Fix: Enable seamless forward and backward transition in VirtualizingCarouselPanel (#14125)

* Update VirtualizingCarouselPanel.cs

Changes the calculation of the "forward" flag for the page transition to enable seamless forward and backward transitions.

* Update VirtualizingCarouselPanel.cs

Added Check for more than 2 items in carousel as there is actually no way to determine the correct transition with only 2 items. If there are only 2 items the transition behaviour is not changed.

* Update VirtualizingCarouselPanelTests.cs

Added Unit Tests for cycling through list forward and backward

src/Avalonia.Controls/VirtualizingCarouselPanel.cs

@@ -158,7 +158,15 @@ namespace Avalonia.Controls
                 GetTransition() is { } transition)
             {
                 _transition = new CancellationTokenSource();
-                transition.Start(_transitionFrom, to, _realizedIndex > _transitionFromIndex, _transition.Token)
+
+                var forward = (_realizedIndex > _transitionFromIndex);
+                if (Items.Count > 2)
+                {
+                    forward = forward || (_transitionFromIndex == Items.Count - 1 && _realizedIndex == 0);
+                    forward = forward && !(_transitionFromIndex == 0 && _realizedIndex == Items.Count - 1);
+                }
+
+                transition.Start(_transitionFrom, to, forward, _transition.Token)
                     .ContinueWith(TransitionFinished, TaskScheduler.FromCurrentSynchronizationContext());
             }
 

tests/Avalonia.Controls.UnitTests/VirtualizingCarouselPanelTests.cs

@@ -147,6 +147,55 @@ namespace Avalonia.Controls.UnitTests
                     Times.Once);
             }
 
+            [Fact]
+            public void Changing_SelectedIndex_transitions_forward_cycle()
+            {
+                using var app = Start();
+                var items = new Control[] { new Button(), new Canvas(), new Label() };
+                var transition = new Mock<IPageTransition>();
+                var (target, carousel) = CreateTarget(items, transition.Object);
+                var cycleindexes = new[] { 1, 2, 0};
+
+                for (int cycleIndex = 0; cycleIndex < cycleindexes.Length; cycleIndex++)
+                {
+                    carousel.SelectedIndex = cycleindexes[cycleIndex];
+                    Layout(target);
+
+                    var index = cycleIndex;
+                    transition.Verify(x => x.Start(
+                            index > 0 ? items[cycleindexes[index - 1]] : items[0],
+                            items[cycleindexes[index]],
+                            true,
+                            It.IsAny<CancellationToken>()),
+                        Times.Once);
+                }
+            }
+
+            [Fact]
+            public void Changing_SelectedIndex_transitions_backward_cycle()
+            {
+                using var app = Start();
+                var items = new Control[] { new Button(), new Canvas(), new Label() };
+                var transition = new Mock<IPageTransition>();
+                var (target, carousel) = CreateTarget(items, transition.Object);
+
+                var cycleindexes = new[] { 2, 1, 0};
+
+                for (int cycleIndex = 0; cycleIndex < cycleindexes.Length; cycleIndex++)
+                {
+                    carousel.SelectedIndex = cycleindexes[cycleIndex];
+                    Layout(target);
+
+                    var index = cycleIndex;
+                    transition.Verify(x => x.Start(
+                            index > 0 ? items[cycleindexes[index - 1]] : items[0],
+                            items[cycleindexes[index]],
+                            false,
+                            It.IsAny<CancellationToken>()),
+                        Times.Once);
+                }
+            }
+
             [Fact]
             public void TransitionFrom_Control_Is_Recycled_When_Transition_Completes()
             {