Adding Switch.

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

@@ -0,0 +1,122 @@
+// 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;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Switch<TSource>(this IAsyncObservable<IAsyncObservable<TSource>> source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<TSource>(async observer =>
+            {
+                var (sink, cancel) = AsyncObserver.Switch(observer);
+
+                var subscription = await source.SubscribeSafeAsync(sink);
+
+                return StableCompositeAsyncDisposable.Create(subscription, cancel);
+            });
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static (IAsyncObserver<IAsyncObservable<TSource>>, IAsyncDisposable) Switch<TSource>(IAsyncObserver<TSource> observer)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            var gate = new AsyncLock();
+
+            var isStopped = false;
+            var hasLatest = false;
+            var latest = 0UL;
+
+            var disposable = new SerialAsyncDisposable();
+
+            return
+                (
+                    Create<IAsyncObservable<TSource>>(
+                        async xs =>
+                        {
+                            ulong id;
+
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                hasLatest = true;
+                                id = unchecked(++latest);
+                            }
+
+                            var innerObserver = Create<TSource>(
+                                async x =>
+                                {
+                                    using (await gate.LockAsync().ConfigureAwait(false))
+                                    {
+                                        if (latest == id)
+                                        {
+                                            await observer.OnNextAsync(x).ConfigureAwait(false);
+                                        }
+                                    }
+                                },
+                                async ex =>
+                                {
+                                    using (await gate.LockAsync().ConfigureAwait(false))
+                                    {
+                                        if (latest == id)
+                                        {
+                                            await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                                        }
+                                    }
+                                },
+                                async () =>
+                                {
+                                    using (await gate.LockAsync().ConfigureAwait(false))
+                                    {
+                                        if (latest == id)
+                                        {
+                                            hasLatest = false;
+
+                                            if (isStopped)
+                                            {
+                                                await observer.OnCompletedAsync().ConfigureAwait(false);
+                                            }
+                                        }
+                                    }
+                                }
+                            );
+
+                            var inner = await xs.SubscribeSafeAsync(innerObserver).ConfigureAwait(false);
+
+                            await disposable.AssignAsync(inner).ConfigureAwait(false);
+                        },
+                        async ex =>
+                        {
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                            }
+                        },
+                        async () =>
+                        {
+                            using (await gate.LockAsync().ConfigureAwait(false))
+                            {
+                                isStopped = true;
+
+                                if (!hasLatest)
+                                {
+                                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                                }
+                            }
+                        }
+                    ),
+                    disposable
+                );
+        }
+    }
+}