ManualResetEvent wait doesn't release after being set

Question

I'm downloading two JSON files from the webs, after which I want to allow loading two pages, but not before. However, the ManualResetEvent that is required to be set in order to load the page never "fires". Even though I know that it gets set, WaitOne never returns.

Method that launches the downloads:

private void Application_Launching(object sender, LaunchingEventArgs e)
{
    PhoneApplicationService.Current.State["doneList"] = new List<int>();
    PhoneApplicationService.Current.State["manualResetEvent"] = new ManualResetEvent(false);

    Helpers.DownloadAndStoreJsonObject<ArticleList>("http://arkad.tlth.se/api/get_posts/", "articleList");
    Helpers.DownloadAndStoreJsonObject<CompanyList>("http://arkad.tlth.se/api/get_posts/?postType=webbkatalog", "catalog");
}

The downloading method, that sets the ManualResetEvent

public static void DownloadAndStoreJsonObject<T>(string url, string objName)
{
    var webClient = new WebClient();
    webClient.DownloadStringCompleted += (sender, e) => 
    {
        if (!string.IsNullOrEmpty(e.Result))
        {
            var obj = ProcessJson<T>(e.Result);
            PhoneApplicationService.Current.State[objName] = obj;


            var doneList = PhoneApplicationService.Current.State["doneList"] as List<int>;
            doneList.Add(0);

            if (doneList.Count == 2)    // Two items loaded
            {
                (PhoneApplicationService.Current.State["manualResetEvent"] as ManualResetEvent).Set();  // Signal that it's done
            }
        }
    };

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

The waiting method (constructor in this case)

public SenastePage()
{
    InitializeComponent();

    if ((PhoneApplicationService.Current.State["doneList"] as List<int>).Count < 2)
    {
        (PhoneApplicationService.Current.State["manualResetEvent"] as ManualResetEvent).WaitOne();
    }
    SenasteArticleList.ItemsSource =  (PhoneApplicationService.Current.State["articleList"] as ArticleList).posts;
}

If I wait before trying to access that constructor, it easily passes the if-statement and doesn't get caught in the WaitOne, but if I call it immediately, I get stuck, and it never returns...

Any ideas?

Answer 1

Blocking the UI thread must be prevented at all costs. Especially when downloading data: don't forget that your application is executing on a phone, which has a very instable network. If the data takes two minutes to load, then the UI will be freezed for two minutes. It would be an awful user experience.

There's many ways to prevent that. For instance, you can keep the same logic but waiting in a background thread instead of the UI thread:

public SenastePage()
{
    // Write the XAML of your page to display the loading animation per default
    InitializeComponent();

    Task.Factory.StartNew(LoadData);
}

private void LoadData()
{
    ((ManualResetEvent)PhoneApplicationService.Current.State["manualResetEvent"]).WaitOne();

    Dispatcher.BeginInvoke(() =>
    {
        SenasteArticleList.ItemsSource = ((ArticleList)PhoneApplicationService.Current.State["articleList"]).posts;

        // Hide the loading animation
    }
}

That's just a quick and dirty way to reach the result you want. You could also rewrite your code using tasks, and using Task.WhenAll to trigger an action when they're all finished.

Answer 2

Perhaps there is a logic problem. In the SenastePage() constructor you are waiting for the set event only if the doneList count is less than two. However, you don't fire the set event until the doneList count is equal to two. You are listening for the set event before it can ever fire.