Using async/await with void method

Question

in my Windows Phone 8 application, I have a LoadData() method in my file MainViewModel.cs.

This method load data from a WCF service with entity framework...

Then, in my pages, I call LoadData()

The LoadData() method :

public void LoadData()
{
    client.GetMoviesCompleted += new EventHandler<ServiceReference1.GetMoviesCompletedEventArgs>(client_GetMoviesCompleted);
    client.GetMoviesAsync();

    client.GetTheatersCompleted += new EventHandler<ServiceReference1.GetTheatersCompletedEventArgs>(client_GetTheatersCompleted);
    client.GetTheatersAsync();

    this.IsDataLoaded = true;
}

With the methods :

private void client_GetMoviesCompleted(object sender, ServiceReference1.GetMoviesCompletedEventArgs e)
{
    Movies = e.Result;
}

private void client_GetTheatersCompleted(object sender, ServiceReference1.GetTheatersCompletedEventArgs e)
{
    Theaters = e.Result;
}

Then in my pages :

 App.ViewModel.LoadData();

The problem is that it doesn't wait until the data is loaded.

Can you help me to use Async/Await the LoadData() method to wait until the data is loaded ?

Thanks

Answer 1

So we'll start with these two methods that convert your existing methods from an event-based model into a task based model. You'll need to modify them slightly to line up with your types as I don't quite have enough information to replicate them completely, but the remaining change should be small:

public static Task<Movie[]> WhenGetMovies(MyClient client)
{
    var tcs = new TaskCompletionSource<Movie[]>();
    Action<object, Movie[]> handler = null;
    handler = (obj, args) =>
    {
        tcs.SetResult(args.Result);
        client.GetMoviesCompleted -= handler;
    };
    client.GetMoviesCompleted += handler;
    client.GetMoviesAsync();
    return tcs.Task;
}
public static Task<Theater[]> WhenGetMovies(MyClient client)
{
    var tcs = new TaskCompletionSource<Theater[]>();
    Action<object, Theater[]> handler = null;
    handler = (obj, args) =>
    {
        tcs.SetResult(args.Result);
        client.GetTheatersCompleted -= handler;
    };
    client.GetTheatersCompleted += handler;
    client.GetTheatersAsync();
    return tcs.Task;
}

Now that we can get tasks that represent the completion of these async operations loading the data is easy:

public async Task LoadData()
{
    var moviesTask = WhenGetMovies(client);
    var theatersTask = WhenGetTheaters(client);
    var movies = await moviesTask;
    var theaters = await theatersTask;
}

Answer 2

The problem is, when you execute your LoadData() method, the runtime don't wait to continue the execution of your method. You can simply do a think like this :

private bool _moviesLoaded;
private bool _theatersLoaded;

private void client_GetMoviesCompleted(object sender, ServiceReference1.GetMoviesCompletedEventArgs e)
{
    Movies = e.Result;
    _moviesLoaded = true;
    TrySetDataIsLoaded();
}

private void client_GetTheatersCompleted(object sender, ServiceReference1.GetTheatersCompletedEventArgs e)
{
    Theaters = e.Result;
    _theatersLoaded = true;
    TrySetDataIsLoaded();
}

private void TrySetDataIsLoaded()
{
     if(_moviesLoaded && _theatersLoaded) this.IsDataLoaded = true;
}

If you want to use async and await, you can try to work with TaskCompletionSource

Answer 3

The bottom line is that you'll need to design and implement a "loading" state for your application. This is true whether you use event-based asynchronous programming (like your current code) or async / await . You should not synchronously block the UI until the loading is complete.

Personally, I like to (synchronously) initialize everything into the "loading" state, and when the asynchronous loading completes, have it update data-bound items on the View Model. The View then transitions to a "ready" state via data binding.

Answer 4

make above two variables (private bool _moviesLoaded;private bool _theatersLoaded;) as properties and set them in completed eventhandlers . and till the set is called use loader and when set is called disable this loader and now you can use this data for your work..