Less allocations in SubscribeOnCtxObservable (#642)

Rx.NET/Source/src/System.Reactive/Concurrency/Synchronization.cs

@@ -102,27 +102,49 @@ namespace System.Reactive.Concurrency
 
         sealed class SubscribeOnCtxObservable<TSource> : ObservableBase<TSource>
         {
-            readonly IObservable<TSource> source;
+            private sealed class Subscription : IDisposable
+            {
+                private readonly IObservable<TSource> _source;
+                private readonly IObserver<TSource> _observer;
+                private readonly SynchronizationContext _context;
 
-            readonly SynchronizationContext context;
+                private IDisposable _cancel;
+
+                public Subscription(IObservable<TSource> source, SynchronizationContext context, IObserver<TSource> observer)
+                {
+                    _source = source;
+                    _context = context;
+                    _observer = observer;
+
+                    context.PostWithStartComplete(
+                        @this =>
+                        {
+                            if (!Disposable.GetIsDisposed(ref @this._cancel))
+                            {
+                                Disposable.SetSingle(ref @this._cancel, new ContextDisposable(@this._context, @this._source.SubscribeSafe(@this._observer)));
+                            }
+                        },
+                        this);
+                }
+
+                public void Dispose()
+                {
+                    Disposable.TryDispose(ref _cancel);
+                }
+            }
+
+            readonly IObservable<TSource> _source;
+            readonly SynchronizationContext _context;
 
             public SubscribeOnCtxObservable(IObservable<TSource> source, SynchronizationContext context)
             {
-                this.source = source;
-                this.context = context;
+                this._source = source;
+                this._context = context;
             }
 
             protected override IDisposable SubscribeCore(IObserver<TSource> observer)
             {
-                var subscription = new SingleAssignmentDisposable();
-                context.PostWithStartComplete(() =>
-                {
-                    if (!subscription.IsDisposed)
-                    {
-                        subscription.Disposable = new ContextDisposable(context, source.SubscribeSafe(observer));
-                    }
-                });
-                return subscription;
+                return new Subscription(_source, _context, observer);
             }
         }