ObserverProgress: An IProgress implementation that forwards to an IObserver.

ObserverProgress.cs

using System;
using System.Threading.Tasks;

namespace Nito.AsyncEx
{
    /// <summary>
    /// A progress implementation that sends progress reports to an observer stream. Optionally ends the stream when the task completes.
    /// </summary>
    /// <typeparam name="T">The type of progress value.</typeparam>
    internal sealed class ObserverProgress<T> : IProgress<T>
    {
        /// <summary>
        /// The observer to pass progress reports to.
        /// </summary>
        private readonly IObserver<T> _observer;

        /// <summary>
        /// Initializes a new instance of the <see cref="ObserverProgress&lt;T&gt;"/> class.
        /// </summary>
        /// <param name="observer">The observer to pass progress reports to. May not be <c>null</c>.</param>
        public ObserverProgress(IObserver<T> observer)
        {
            _observer = observer;
        }

        void IProgress<T>.Report(T value)
        {
            _observer.OnNext(value);
        }

        /// <summary>
        /// Watches the task, and completes the observer (via <see cref="IObserver{T}.OnError"/> or <see cref="IObserver{T}.OnCompleted"/>) when the task completes.
        /// </summary>
        /// <param name="task">The task to watch. May not be <c>null</c>.</param>
        public void ObserveTaskForCompletion(Task task)
        {
            task.ContinueWith(_ =>
            {
                if (task.IsFaulted)
                    _observer.OnError(task.Exception.InnerException);
                else
                    _observer.OnCompleted();
            }, TaskScheduler.Default);
        }
    }
}

Sooo nice!