Simple Async ExternalEventHandler for Revit API

AsyncExternalEventHandler.cs

using Autodesk.Revit.UI;
using System;
using System.Threading.Tasks;

public class AsyncExternalEventHandler : IExternalEventHandler
{
    private readonly Action<UIApplication> execute;
    private readonly ExternalEvent externalEvent;
    private TaskCompletionSource<bool> eventCompleted;
    public AsyncExternalEventHandler(Action<UIApplication> execute)
    {
        this.execute = execute;
        this.externalEvent = ExternalEvent.Create(this);
    }

    public Task RaiseAsync()
    {
        eventCompleted = new TaskCompletionSource<bool>();
        externalEvent.Raise();
        return eventCompleted.Task;
    }

    public void Execute(UIApplication app)
    {
        try
        {
            execute.Invoke(app);
            eventCompleted.TrySetResult(true);
        }
        catch (Exception ex)
        {
            eventCompleted.TrySetException(ex);
        }
    }

    public string GetName()
    {
        return this.GetType().Name;
    }
}

This command show the title of the document two times after a second wait.

AsyncExternalEventHandler need to be created inside the Revit API Context

using Autodesk.Revit.Attributes;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using System.Threading.Tasks;

[Transaction(TransactionMode.Manual)]
public class CommandAsync : IExternalCommand
{
    public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elementSet)
    {
        UIApplication uiapp = commandData.Application;

        var showTitleHandler = new AsyncExternalEventHandler(ShowTitle);

        Task.Run(async () =>
        {
            await Task.Delay(1000);
            await showTitleHandler.RaiseAsync();
            await Task.Delay(1000);
            await showTitleHandler.RaiseAsync();
        });

        return Result.Succeeded;
    }

    public static void ShowTitle(UIApplication app)
    {
        Document document = app.ActiveUIDocument.Document;
        TaskDialog.Show("Title", document.Title);
    }
}