Adding Defer.

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

@@ -0,0 +1,43 @@
+// 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.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Defer<TSource>(Func<IAsyncObservable<TSource>> observableFactory)
+        {
+            if (observableFactory == null)
+                throw new ArgumentNullException(nameof(observableFactory));
+
+            return Defer(() => Task.FromResult(observableFactory()));
+        }
+
+        public static IAsyncObservable<TSource> Defer<TSource>(Func<Task<IAsyncObservable<TSource>>> observableFactory)
+        {
+            if (observableFactory == null)
+                throw new ArgumentNullException(nameof(observableFactory));
+
+            return Create<TSource>(async observer =>
+            {
+                var source = default(IAsyncObservable<TSource>);
+
+                try
+                {
+                    source = await observableFactory().ConfigureAwait(false);
+                }
+                catch (Exception ex)
+                {
+                    await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                    return AsyncDisposable.Nop;
+                }
+
+                return await source.SubscribeAsync(observer).ConfigureAwait(false);
+            });
+        }
+    }
+}