Merge pull request #3198 from mstr2/transitioninstance

TransitionInstance with zero duration is now completed on first tick

src/Avalonia.Animation/Properties/AssemblyInfo.cs

@@ -10,3 +10,4 @@ using System.Runtime.CompilerServices;
 [assembly: XmlnsDefinition("https://github.com/avaloniaui", "Avalonia.Animation.Animators")]
 
 [assembly: InternalsVisibleTo("Avalonia.LeakTests")]
+[assembly: InternalsVisibleTo("Avalonia.Animation.UnitTests")]

src/Avalonia.Animation/TransitionInstance.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Animation
 
         private void TimerTick(TimeSpan t)
         {
-            var interpVal = (double)t.Ticks / _duration.Ticks;
+            var interpVal = _duration.Ticks == 0 ? 1d : (double)t.Ticks / _duration.Ticks;
 
             // Clamp interpolation value.
             if (interpVal >= 1d | interpVal < 0d)

tests/Avalonia.Animation.UnitTests/TransitionsTests.cs

@@ -1,14 +1,7 @@
 using System;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using Avalonia.Animation;
 using Avalonia.Controls;
-using Avalonia.Styling;
 using Avalonia.UnitTests;
-using Avalonia.Data;
 using Xunit;
-using Avalonia.Animation.Easings;
 
 namespace Avalonia.Animation.UnitTests
 {
@@ -69,5 +62,26 @@ namespace Avalonia.Animation.UnitTests
                 Assert.Equal(0, border.Opacity);
             }
         }
+
+        [Fact]
+        public void TransitionInstance_With_Zero_Duration_Is_Completed_On_First_Tick()
+        {
+            var clock = new MockGlobalClock();
+
+            using (UnitTestApplication.Start(new TestServices(globalClock: clock)))
+            {
+                int i = 0;
+                var inst = new TransitionInstance(clock, TimeSpan.Zero).Subscribe(nextValue =>
+                {
+                    switch (i++)
+                    {
+                        case 0: Assert.Equal(0, nextValue); break;
+                        case 1: Assert.Equal(1d, nextValue); break;
+                    }
+                });
+
+                clock.Pulse(TimeSpan.FromMilliseconds(10));
+            }
+        }
     }
 }