From 44cf7f24db15c9f94c33f3ce0bf00d7516f59e55 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 5 Feb 2020 12:44:42 +0100
Subject: [PATCH] Expose API for batch updates.

---
 src/Avalonia.Controls/SelectionModel.cs       | 31 ++++++++++++++-----
 .../SelectionModelTests.cs                    | 27 ++++++++++++++++
 2 files changed, 51 insertions(+), 7 deletions(-)

diff --git a/src/Avalonia.Controls/SelectionModel.cs b/src/Avalonia.Controls/SelectionModel.cs
index c0ae67ff91..bc650907d9 100644
--- a/src/Avalonia.Controls/SelectionModel.cs
+++ b/src/Avalonia.Controls/SelectionModel.cs
@@ -17,6 +17,7 @@ namespace Avalonia.Controls
     {
         private readonly SelectionNode _rootNode;
         private bool _singleSelect;
+        private int _operationCount;
         private IReadOnlyList<IndexPath>? _selectedIndicesCached;
         private IReadOnlyList<object?>? _selectedItemsCached;
         private SelectionModelChildrenRequestedEventArgs? _childrenRequestedEventArgs;
@@ -509,6 +510,8 @@ namespace Avalonia.Controls
             ClearSelection(resetAnchor: true);
         }
 
+        public IDisposable Update() => new Operation(this);
+
         protected void OnPropertyChanged(string propertyName)
         {
             RaisePropertyChanged(propertyName);
@@ -732,19 +735,33 @@ namespace Avalonia.Controls
                 });
         }
 
-        private void BeginOperation() => _rootNode.BeginOperation();
+        private void BeginOperation()
+        {
+            if (_operationCount++ == 0)
+            {
+                _rootNode.BeginOperation();
+            }
+        }
 
         private void EndOperation()
         {
-            var changes = new List<SelectionNodeOperation>();
-            _rootNode.EndOperation(changes);
+            if (_operationCount == 0)
+            {
+                throw new AvaloniaInternalException("No selection operation in progress.");
+            }
 
             SelectionModelSelectionChangedEventArgs? e = null;
-            
-            if (changes.Count > 0)
+
+            if (--_operationCount == 0)
             {
-                var changeSet = new SelectionModelChangeSet(changes);
-                e = changeSet.CreateEventArgs();
+                var changes = new List<SelectionNodeOperation>();
+                _rootNode.EndOperation(changes);
+
+                if (changes.Count > 0)
+                {
+                    var changeSet = new SelectionModelChangeSet(changes);
+                    e = changeSet.CreateEventArgs();
+                }
             }
 
             OnSelectionChanged(e);
diff --git a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
index 1950da2818..c76dc890a9 100644
--- a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
@@ -1466,6 +1466,33 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(1, raised);
         }
 
+        [Fact]
+        public void Can_Batch_Update()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(1);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Equal(new[] { new IndexPath(1) }, e.DeselectedIndices);
+                Assert.Equal(new object[] { 1 }, e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(4) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 4 }, e.SelectedItems);
+                ++raised;
+            };
+
+            using (target.Update())
+            {
+                target.Deselect(1);
+                target.Select(4);
+            }
+
+            Assert.Equal(1, raised);
+        }
+
         private int GetSubscriberCount(AvaloniaList<object> list)
         {
             return ((INotifyCollectionChangedDebug)list).GetCollectionChangedSubscribers()?.Length ?? 0;