Adding WindowAsyncObservable.

AsyncRx.NET/System.Reactive.Async.Linq/System/Reactive/Linq/WindowAsyncObservable.cs

@@ -0,0 +1,34 @@
+// 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.Reactive.Disposables;
+using System.Reactive.Subjects;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    internal sealed class WindowAsyncObservable<TSource> : AsyncObservableBase<TSource>
+    {
+        private readonly IAsyncSubject<TSource> _subject;
+        private readonly RefCountAsyncDisposable _disposable;
+
+        public WindowAsyncObservable(IAsyncSubject<TSource> subject, RefCountAsyncDisposable disposable)
+        {
+            _subject = subject;
+            _disposable = disposable;
+        }
+
+        protected override async Task<IAsyncDisposable> SubscribeAsyncCore(IAsyncObserver<TSource> observer)
+        {
+            if (_disposable != null)
+            {
+                var d = await _disposable.GetDisposableAsync().ConfigureAwait(false);
+                var s = await _subject.SubscribeAsync(observer).ConfigureAwait(false);
+                return StableCompositeAsyncDisposable.Create(d, s);
+            }
+
+            return await _subject.SubscribeAsync(observer).ConfigureAwait(false);
+        }
+    }
+}