Adding Scan.

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

@@ -0,0 +1,141 @@
+// 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.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Scan<TSource>(this IAsyncObservable<TSource> source, Func<TSource, TSource, TSource> func)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.Scan(observer, func)));
+        }
+
+        public static IAsyncObservable<TSource> Scan<TSource>(this IAsyncObservable<TSource> source, Func<TSource, TSource, Task<TSource>> func)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.Scan(observer, func)));
+        }
+
+        public static IAsyncObservable<TResult> Scan<TSource, TResult>(this IAsyncObservable<TSource> source, TResult seed, Func<TResult, TSource, TResult> func)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Create<TResult>(observer => source.SubscribeAsync(AsyncObserver.Scan(observer, seed, func)));
+        }
+
+        public static IAsyncObservable<TResult> Scan<TSource, TResult>(this IAsyncObservable<TSource> source, TResult seed, Func<TResult, TSource, Task<TResult>> func)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Create<TResult>(observer => source.SubscribeAsync(AsyncObserver.Scan(observer, seed, func)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> Scan<TSource>(IAsyncObserver<TSource> observer, Func<TSource, TSource, TSource> func)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Scan(observer, (a, x) => Task.FromResult(func(a, x)));
+        }
+
+        public static IAsyncObserver<TSource> Scan<TSource>(IAsyncObserver<TSource> observer, Func<TSource, TSource, Task<TSource>> func)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            var hasValue = false;
+            var value = default(TSource);
+
+            return Create<TSource>(
+                async x =>
+                {
+                    if (hasValue)
+                    {
+                        try
+                        {
+                            value = await func(value, x).ConfigureAwait(false);
+                        }
+                        catch (Exception ex)
+                        {
+                            await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                            return;
+                        }
+                    }
+                    else
+                    {
+                        value = x;
+                        hasValue = true;
+                    }
+
+                    await observer.OnNextAsync(value).ConfigureAwait(false);
+                },
+                observer.OnErrorAsync,
+                observer.OnCompletedAsync
+            );
+        }
+
+        public static IAsyncObserver<TSource> Scan<TSource, TResult>(IAsyncObserver<TResult> observer, TResult seed, Func<TResult, TSource, TResult> func)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            return Scan<TSource, TResult>(observer, seed, (a, x) => Task.FromResult(func(a, x)));
+        }
+
+        public static IAsyncObserver<TSource> Scan<TSource, TResult>(IAsyncObserver<TResult> observer, TResult seed, Func<TResult, TSource, Task<TResult>> func)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (func == null)
+                throw new ArgumentNullException(nameof(func));
+
+            var value = seed;
+
+            return Create<TSource>(
+                async x =>
+                {
+                    try
+                    {
+                        value = await func(value, x).ConfigureAwait(false);
+                    }
+                    catch (Exception ex)
+                    {
+                        await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                        return;
+                    }
+
+                    await observer.OnNextAsync(value).ConfigureAwait(false);
+                },
+                observer.OnErrorAsync,
+                observer.OnCompletedAsync
+            );
+        }
+    }
+}