Add Prepend.

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

@@ -0,0 +1,66 @@
+// 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.Concurrency;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Prepend<TSource>(this IAsyncObservable<TSource> source, TSource element)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<TSource>(async observer => await source.SubscribeAsync(await AsyncObserver.Prepend(observer, element).ConfigureAwait(false)).ConfigureAwait(false));
+        }
+
+        public static IAsyncObservable<TSource> Prepend<TSource>(this IAsyncObservable<TSource> source, TSource element, IAsyncScheduler scheduler)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (scheduler == null)
+                throw new ArgumentNullException(nameof(scheduler));
+
+            return Create<TSource>(async observer => await source.SubscribeAsync(await AsyncObserver.Prepend(observer, element, scheduler).ConfigureAwait(false)).ConfigureAwait(false));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static Task<IAsyncObserver<TSource>> Prepend<TSource>(IAsyncObserver<TSource> observer, TSource element)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            return Core();
+
+            async Task<IAsyncObserver<TSource>> Core()
+            {
+                await observer.OnNextAsync(element);
+
+                return Create<TSource>(
+                    observer.OnNextAsync,
+                    observer.OnErrorAsync,
+                    async () =>
+                    {
+                        await observer.OnNextAsync(element).ConfigureAwait(false);
+                        await observer.OnCompletedAsync().ConfigureAwait(false);
+                    }
+                );
+            }
+        }
+
+        public static Task<IAsyncObserver<TSource>> Prepend<TSource>(IAsyncObserver<TSource> observer, TSource element, IAsyncScheduler scheduler)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (scheduler == null)
+                throw new ArgumentNullException(nameof(scheduler));
+
+            throw new NotImplementedException();
+        }
+    }
+}