Adding WithLatestFrom.

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

@@ -0,0 +1,135 @@
+// 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;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TResult> WithLatestFrom<TFirst, TSecond, TResult>(this IAsyncObservable<TFirst> first, IAsyncObservable<TSecond> second, Func<TFirst, TSecond, TResult> resultSelector)
+        {
+            if (first == null)
+                throw new ArgumentNullException(nameof(first));
+            if (second == null)
+                throw new ArgumentNullException(nameof(second));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            return Create<TResult>(async observer =>
+            {
+                var (firstObserver, secondObserver) = AsyncObserver.WithLatestFrom(observer, resultSelector);
+
+                var firstSubscription = await first.SubscribeSafeAsync(firstObserver);
+                var secondSubscription = await second.SubscribeSafeAsync(secondObserver);
+
+                return StableCompositeAsyncDisposable.Create(firstSubscription, secondSubscription);
+            });
+        }
+
+        public static IAsyncObservable<TResult> WithLatestFrom<TFirst, TSecond, TResult>(this IAsyncObservable<TFirst> first, IAsyncObservable<TSecond> second, Func<TFirst, TSecond, Task<TResult>> resultSelector)
+        {
+            if (first == null)
+                throw new ArgumentNullException(nameof(first));
+            if (second == null)
+                throw new ArgumentNullException(nameof(second));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            return Create<TResult>(async observer =>
+            {
+                var (firstObserver, secondObserver) = AsyncObserver.WithLatestFrom(observer, resultSelector);
+
+                var firstSubscription = await first.SubscribeSafeAsync(firstObserver);
+                var secondSubscription = await second.SubscribeSafeAsync(secondObserver);
+
+                return StableCompositeAsyncDisposable.Create(firstSubscription, secondSubscription);
+            });
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static (IAsyncObserver<TFirst>, IAsyncObserver<TSecond>) WithLatestFrom<TFirst, TSecond, TResult>(IAsyncObserver<TResult> observer, Func<TFirst, TSecond, TResult> resultSelector)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            return WithLatestFrom<TFirst, TSecond, TResult>(observer, (x, y) => Task.FromResult(resultSelector(x, y)));
+        }
+
+        public static (IAsyncObserver<TFirst>, IAsyncObserver<TSecond>) WithLatestFrom<TFirst, TSecond, TResult>(IAsyncObserver<TResult> observer, Func<TFirst, TSecond, Task<TResult>> resultSelector)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            var gate = new AsyncLock();
+
+            async Task OnErrorAsync(Exception ex)
+            {
+                using (await gate.LockAsync().ConfigureAwait(false))
+                {
+                    await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                }
+            }
+
+            var hasLatest = false;
+            var latest = default(TSecond);
+
+            return
+                (
+                    Create<TFirst>(
+                        async x =>
+                        {
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                if (hasLatest)
+                                {
+                                    var res = default(TResult);
+
+                                    try
+                                    {
+                                        res = await resultSelector(x, latest).ConfigureAwait(false);
+                                    }
+                                    catch (Exception ex)
+                                    {
+                                        await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                                        return;
+                                    }
+
+                                    await observer.OnNextAsync(res).ConfigureAwait(false);
+                                }
+                            }
+                        },
+                        OnErrorAsync,
+                        async () =>
+                        {
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                await observer.OnCompletedAsync().ConfigureAwait(false);
+                            }
+                        }
+                    ),
+                    Create<TSecond>(
+                        async y =>
+                        {
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                hasLatest = true;
+                                latest = y;
+                            }
+                        },
+                        OnErrorAsync,
+                        () => Task.CompletedTask
+                    )
+                );
+        }
+    }
+}