reactive/AsyncRx.NET/System.Reactive.Async.Interfaces/System/IAsyncObserver.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

using System.Threading.Tasks;

namespace System
{
    public interface IAsyncObserver<in T>
    {
        Task OnNextAsync(T value);
        Task OnErrorAsync(Exception error);
        Task OnCompletedAsync();
    }
}