Adding Concat.

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

@@ -65,7 +65,7 @@ namespace System.Reactive.Linq
             });
         }
 
-        public static IAsyncObservable<TSource> Catch<TSource>(params IAsyncObservable<TSource>[] sources) => Catch<TSource>((IEnumerable<IAsyncObservable<TSource>>)sources);
+        public static IAsyncObservable<TSource> Catch<TSource>(params IAsyncObservable<TSource>[] sources) => Catch((IEnumerable<IAsyncObservable<TSource>>)sources);
 
         public static IAsyncObservable<TSource> Catch<TSource>(this IEnumerable<IAsyncObservable<TSource>> sources)
         {
@@ -78,7 +78,7 @@ namespace System.Reactive.Linq
 
                 if (!enumerator.MoveNext())
                 {
-                    return AsyncDisposable.Nop;
+                    return AsyncDisposable.Nop; // REVIEW: Is Never behavior right here?
                 }
 
                 var source = enumerator.Current;
@@ -203,7 +203,7 @@ namespace System.Reactive.Linq
 
                     if (handler == null)
                     {
-                        await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                        await observer.OnErrorAsync(ex).ConfigureAwait(false); // REVIEW: Is Throw behavior right here?
                         return;
                     }
 

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

@@ -0,0 +1,138 @@
+// 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.Disposables;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    // TODO: Implement tail call behavior to flatten Concat chains.
+
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Concat<TSource>(this IAsyncObservable<TSource> first, IAsyncObservable<TSource> second)
+        {
+            if (first == null)
+                throw new ArgumentNullException(nameof(first));
+            if (second == null)
+                throw new ArgumentNullException(nameof(second));
+
+            return Create<TSource>(async observer =>
+            {
+                var (sink, inner) = AsyncObserver.Concat(observer, second);
+
+                var subscription = await first.SubscribeSafeAsync(sink).ConfigureAwait(false);
+
+                return StableCompositeAsyncDisposable.Create(subscription, inner);
+            });
+        }
+
+        public static IAsyncObservable<TSource> Concat<TSource>(params IAsyncObservable<TSource>[] sources) => Concat((IEnumerable<IAsyncObservable<TSource>>)sources);
+
+        public static IAsyncObservable<TSource> Concat<TSource>(this IEnumerable<IAsyncObservable<TSource>> sources)
+        {
+            if (sources == null)
+                throw new ArgumentNullException(nameof(sources));
+
+            return Create<TSource>(async observer =>
+            {
+                var enumerator = sources.GetEnumerator();
+
+                if (!enumerator.MoveNext())
+                {
+                    return AsyncDisposable.Nop; // REVIEW: Is Never behavior right here?
+                }
+
+                var source = enumerator.Current;
+
+                var (sink, inner) = AsyncObserver.Concat(observer, enumerator);
+
+                var subscription = await source.SubscribeSafeAsync(sink).ConfigureAwait(false);
+
+                return StableCompositeAsyncDisposable.Create(subscription, inner);
+            });
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static (IAsyncObserver<TSource>, IAsyncDisposable) Concat<TSource>(IAsyncObserver<TSource> observer, IAsyncObservable<TSource> second)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (second == null)
+                throw new ArgumentNullException(nameof(second));
+
+            var subscription = new SingleAssignmentAsyncDisposable();
+
+            var sink = Create<TSource>(
+                observer.OnNextAsync,
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    var secondSubscription = await second.SubscribeSafeAsync(observer).ConfigureAwait(false);
+
+                    await subscription.AssignAsync(secondSubscription).ConfigureAwait(false);
+                }
+            );
+
+            return (sink, subscription);
+        }
+
+        public static (IAsyncObserver<TSource>, IAsyncDisposable) Concat<TSource>(IAsyncObserver<TSource> observer, IEnumerator<IAsyncObservable<TSource>> handlers)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (handlers == null)
+                throw new ArgumentNullException(nameof(handlers));
+
+            var innerSubscription = new SerialAsyncDisposable();
+
+            IAsyncObserver<TSource> sink = null;
+
+            sink = Create<TSource>(
+                observer.OnNextAsync,
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    var next = default(IAsyncObservable<TSource>);
+
+                    try
+                    {
+                        if (handlers.MoveNext())
+                        {
+                            next = handlers.Current;
+                        }
+                    }
+                    catch (Exception err)
+                    {
+                        await observer.OnErrorAsync(err).ConfigureAwait(false);
+                        return;
+                    }
+
+                    if (next == null)
+                    {
+                        await observer.OnCompletedAsync().ConfigureAwait(false); // REVIEW: Is Empty behavior right here?
+                        return;
+                    }
+
+                    var nextSubscription = await next.SubscribeSafeAsync(sink).ConfigureAwait(false);
+
+                    await innerSubscription.AssignAsync(nextSubscription).ConfigureAwait(false);
+                }
+            );
+
+            var disposeEnumerator = AsyncDisposable.Create(() =>
+            {
+                handlers.Dispose();
+                return Task.CompletedTask;
+            });
+
+            var subscription = StableCompositeAsyncDisposable.Create(innerSubscription, disposeEnumerator);
+
+            return (sink, subscription);
+        }
+    }
+}