Async await and OnNavigatedTo #wpdev

Update: this only applies to #wpdev and using async CTP for async task mechanism. I have had a lot of feedback and after a bit of rework, i can do it right in #win8dev. see this post https://invokeit.wordpress.com/2012/10/24/async-task-mechanism-and-onnavigatedto-part-2-win8dev/

I am sure i am not the only one with bad coding habits. Maybe its just me or maybe i am more common than i’d like to think but here’s how i deal with loading data i require per page.

When a XAML page is called, i tend to override OnNavigatedTo event. This is where i put all the calls to load / fire data. If i am going to make web service calls or read from storage, it tends to happen in OnNavigatedTo method.

Over the last few months, i have migrated 2 of my apps from Windows Phone to Windows 8 and for the 3rd, i have taken a slightly different approach. Instead of jumping straight and porting it, i put the app in #WPDev under scissors and i am making code easily shareable between the two.

Step 1: Install Async CTP for VS 2010 (make sure you pray to whatever thing you believe in).. if that doesn’t work.. try threatening whatever thing you believe in.. if you are lucky Async CTP might just install 🙂

Step 2: Convert event driven async code to async await type code. For WebClient i found a helper class, you can download it here http://softwareblog.alcedo.com/post/2012/01/25/TaskCompletionSource-in-real-life-(part-2-of-2).aspx. It exposes awaitable Web Client and I used that to convert all other Event drive code to Task driven code

Here’s where the problem started. I tend to fire off the async event driven calls in OnNavigatedTo
If i change it to async await.. it doesn’t really work (at least not how i want it to). I have noticed the same with async await on Windows 8.

AsyncDataSourceObj aeo = new AsyncDataSourceObj();
protected override void OnNavigatedTo(NavigationEventArgs e)
{
    aeo.UseEventStyleCallCompleted += OnUseEventStyleCallCompleted;
    aeo.UseEventStyleCall();
}

private void OnUseEventStyleCallCompleted( /* args */ )
{
    // show the data
    // make sure you use Dispatcher when touching UI objects
}

Now suppose we modified the above code to use async await

AsyncDataSourceObj aeo = new AsyncDataSourceObj();
protected async override void OnNavigatedTo(NavigationEventArgs e)
{
    MyDataType mydata = await aeo.UseAsyncAwaitStyleCall();

    // now show the data.
}

This doesn’t seem to work correctly… maybe because i am declaring OnNavigatedTo async or maybe because i am making it pause and continue in a fashion it wasn’t supposed to… well i got tired eventually and this is what i settled for something that looks like this

AsyncDataSourceObj aeo = new AsyncDataSourceObj();
protected override void OnNavigatedTo(NavigationEventArgs e)
{
    ThreadPool.QueueUserWorkItem(new WaitCallback(f =>
    {
        Task<MyDataType> tmydata = aeo.UseAsyncAwaitStyleCall();
        tmydata.Wait();

        // now show the data.
        // use dispatcher to manipulate UI controls
    }));
}

this is a compromise between a purist Async Await usage and actually doing things the way i like it. And just FYI, it works beautifully

I haven’t done any hacks.. these methods are exposed.. i have just spawned off the async wait on a Threadpool thread myself.. sure the system could have done it but someone it didn’t like the way i did it. Oh well

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