Adding DeferAsync.

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

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information. 
 
 using System.Reactive.Disposables;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace System.Reactive.Linq
@@ -17,6 +18,8 @@ namespace System.Reactive.Linq
             return Defer(() => Task.FromResult(observableFactory()));
         }
 
+        public static IAsyncObservable<TSource> DeferAsync<TSource>(Func<Task<IAsyncObservable<TSource>>> observableFactory) => Defer(observableFactory);
+
         public static IAsyncObservable<TSource> Defer<TSource>(Func<Task<IAsyncObservable<TSource>>> observableFactory)
         {
             if (observableFactory == null)
@@ -39,5 +42,15 @@ namespace System.Reactive.Linq
                 return await source.SubscribeSafeAsync(observer).ConfigureAwait(false);
             });
         }
+
+        public static IAsyncObservable<TSource> DeferAsync<TSource>(Func<CancellationToken, Task<IAsyncObservable<TSource>>> observableFactory) => DeferAsync(observableFactory);
+
+        public static IAsyncObservable<TSource> Defer<TSource>(Func<CancellationToken, Task<IAsyncObservable<TSource>>> observableFactory)
+        {
+            if (observableFactory == null)
+                throw new ArgumentNullException(nameof(observableFactory));
+
+            return StartAsync(observableFactory).Merge();
+        }
     }
 }