Question

I have been experimenting with WP7 apps today and have hit a bit of a wall.
I like to have seperation between the UI and the main app code but Ive hit a wall.

I have succesfully implemented a webclient request and gotten a result, but because the call is async I dont know how to pass this backup to the UI level. I cannot seem to hack in a wait for response to complete or anything.
I must be doing something wrong.

(this is the xbox360Voice library that I have for download on my website: http://www.jamesstuddart.co.uk/Projects/ASP.Net/Xbox_Feeds/ which I am porting to WP7 as a test)

here is the backend code snippet:

    internal const string BaseUrlFormat = "http://www.360voice.com/api/gamertag-profile.asp?tag={0}";
    internal static string ResponseXml { get; set; }
    internal static WebClient Client = new WebClient();

    public static XboxGamer? GetGamer(string gamerTag)
    {
        var url = string.Format(BaseUrlFormat, gamerTag);

        var response = GetResponse(url, null, null);

        return SerializeResponse(response);
    }

    internal static XboxGamer? SerializeResponse(string response)
    {
        if (string.IsNullOrEmpty(response))
        {
            return null;
        }

        var tempGamer = new XboxGamer();
        var gamer = (XboxGamer)SerializationMethods.Deserialize(tempGamer, response);

        return gamer;
    }

    internal static string GetResponse(string url, string userName, string password)
    {


            if (!string.IsNullOrEmpty(userName) && !string.IsNullOrEmpty(password))
            {
                Client.Credentials = new NetworkCredential(userName, password);
            }

            try
            {
                Client.DownloadStringCompleted += ClientDownloadStringCompleted;
                Client.DownloadStringAsync(new Uri(url));

                return ResponseXml;
            }
            catch (Exception ex)
            {
                return null;
            }
        }



    internal static void ClientDownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            ResponseXml = e.Result;
        }
    }

and this is the front end code:

public void GetGamerDetails()
{
    var xboxManager = XboxFactory.GetXboxManager("DarkV1p3r");
    var xboxGamer = xboxManager.GetGamer();

    if (xboxGamer.HasValue)
    {
        var profile = xboxGamer.Value.Profile[0];
        imgAvatar.Source = new BitmapImage(new Uri(profile.ProfilePictureMiniUrl));
        txtUserName.Text = profile.GamerTag;
        txtGamerScore.Text = int.Parse(profile.GamerScore).ToString("G 0,000");
        txtZone.Text = profile.PlayerZone;
    }
    else
    {
        txtUserName.Text = "Failed to load data";
    }
}

Now I understand I need to place something in ClientDownloadStringCompleted but I am unsure what.

Answer 1

The problem you have is that as soon as an asynchronous operation is introduced in to the code path the entire code path needs to become asynchronous.

• Because GetResponse calls DownloadStringAsync it must become asynchronous, it can’t return a string, it can only do that on a callback
• Because GetGamer calls GetResponse which is now asynchronous it can’t return a XboxGamer, it can only do that on a callback
• Because GetGamerDetails calls GetGamer which is now asynchronous it can’t continue with its code following the call, it can only do that after it has received a call back from GetGamer.
• Because GetGamerDetails is now asynchronous anything call it must also acknowledge this behaviour.
• …. this continues all the way up to the top of the chain where a user event will have occured.

Here is some air code that knocks some asynchronicity in to the code.

public static void GetGamer(string gamerTag, Action<XboxGamer?> completed) 
{ 
    var url = string.Format(BaseUrlFormat, gamerTag); 

    var response = GetResponse(url, null, null, (response) =>
    {
        completed(SerializeResponse(response));
    }); 
} 


internal static string GetResponse(string url, string userName, string password, Action<string> completed)      
{      

   WebClient client = new WebClient();
   if (!string.IsNullOrEmpty(userName) && !string.IsNullOrEmpty(password))      
   {      
       client.Credentials = new NetworkCredential(userName, password);      
   }      

   try      
   {      
        client.DownloadStringCompleted += (s, args) =>
        {
           // Messy error handling needed here, out of scope
           completed(args.Result);
        };
        client.DownloadStringAsync(new Uri(url));        
   }      
   catch     
   {      
      completed(null);      
   }      
}      


public void GetGamerDetails()              
{              
    var xboxManager = XboxFactory.GetXboxManager("DarkV1p3r");              
    xboxManager.GetGamer( (xboxGamer) =>              
    {
         // Need to move to the main UI thread.
         Dispatcher.BeginInvoke(new Action<XboxGamer?>(DisplayGamerDetails), xboxGamer);
    });

} 

void DisplayGamerDetails(XboxGamer? xboxGamer)
{
    if (xboxGamer.HasValue)              
    {              
        var profile = xboxGamer.Value.Profile[0];              
        imgAvatar.Source = new BitmapImage(new Uri(profile.ProfilePictureMiniUrl));              
        txtUserName.Text = profile.GamerTag;              
        txtGamerScore.Text = int.Parse(profile.GamerScore).ToString("G 0,000");              
        txtZone.Text = profile.PlayerZone;              
    }              
    else              
    {              
        txtUserName.Text = "Failed to load data";              
    }         
}

As you can see async programming can get realy messy.