Add some unit tests for Incement / Decrement

tests/Avalonia.Controls.UnitTests/NumericUpDownTests.cs

@@ -1,8 +1,10 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
+using Avalonia.Interactivity;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Xunit;
@@ -43,6 +45,38 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Theory]
+        [MemberData(nameof(Increment_Decrement_TestData))]
+        public void Increment_Decrement_Tests(decimal min, decimal max, decimal? value, SpinDirection direction,
+            decimal? expected)
+        {
+            var control = CreateControl();
+            control.Minimum = min;
+            control.Maximum = max;
+            control.Value = value;
+
+            var spinner = GetSpinner(control);
+
+            spinner.RaiseEvent(new SpinEventArgs(Spinner.SpinEvent, direction));
+            
+            Assert.Equal(control.Value, expected);
+        }
+
+        public static IEnumerable<object[]> Increment_Decrement_TestData()
+        {
+            // if min and max are not defined and value was null, 0 should be ne new value after spin
+            yield return new object[] { decimal.MinValue, decimal.MaxValue, null, SpinDirection.Decrease, 0m };
+            yield return new object[] { decimal.MinValue, decimal.MaxValue, null, SpinDirection.Increase, 0m };
+            
+            // if no value was defined, but Min or Max are defined, use these as the new value
+            yield return new object[] { -400m, -200m, null, SpinDirection.Decrease, -200m };
+            yield return new object[] { 200m, 400m, null, SpinDirection.Increase, 200m };
+            
+            // Value should be clamped to Min / Max after spinning
+            yield return new object[] { 200m, 400m, 5m, SpinDirection.Increase, 200m };
+            yield return new object[] { 200m, 400m, 200m, SpinDirection.Decrease, 200m };
+        }
+        
         private void RunTest(Action<NumericUpDown, TextBox> test)
         {
             using (UnitTestApplication.Start(Services))
@@ -76,6 +110,14 @@ namespace Avalonia.Controls.UnitTests
                           .OfType<TextBox>()
                           .First();
         }
+        
+        private static ButtonSpinner GetSpinner(NumericUpDown control)
+        {
+            return control.GetTemplateChildren()
+                .OfType<ButtonSpinner>()
+                .First();
+        }
+        
         private static IControlTemplate CreateTemplate()
         {
             return new FuncControlTemplate<NumericUpDown>((control, scope) =>