Don't build the selected text just to check whether a selection exists (#22100)

* perf(text): test the selection by index instead of building it

UpdateCommandStates only needs to know whether the selection is empty, but it
called GetSelection, which allocates a substring on every selection change. Test
the indices instead, so a selection change no longer allocates.

Requested by @Gillibald in #21492.

* perf(text): read the selection properties once in HasSelection

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -539,9 +539,28 @@ namespace Avalonia.Controls
 
         private void UpdateCommandStates()
         {
-            var text = GetSelection();
+            CanCopy = HasSelection();
+        }
+
+        /// <summary>
+        /// Reports the same emptiness conditions as <see cref="GetSelection"/>, without building
+        /// the selected string.
+        /// </summary>
+        private bool HasSelection()
+        {
+            var selectionStart = SelectionStart;
+            var selectionEnd = SelectionEnd;
+            var start = Math.Min(selectionStart, selectionEnd);
+            var end = Math.Max(selectionStart, selectionEnd);
+
+            if (start == end)
+            {
+                return false;
+            }
+
+            var textLength = (HasComplexContent ? Inlines?.Text : Text)?.Length ?? 0;
 
-            CanCopy = !string.IsNullOrEmpty(text);
+            return textLength > 0 && end <= textLength;
         }
 
         private string GetSelection()

src/Avalonia.Controls/TextBox.cs

@@ -1112,10 +1112,9 @@ namespace Avalonia.Controls
 
         private void UpdateCommandStates()
         {
-            var text = GetSelection();
-            var isSelectionNullOrEmpty = string.IsNullOrEmpty(text);
-            CanCopy = !IsPasswordBox && !isSelectionNullOrEmpty;
-            CanCut = !IsPasswordBox && !isSelectionNullOrEmpty && !IsReadOnly;
+            var hasSelection = HasSelection();
+            CanCopy = !IsPasswordBox && hasSelection;
+            CanCut = !IsPasswordBox && hasSelection && !IsReadOnly;
             CanPaste = !IsReadOnly;
         }
 
@@ -2450,6 +2449,24 @@ namespace Avalonia.Controls
             return false;
         }
 
+        /// <summary>
+        /// Reports the same emptiness conditions as <see cref="GetSelection"/>, without building
+        /// the selected string.
+        /// </summary>
+        private bool HasSelection()
+        {
+            var (start, end) = GetSelectionRange();
+
+            if (start == end)
+            {
+                return false;
+            }
+
+            var textLength = Text?.Length ?? 0;
+
+            return textLength > 0 && end <= textLength;
+        }
+
         private string GetSelection()
         {
             var text = Text;

tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs

@@ -273,6 +273,51 @@ namespace Avalonia.Controls.UnitTests
             Assert.IsType<InvalidOperationException>(Record.Exception(syncContext.ExecutePostedCallbacks));
         }
 
+        [Theory]
+        [InlineData(2, 2, false)]
+        [InlineData(1, 3, true)]
+        [InlineData(3, 1, true)]
+        [InlineData(0, 4, true)]
+        public void CanCopy_Tracks_Whether_Selection_Covers_Any_Character(int start, int end, bool expected)
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock { Text = "abcd" };
+
+                target.Measure(Size.Infinity);
+
+                target.SelectionStart = start;
+                target.SelectionEnd = end;
+
+                Assert.Equal(expected, target.CanCopy);
+            }
+        }
+
+        [Fact]
+        public void CanCopy_Tracks_Selection_Over_Inlines()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock();
+
+                target.Inlines!.Add(new Run("foo"));
+                target.Inlines!.Add(new Run("bar"));
+
+                target.Measure(Size.Infinity);
+
+                Assert.False(target.CanCopy);
+
+                target.SelectionStart = 2;
+                target.SelectionEnd = 5;
+
+                Assert.True(target.CanCopy);
+
+                target.ClearSelection();
+
+                Assert.False(target.CanCopy);
+            }
+        }
+
         private static TestServices ClipboardServices
             => TestServices.MockThreadingInterface.With(
                 assetLoader: new StandardAssetLoader(),