fix #2866 CanExecute Scheduler (#2880)

Author: ChrisPulman

src/ReactiveUI.Tests/Commands/ReactiveCommandTest.cs

@@ -9,6 +9,7 @@
 using System.Reactive.Concurrency;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
+using System.Threading;
 using System.Threading.Tasks;
 using System.Windows.Input;
 using DynamicData;
@@ -1183,5 +1184,29 @@ public async Task ReactiveCommandCreateFromTaskHandlesTaskExceptionAsync()
             Assert.False(isExecuting);
             Assert.Equal("break execution", fail?.Message);
         }
+
+        [Fact]
+        public async Task ReactiveCommandExecutesFromInvokeCommand()
+        {
+            var semaphore = new SemaphoreSlim(0);
+            var command = ReactiveCommand.Create(() => semaphore.Release());
+
+            Observable.Return(Unit.Default)
+                      .InvokeCommand(command);
+
+            var result = 0;
+            var task = semaphore.WaitAsync();
+            if (await Task.WhenAny(Task.Delay(TimeSpan.FromMilliseconds(100)), task).ConfigureAwait(true) == task)
+            {
+                result = 1;
+            }
+            else
+            {
+                result = -1;
+            }
+
+            await Task.Delay(200).ConfigureAwait(false);
+            Assert.Equal(1, result);
+        }
     }
 }

src/ReactiveUI/ReactiveCommand/ReactiveCommand.cs

@@ -766,13 +766,14 @@ protected internal ReactiveCommand(
                 .CombineLatest(_isExecuting, (canEx, isEx) => canEx && !isEx)
                 .DistinctUntilChanged()
                 .Replay(1)
-                .RefCount()
-                .ObserveOn(_canExecuteScheduler);
+                .RefCount();
 
             _results = _synchronizedExecutionInfo.Where(x => x.Demarcation == ExecutionDemarcation.Result)
                 .Select(x => x.Result);
 
-            _canExecuteSubscription = _canExecute.Subscribe(OnCanExecuteChanged);
+            _canExecuteSubscription = _canExecute
+                .ObserveOn(_canExecuteScheduler)
+                .Subscribe(OnCanExecuteChanged);
         }
 
         private enum ExecutionDemarcation