { 
     wcfSubCategories = e.Result.ToList(); 
    } 

Answer 1

是的，然後才能與算法的繼續執行下一步，你需要已經得到了前面的步驟，它可以是很難當你必須使用異步方法的結果。

如果這不在UI線程上發生，那麼您可以阻止並等待響應。例如，已分別「已完成」的方法信號（使用任何同步原語都可以在Silverlight，我不知道隨便例如，如果ManualResetEvent的是存在的，如果是的話，必須完成的回調調用.Set()），然後讓你的主要PopulateControl方法調用FooAsync（）調用，然後阻塞，直到ManualResetEvent信號（通過調用.Wait()）。

如果這是在UI線程，你真的需要寫一個無阻塞的解決方案，這是非常非常難的C＃代碼正確這件事。您可以考慮使用F＃，而async爲非阻塞調用提供了一個很好的編程模型。

編輯：

僞代碼示例阻塞的結果：

// class-level 
ManualResetEvent mre = new ManualResetEvent(false); 
// some method that needs to make WCF call and use results 
void Blah() { 
    // yadda yadda 
    proxy.FooCompleted += (o,ea) => { ... mre.Set(); }; 
    proxy.FooAsync(...); 
    mre.WaitOne(); // block until FooCompleted 
    // use results from FooCompleted now that they're here 
    // mre.Reset() if you intend to use it again later 
} 

我用的λ爲FooCompleted，但使用一個單獨的方法等你已經是沒關係。

Answer 2

另外，對於每一個異步方法您使用來填充你可以創建一個輔助方法，將返回IObservable的集合，然後使用Linq查詢對結果進行分組。

如：

private IObservable<Photographer> GetPhotographerNames() 
{ 
    var photographers = Observable 
     .FromEvent<GetPhotographerNamesCompletedEventArgs>(proxy, "GetPhotographerNamesCompleted") 
     .Prune() 
     .SelectMany(e => e.EventArgs.Result.ToObservable()); 

    proxy.GetPhotographerNamesAsync(); 

    return photographers; 
} 

而且類似：

private IObservable<Category> GetCategoryNamesFilteredByPhotographer(int photographerId)  { ... } 
private IObservable<SubCategory> GetSubCategoryNamesFilteredByCategory(int photographerId, int categoryId) { ... } 

現在你可以寫一個LINQ查詢：

var pcs = from p in GetPhotographerNames() 
      from c in GetCategoryNamesFilteredByPhotographer(p.PhotographerId) 
      from s in GetSubCategoryNamesFilteredByCategory(p.PhotographerId, c.CategoryId) 
      select new {p, c, s}; 

此查詢將返回三元組列表（攝影師，類別，子類別）現在，您只需要訂閱它並將其聚合到您在其上使用的對象e客戶端應該是非常簡單的。