我怎样才能做一个异步任务同步

Question

我想从两个不同的网站获取两种类型的数据，并将其绑定到一个列表，但我与异步的问题，我想要做的是从一个rss将它添加到列表中，然后从另一个网站获取信息将其添加到列表中，然后将这两个添加到绑定的可观察集合中。但DownloadStringAsync已经过时运行了，并且应用程序崩溃了。你能帮我吗？

我的代码是所有我相信lamdba会比你的情况的回调更好的

private static ObservableCollection<Top> top= new ObservableCollection<Top>(); 
    private static ObservableCollection<string> place= new ObservableCollection<string>(); 
    // Constructor 
    public MainPage() 
    { 
     InitializeComponent(); 
     if (NetworkInterface.GetIsNetworkAvailable()) 
     { 
      LoadSiteContent_A(url1); 

      LoadSiteContent_B(url2); 


     } 
     else 
      MessageBox.Show("No Internet Connection, please connect to use this applacation"); 



     listBox.ItemsSource = top; //trying to bind listbox after web calls 
    } 





    public void LoadSiteContent_A(string url) 
    { 

      //create a new WebClient object 
      WebClient clientC = new WebClient(); 


      clientC.DownloadStringCompleted += new DownloadStringCompletedEventHandler(a_DownloadStringCompleted); 
      clientC.DownloadStringAsync(new Uri(url)); 


    } 

    public void LoadSiteContent_B(string url) 
    { 
      //create a new WebClient object 
      WebClient clientC = new WebClient(); 


      clientC.DownloadStringCompleted += new DownloadStringCompletedEventHandler(b_DownloadStringCompleted); 
      clientC.DownloadStringAsync(new Uri(url)); 


    } 

    public void a_DownloadStringCompleted(Object sender, DownloadStringCompletedEventArgs e) 
    { 
     string testString = ""; 


     if (!e.Cancelled && e.Error == null) 
     { 
      string str; 

      str = (string)e.Result; 

      //Various operations and parsing 


        place.Add(testString); 

      } 


      } 


    } 
    public void b_DownloadStringCompleted(Object sender, DownloadStringCompletedEventArgs e) 
    { 

     string testMatch = ""; 


     if (!e.Cancelled && e.Error == null) 
     { 
      string str; 
      // Size the control to fill the form with a margin 
      str = (string)e.Result; 

       //Various operations and parsing 



       top.Add(new Top(testMatch,(place.Count+1))); 


      } 



    } 


public class TopUsers 
{ 
    public string TopUsername { get; set; } 
    public int Place { get; set; } 

    public TopUsers(string topusername, int place) 
    { 
     this.TopUsername = topusername; 
     this.Place = place; 

    } 
} 


} 

Answer 1

第一。 为了同步下载，您必须在LoadSiteContent_A的完整事件中调用LoadSiteContent_B。

private static ObservableCollection<Top> top= new ObservableCollection<Top>(); 
    private static ObservableCollection<string> place= new ObservableCollection<string>(); 
    private string _url1; 
    private string _url2; 
    // Constructor 
    public MainPage(string url1, string url2) 
    { 
     InitializeComponent(); 
     if (NetworkInterface.GetIsNetworkAvailable()) 
     { 
      _url1 = url1; 
      _url2 = url2; 
      LoadSiteContent_A(url1); 
     } 
     else 
      MessageBox.Show("No Internet Connection, please connect to use this applacation"); 
     listBox.ItemsSource = top; //trying to bind listbox after web calls 
    } 

    public void LoadSiteContent_A(string url) 
    { 
      //create a new WebClient object 
      WebClient clientC = new WebClient(); 
      clientC.DownloadStringCompleted += (sender, e) => { 
      string testString = ""; 
      if (!e.Cancelled && e.Error == null) 
      { 
       string str; 
       str = (string)e.Result; 
       //Various operations and parsing 
       place.Add(testString); 
       LoadSiteContent_B(_url2); 
       } 
      }; 

      clientC.DownloadStringAsync(new Uri(url)); 
    } 

    public void LoadSiteContent_B(string url) 
    { 
      //create a new WebClient object 
      WebClient clientC = new WebClient(); 
      clientC.DownloadStringCompleted += (sender, e) => {/*do whatever you need*/}; 
      clientC.DownloadStringAsync(new Uri(url)); 
    } 


public class TopUsers 
{ 
    public string TopUsername { get; set; } 
    public int Place { get; set; } 

    public TopUsers(string topusername, int place) 
    { 
     this.TopUsername = topusername; 
     this.Place = place; 

    } 
} 


} 

Answer 2

这是更多的替代答案（AlexTheo的解决方案应该工作）。

当他们给我们（WP开发者）提供新的异步内容时，所有这些变得更加容易。

你的代码可以这样写：

public async MainPage() 
{ 
    InitializeComponent(); 
    DoAsyncLoad(); 
} 
private async Task DoAsyncLoad() // note use of "async" keyword 
{ 
    if (NetworkInterface.GetIsNetworkAvailable()) 
    { 
     await LoadSiteContent_A(""); 
     await LoadSiteContent_B(""); 
    } 
    else 
     MessageBox.Show("No Internet Connection, please connect to use this applacation"); 

    listBox.ItemsSource = top; //trying to bind listbox after web calls 
} 

public async Task LoadSiteContent_A(string url) 
{ 
    //create a new WebClient object 
    WebClient clientC = new WebClient(); 

    var result = await clientC.DownloadStringTaskAsync(new Uri(url)); 
    // No need for a Lambda or setting up an event 

    var testString = result; // result is the string you were waiting for (will be empty of canceled or errored) 
} 
public async Task LoadSiteContent_B(string url) 
{ 
    //create a new WebClient object 
    WebClient clientC = new WebClient(); 

    var result = await clientC.DownloadStringTaskAsync(new Uri(url)); 
    // Again, no need for a Lambda or setting up an event (code is simpler as a result) 
    top.Add(new Top(testMatch, place.Count + 1)); 
} 

有更多的代码更改你将不得不作出（使用HTTP调用的异步版本并标记LoadSiteContent_A/B为异步--and设置任务的返回）。

顺便说一句，你实际上可以加载最新的Async-CTP3并发布这种方式编写的WP代码。尽管大多数人对CTP有点害怕。

我写了这一篇博客文章，你可以看看这里 - http://www.jaykimble.net/metro-nuggets-async-is-your-friend.aspx

Answer 3

我不会试图让他们的其他类似后一个。通过一个接一个地“堆叠”它们，就像你一开始就失去了异步调用的所有优点。不仅如此，在像Windows Phone这样的移动平台上，您必须记住网络呼叫会排队以便有效使用天线。当你同时拨打这两个电话时，他们在相同的天线连接期间执行的可能性要高得多，这是一件“好事”。

接下来，您的每个回调都实际上更新完全独立的集合。 A正在更新place集合，B正在更新top集合。所以这不是两个人以任何方式彼此相处的问题。

我在这里看到的唯一真正的问题就是您正在更新设置为listBox.ItemsSource的top集合。您需要将绑定数据的更新封送回UI线程（又名Dispatcher线程），以便绑定到它们的控件将在正确的线程上更新。

所以，你应该做你的任何代码的唯一改变是元帅增加了新的项目到top收集回Dispatcher线程在B回调。这将是这样的：

public void b_DownloadStringCompleted(Object sender, DownloadStringCompletedEventArgs e) 
{ 
    string testMatch = ""; 

    if(!e.Cancelled && e.Error == null) 
    { 
     string str; 
     // Size the control to fill the form with a margin 
     str = (string)e.Result; 

     //Various operations and parsing 

     Top newTop = new Top(testMatch,(place.Count+1)); 

     Dispatcher.Invoke(() => 
     { 
      top.Add(newTop); 
     }); 
    } 
} 

有了这个，所有的工作仍然异步/同步，除了在您的项目添加到top收集的小小的一部分。

Answer 4

即使使用lambda，也有一个更加优雅的解决方案 - 使用自定义Action，其中T是数据类型。

例如：

public void LoadSiteContent_A(string url, Action<string> onCompletion) 
{ 
     //create a new WebClient object 
     WebClient clientC = new WebClient(); 

     clientC.DownloadStringCompleted += (s,e) => 
     { 
      onCompletion(e.Result); 
     }; 
     clientC.DownloadStringAsync(new Uri(url)); 
} 

当你调用这个方法，你可以通过这样的动作：

LoadSiteContent_a(yourUrlWhatever, data => 
{ 
    // DO SOMETHING WITH DATA 
});