Adding If.

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

@@ -0,0 +1,72 @@
+// 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> If<TSource>(Func<bool> condition, IAsyncObservable<TSource> thenSource) => If(condition, thenSource, Empty<TSource>());
+
+        public static IAsyncObservable<TSource> If<TSource>(Func<bool> condition, IAsyncObservable<TSource> thenSource, IAsyncScheduler scheduler) => If(condition, thenSource, Empty<TSource>(scheduler));
+
+        public static IAsyncObservable<TSource> If<TSource>(Func<bool> condition, IAsyncObservable<TSource> thenSource, IAsyncObservable<TSource> elseSource)
+        {
+            if (condition == null)
+                throw new ArgumentNullException(nameof(condition));
+            if (thenSource == null)
+                throw new ArgumentNullException(nameof(thenSource));
+            if (elseSource == null)
+                throw new ArgumentNullException(nameof(elseSource));
+
+            return Create<TSource>(observer =>
+            {
+                var b = default(bool);
+
+                try
+                {
+                    b = condition();
+                }
+                catch (Exception ex)
+                {
+                    return Throw<TSource>(ex).SubscribeAsync(observer);
+                }
+
+                return (b ? thenSource : elseSource).SubscribeSafeAsync(observer);
+            });
+        }
+
+        public static IAsyncObservable<TSource> If<TSource>(Func<Task<bool>> condition, IAsyncObservable<TSource> thenSource) => If(condition, thenSource, Empty<TSource>());
+
+        public static IAsyncObservable<TSource> If<TSource>(Func<Task<bool>> condition, IAsyncObservable<TSource> thenSource, IAsyncScheduler scheduler) => If(condition, thenSource, Empty<TSource>(scheduler));
+
+        public static IAsyncObservable<TSource> If<TSource>(Func<Task<bool>> condition, IAsyncObservable<TSource> thenSource, IAsyncObservable<TSource> elseSource)
+        {
+            if (condition == null)
+                throw new ArgumentNullException(nameof(condition));
+            if (thenSource == null)
+                throw new ArgumentNullException(nameof(thenSource));
+            if (elseSource == null)
+                throw new ArgumentNullException(nameof(elseSource));
+
+            return Create<TSource>(async observer =>
+            {
+                var b = default(bool);
+
+                try
+                {
+                    b = await condition().ConfigureAwait(false);
+                }
+                catch (Exception ex)
+                {
+                    return await Throw<TSource>(ex).SubscribeAsync(observer).ConfigureAwait(false);
+                }
+
+                return await (b ? thenSource : elseSource).SubscribeSafeAsync(observer).ConfigureAwait(false);
+            });
+        }
+    }
+}