RunAsync - How do I await the completion of work on the UI thread?

Question

When awaiting Dispatcher.RunAsync the continuation occurs when the work is scheduled, not when the work has completed. How can I await the work completing?

Answer 1

A nice way to work the clean way @StephenCleary suggests even if you have to start from a worker thread for some reason, is to use a simple helper object. With the object below you can write code like this:

    await DispatchToUIThread.Awaiter;
    // Now you're running on the UI thread, so this code is safe:
    this.textBox.Text = text;

In your App.OnLaunched you have to initialize the object:

    DispatchToUIThread.Initialize(rootFrame.Dispatcher);

The theory behind the code below you can find at await anything;

public class DispatchToUIThread : INotifyCompletion
{
    private readonly CoreDispatcher dispatcher;

    public static DispatchToUIThread Awaiter { get; private set; }

    private DispatchToUIThread(CoreDispatcher dispatcher)
    {
        this.dispatcher = dispatcher;
    }

    [CLSCompliant(false)]
    public static void Initialize(CoreDispatcher dispatcher)
    {
        if (dispatcher == null) throw new ArgumentNullException("dispatcher");
        Awaiter = new DispatchToUIThread(dispatcher);
    }

    public DispatchToUIThread GetAwaiter()
    {
        return this;
    }

    public bool IsCompleted
    {
        get { return this.dispatcher.HasThreadAccess; }
    }

    public async void OnCompleted(Action continuation)
    {
        if (continuation == null) throw new ArgumentNullException("continuation");
        await this.dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => continuation());
    }

    public void GetResult() { }
}