Adding ObserveOn.

AsyncRx.NET/System.Reactive.Async.Linq/System/Reactive/Linq/Operators/Delay.cs

@@ -90,12 +90,12 @@ namespace System.Reactive.Linq
 
                         await scheduler.Delay(delay, ct).RendezVous(scheduler);
 
-                        await observer.OnNextAsync(next.Value).ConfigureAwait(false);
+                        await observer.OnNextAsync(next.Value).RendezVous(scheduler);
                     }
 
                     if (queue.Count == 0 && isDone)
                     {
-                        await observer.OnCompletedAsync().ConfigureAwait(false);
+                        await observer.OnCompletedAsync().RendezVous(scheduler);
                         break;
                     }
                 }

AsyncRx.NET/System.Reactive.Async.Linq/System/Reactive/Linq/Operators/ObserveOn.cs

@@ -0,0 +1,113 @@
+// 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.Collections.Generic;
+using System.Reactive.Concurrency;
+using System.Reactive.Disposables;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> ObserveOn<TSource>(this IAsyncObservable<TSource> source, IAsyncScheduler scheduler)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (scheduler == null)
+                throw new ArgumentNullException(nameof(scheduler));
+
+            return Create<TSource>(async observer =>
+            {
+                var (sink, drain) = await AsyncObserver.ObserveOn(observer, scheduler).ConfigureAwait(false);
+
+                var subscription = await source.SubscribeAsync(sink).ConfigureAwait(false);
+
+                return StableCompositeAsyncDisposable.Create(subscription, drain);
+            });
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static async Task<(IAsyncObserver<TSource>, IAsyncDisposable)> ObserveOn<TSource>(this IAsyncObserver<TSource> observer, IAsyncScheduler scheduler)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (scheduler == null)
+                throw new ArgumentNullException(nameof(scheduler));
+
+            var semaphore = new SemaphoreSlim(0);
+
+            var gate = new AsyncLock();
+
+            var queue = new Queue<TSource>();
+            var error = default(Exception);
+            var isDone = false;
+
+            var drain = await scheduler.ScheduleAsync(async ct =>
+            {
+                while (!ct.IsCancellationRequested)
+                {
+                    await semaphore.WaitAsync(ct).RendezVous(scheduler);
+
+                    if (queue.Count > 0)
+                    {
+                        var next = queue.Dequeue();
+
+                        await observer.OnNextAsync(next).RendezVous(scheduler);
+                    }
+
+                    if (queue.Count == 0)
+                    {
+                        if (isDone)
+                        {
+                            await observer.OnCompletedAsync().RendezVous(scheduler);
+                            break;
+                        }
+
+                        if (error != null)
+                        {
+                            await observer.OnErrorAsync(error).RendezVous(scheduler);
+                            break;
+                        }
+                    }
+                }
+            }).ConfigureAwait(false);
+
+            var sink = Create<TSource>(
+                async x =>
+                {
+                    using (await gate.LockAsync().ConfigureAwait(false))
+                    {
+                        queue.Enqueue(x);
+                    }
+
+                    semaphore.Release(1);
+                },
+                async ex =>
+                {
+                    using (await gate.LockAsync().ConfigureAwait(false))
+                    {
+                        error = ex;
+                    }
+
+                    semaphore.Release(1);
+                },
+                async () =>
+                {
+                    using (await gate.LockAsync().ConfigureAwait(false))
+                    {
+                        isDone = true;
+                    }
+
+                    semaphore.Release(1);
+                }
+            );
+
+            return (sink, drain);
+        }
+    }
+}