Adding RefCountAsyncDisposable.

AsyncRx.NET/System.Reactive.Async.Disposables/System/Reactive/Disposables/RefCountAsyncDisposable.cs

@@ -0,0 +1,103 @@
+// 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;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Disposables
+{
+    public sealed class RefCountAsyncDisposable : IAsyncDisposable
+    {
+        private readonly AsyncLock _gate = new AsyncLock();
+        private IAsyncDisposable _disposable;
+        private bool _primaryDisposed;
+        private int _count;
+
+        public RefCountAsyncDisposable(IAsyncDisposable disposable)
+        {
+            if (disposable == null)
+                throw new ArgumentNullException(nameof(disposable));
+
+            _disposable = disposable;
+            _primaryDisposed = false;
+            _count = 0;
+        }
+
+        public async Task<IAsyncDisposable> GetDisposableAsync()
+        {
+            using (await _gate.LockAsync().ConfigureAwait(false))
+            {
+                if (_disposable == null)
+                {
+                    return AsyncDisposable.Nop;
+                }
+                else
+                {
+                    _count++;
+                    return new Inner(this);
+                }
+            }
+        }
+
+        public async Task DisposeAsync()
+        {
+            var disposable = default(IAsyncDisposable);
+
+            using (await _gate.LockAsync().ConfigureAwait(false))
+            {
+                if (_disposable != null && !_primaryDisposed)
+                {
+                    _primaryDisposed = true;
+
+                    if (_count == 0)
+                    {
+                        disposable = _disposable;
+                        _disposable = null;
+                    }
+                }
+            }
+
+            if (disposable != null)
+            {
+                await disposable.DisposeAsync().ConfigureAwait(false);
+            }
+        }
+
+        private async Task ReleaseAsync()
+        {
+            var disposable = default(IAsyncDisposable);
+
+            using (await _gate.LockAsync().ConfigureAwait(false))
+            {
+                if (_disposable != null)
+                {
+                    _count--;
+
+                    if (_primaryDisposed && _count == 0)
+                    {
+                        disposable = _disposable;
+                        _disposable = null;
+                    }
+                }
+            }
+
+            if (disposable != null)
+            {
+                await disposable.DisposeAsync().ConfigureAwait(false);
+            }
+        }
+
+        private sealed class Inner : IAsyncDisposable
+        {
+            private RefCountAsyncDisposable _parent;
+
+            public Inner(RefCountAsyncDisposable parent)
+            {
+                _parent = parent;
+            }
+
+            public Task DisposeAsync() => Interlocked.Exchange(ref _parent, null)?.ReleaseAsync() ?? Task.CompletedTask;
+        }
+    }
+}