UI-thread-task-await-from-background-thread.md

Await a UI task sent from a background thread

Enables code running on a background thread to await a task that must run on the UI thread.

Normally, you can update your UI from a background thread by calling Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => /* update the UI */). However, this merely schedules the work on the UI thread and returns immediately, even if the task is to await user input in a popup box.

This method uses TaskCompletionSource in combination with RunAsync to return a Task that you can await from your background thread, thereby pausing execution until the UI task completes.

using System;
using System.Threading.Tasks;
using Windows.UI.Core;

public static Task RunAwaitableAsync(CoreDispatcher dispatcher, Func<Task> task)
{
    var completion = new TaskCompletionSource<bool>();
    var action = dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
    {
        try
        {
            await task.Invoke();
            completion.TrySetResult(true);
        }
        catch (Exception e)
        {
            completion.TrySetException(e);
        }
    });
    return completion.Task;
}

Usage

public async Task MyMethod()
{
    System.Diagnostics.Debug.WriteLine("MyMethod() is running");
    await RunAwaitableAsync(CoreWindow.GetForCurrentThread().Dispatcher, async () =>
    {
        var popup = new Windows.UI.Popups.MessageDialog("Alert", 
            "MyMethod() is paused. Tap 'Close' to continue running MyMethod()");
        await popup.ShowAsync();
    });
    System.Diagnostics.Debug.WriteLine("MyMethod() is running again");
}

See also

CoreDispatcher class
CoreDispatcher.RunAsync method
TaskCompletionSource class
Lambda expressions (anonymous methods using the "=>" syntax)