Adding DefaultIfEmpty.

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

@@ -0,0 +1,64 @@
+// 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;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> DefaultIfEmpty<TSource>(this IAsyncObservable<TSource> source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.DefaultIfEmpty<TSource>(observer)));
+        }
+
+        public static IAsyncObservable<TSource> DefaultIfEmpty<TSource>(this IAsyncObservable<TSource> source, TSource defaultValue)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.DefaultIfEmpty<TSource>(observer, defaultValue)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(this IAsyncObserver<TSource> observer)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            return DefaultIfEmpty(observer, default(TSource));
+        }
+
+        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(this IAsyncObserver<TSource> observer, TSource defaultValue)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            var hasValue = false;
+
+            return Create<TSource>(
+                x =>
+                {
+                    hasValue = true;
+                    return observer.OnNextAsync(x);
+                },
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    if (!hasValue)
+                    {
+                        await observer.OnNextAsync(defaultValue).ConfigureAwait(false);
+                    }
+
+                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                }
+            );
+        }
+    }
+}