Handle null Items in SelectingItemsControl.

Perspex.Controls/Primitives/SelectingItemsControl.cs

@@ -18,7 +18,7 @@ namespace Perspex.Controls.Primitives
             PerspexProperty.Register<SelectingItemsControl, int>("SelectedIndex", coerce: CoerceSelectedIndex);
 
         public static readonly PerspexProperty<object> SelectedItemProperty =
-            PerspexProperty.Register<SelectingItemsControl, object>("SelectedItem");
+            PerspexProperty.Register<SelectingItemsControl, object>("SelectedItem", coerce: CoerceSelectedItem);
 
         static SelectingItemsControl()
         {
@@ -30,7 +30,16 @@ namespace Perspex.Controls.Primitives
 
                 if (control != null)
                 {
-                    control.SelectedItem = control.Items.ElementAt((int)x.NewValue);
+                    var index = (int)x.NewValue;
+
+                    if (index == -1)
+                    {
+                        control.SelectedItem = null;
+                    }
+                    else
+                    {
+                        control.SelectedItem = control.Items.ElementAt((int)x.NewValue);
+                    }
                 }
             });
 
@@ -156,8 +165,32 @@ namespace Perspex.Controls.Primitives
                 }
                 else if (value > -1)
                 {
-                    var count = control.Items.Count();
-                    return Math.Min(value, count - 1);
+                    var items = control.Items;
+
+                    if (items != null)
+                    {
+                        var count = items.Count();
+                        return Math.Min(value, count - 1);
+                    }
+                    else
+                    {
+                        return -1;
+                    }
+                }
+            }
+
+            return value;
+        }
+
+        private static object CoerceSelectedItem(PerspexObject o, object value)
+        {
+            var control = o as SelectingItemsControl;
+
+            if (control != null)
+            {
+                if (value != null && (control.Items == null || control.Items.IndexOf(value) == -1))
+                {
+                    return -1;
                 }
             }
 
@@ -189,7 +222,12 @@ namespace Perspex.Controls.Primitives
             }
             else
             {
-                this.SelectedIndex = this.Items.IndexOf(selected);
+                var items = this.Items;
+
+                if (items != null)
+                {
+                    this.SelectedIndex = items.IndexOf(selected);
+                }
             }
         }
     }

Tests/Perspex.Controls.UnitTests/Primitives/SelectingItemsControlTests.cs

@@ -25,8 +25,11 @@ namespace Perspex.Controls.Primitives.UnitTests
             var target = new Target
             {
                 Items = items,
+                Template = this.Template(),
             };
 
+            target.ApplyTemplate();
+
             Assert.False(items[0].IsSelected);
             Assert.False(items[1].IsSelected);
         }
@@ -43,9 +46,9 @@ namespace Perspex.Controls.Primitives.UnitTests
             var target = new Target
             {
                 Items = items,
+                Template = this.Template(),
             };
 
-            target.Template = this.Template();
             target.ApplyTemplate();
             target.SelectedItem = items[1];
 
@@ -65,15 +68,43 @@ namespace Perspex.Controls.Primitives.UnitTests
             var target = new Target
             {
                 Items = items,
+                Template = this.Template(),
             };
 
-            target.Template = this.Template();
             target.ApplyTemplate();
             target.SelectedItem = items[1];
 
+            Assert.Equal(items[1], target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
         }
 
+        [Fact]
+        public void Setting_SelectedItem_To_Not_Present_Item_Should_Clear_Selection()
+        {
+            var items = new[]
+            {
+                new Item(),
+                new Item(),
+            };
+
+            var target = new Target
+            {
+                Items = items,
+                Template = this.Template(),
+            };
+
+            target.ApplyTemplate();
+            target.SelectedItem = items[1];
+
+            Assert.Equal(items[1], target.SelectedItem);
+            Assert.Equal(1, target.SelectedIndex);
+
+            target.SelectedItem = new Item();
+
+            Assert.Equal(null, target.SelectedItem);
+            Assert.Equal(-1, target.SelectedIndex);
+        }
+
         [Fact]
         public void Setting_SelectedIndex_Should_Set_SelectedItem()
         {
@@ -86,9 +117,9 @@ namespace Perspex.Controls.Primitives.UnitTests
             var target = new Target
             {
                 Items = items,
+                Template = this.Template(),
             };
 
-            target.Template = this.Template();
             target.ApplyTemplate();
             target.SelectedIndex = 1;
 
@@ -107,15 +138,41 @@ namespace Perspex.Controls.Primitives.UnitTests
             var target = new Target
             {
                 Items = items,
+                Template = this.Template(),
             };
 
-            target.Template = this.Template();
             target.ApplyTemplate();
             target.SelectedIndex = 2;
 
             Assert.Equal(1, target.SelectedIndex);
         }
 
+        [Fact]
+        public void Setting_SelectedIndex_With_No_Items_Should_Not_Throw_Exception()
+        {
+            var target = new Target
+            {
+                Template = this.Template(),
+            };
+
+            target.ApplyTemplate();
+            target.SelectedIndex = 2;
+
+            Assert.Equal(-1, target.SelectedIndex);
+        }
+
+        [Fact]
+        public void Setting_SelectedItem_With_No_Items_Should_Not_Throw_Exception()
+        {
+            var target = new Target
+            {
+                Template = this.Template(),
+            };
+
+            target.ApplyTemplate();
+            target.SelectedItem = new Item();
+        }
+
         [Fact]
         public void PointerPressed_Event_Should_Be_Handled()
         {