Adding IsEmpty.

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

@@ -0,0 +1,28 @@
+// 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. 
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<bool> IsEmpty<TSource>(this IAsyncObservable<TSource> source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<bool>(observer => source.SubscribeAsync(AsyncObserver.IsEmpty<TSource>(observer)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> IsEmpty<TSource>(IAsyncObserver<bool> observer)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            return Any<TSource>(Select<bool, bool>(observer, b => !b));
+        }
+    }
+}