Fixed that .SortAndPage() would not send a downstream changeset upon change of the comprer, when the current page includes all items. (#967)

Author: JakenVeina

src/DynamicData.Tests/Cache/SortAndPageFixture.cs

@@ -40,8 +40,32 @@ public void ChangeComparer()
         // change the comparer 
         _comparerSubject.OnNext(_descComparer);
 
+        Aggregator.Messages.Cast<IChangeSet<Person, string, PageContext<Person>>>().LastOrDefault().Context.Comparer.Should().Be(_descComparer);
         expectedResult = people.OrderBy(p => p, _descComparer).Take(25).ToList();
-        actualResult = Aggregator.Data.Items.OrderBy(p => p, Comparer);
+        actualResult = Aggregator.Data.Items.OrderBy(p => p, _descComparer);
+        actualResult.Should().BeEquivalentTo(expectedResult);
+    }
+
+    [Fact]
+    public void ChangeComparerWithOnlyOnePage()
+    {
+        PageRequests.OnNext(new PageRequest(page: 1, size: 200));
+        
+        var people = Enumerable.Range(1, 100).Select(i => new Person($"P{i:000}", i)).OrderBy(p => Guid.NewGuid());
+        Source.AddOrUpdate(people);
+
+        // for first batch, it should use the results of the _PageRequests subject (if a behaviour subject is used).
+        var expectedResult = people.OrderBy(p => p, Comparer).ToList();
+        var actualResult = Aggregator.Data.Items.OrderBy(p => p, Comparer);
+        actualResult.Should().BeEquivalentTo(expectedResult);
+        var changesetCount = Aggregator.Messages.Count;
+
+        // change the comparer 
+        _comparerSubject.OnNext(_descComparer);
+
+        Aggregator.Messages.Cast<IChangeSet<Person, string, PageContext<Person>>>().LastOrDefault().Context.Comparer.Should().Be(_descComparer);
+        expectedResult = people.OrderBy(p => p, _descComparer).ToList();
+        actualResult = Aggregator.Data.Items.OrderBy(p => p, _descComparer);
         actualResult.Should().BeEquivalentTo(expectedResult);
     }
 }

src/DynamicData/Cache/Internal/SortAndPage.cs

@@ -110,12 +110,11 @@ public IObservable<IChangeSet<TObject, TKey, PageContext<TObject>>> Run() =>
                         return ApplyPagedChanges(changes);
                     });
 
-                return
-                    comparerChanged
-                        .Merge(paramsChanged)
-                        .Merge(dataChange)
-                        .Where(changes => changes.Count is not 0)
-                        .SubscribeSafe(observer);
+                return Observable.Merge(
+                        comparerChanged.Skip(1),
+                        paramsChanged.Where(changes => changes.Count is not 0),
+                        dataChange.Where(changes => changes.Count is not 0))
+                    .SubscribeSafe(observer);
 
                 ChangeSet<TObject, TKey, PageContext<TObject>> ApplyPagedChanges(IChangeSet<TObject, TKey>? changeSet = null)
                 {