.Net通过TaskFactory.FromAsync简化APM

    static IEnumerable<Task> CopyStreamAsync(Stream input, Stream output)
    {
        var buffer = new byte[0x2000];
        while (true)
        {
            var readTask = Task<int>.Factory.FromAsync(input.BeginRead, input.EndRead, buffer, 0, buffer.Length, null);
            yield return readTask;

            if (readTask.Result == 0)
                break;

            yield return Task.Factory.FromAsync(output.BeginWrite, output.EndWrite, buffer, 0, buffer.Length, null);

        }
    }

    foreach (var task in CopyStreamAsync(input, output))
    {
        task.Wait();
    }

    public static Task Iterate(this TaskFactory factory, IEnumerable<Task> asyncIterator)
    {
        var scheduler = factory.Scheduler ?? TaskScheduler.Current;

        // Get an enumerator from the enumerable
        var enumerator = asyncIterator.GetEnumerator();
        if (enumerator == null) throw new InvalidOperationException();

        // Create the task to be returned to the caller. And ensure
        // that when everything is done, the enumerator is cleaned up.
        var trs = new TaskCompletionSource<object>(factory.CreationOptions);
        trs.Task.ContinueWith(_ => enumerator.Dispose(), scheduler);

        // This will be called every time more work can be done.
        Action<Task> recursiveBody = null;
        recursiveBody = antecedent =>
        {
            try
            {
                // If the previous task completed with any exceptions, bail
                if (antecedent != null && antecedent.IsFaulted)
                    trs.TrySetException(antecedent.Exception);

                else if (trs.Task.IsCanceled) trs.TrySetCanceled();

                else if (enumerator.MoveNext())
                    enumerator.Current.ContinueWith(recursiveBody, scheduler);

                // Otherwise, we're done!
                else trs.TrySetResult(null);
            }
            // If MoveNext throws an exception, propagate that to the user
            catch (Exception exc) { trs.TrySetException(exc); }
        };

        // Get things started by launching the first task
        factory.StartNew(_ => recursiveBody(null), scheduler);

        // Return the representative task to the user
        return trs.Task;
    }