Added CollapseAll.

Author: grokys

src/Avalonia.Controls.TreeDataGrid/HierarchicalTreeDataGridSource.cs

@@ -105,6 +105,7 @@ public void Dispose()
         }
 
         public void Collapse(IndexPath index) => GetOrCreateRows().Collapse(index);
+        public void CollapseAll() => GetOrCreateRows().CollapseAll();
         public void Expand(IndexPath index) => GetOrCreateRows().Expand(index);
         public void ExpandAll() => GetOrCreateRows().ExpandRecursive(null);
         public void ExpandRecursive(Func<TModel, bool> filter) => GetOrCreateRows().ExpandRecursive(filter);

src/Avalonia.Controls.TreeDataGrid/Models/TreeDataGrid/HierarchicalRows.cs

@@ -131,6 +131,31 @@ public void Collapse(IndexPath index)
             }
         }
 
+        internal void CollapseAll()
+        {
+            static void Collapse(IReadOnlyList<HierarchicalRow<TModel>> rows)
+            {
+                for (var i = 0; i < rows.Count; ++i)
+                {
+                    var row = rows[i];
+
+                    if (row.Children is { } children)
+                        Collapse(children);
+
+                    row.IsExpanded = false;
+                }
+            }
+
+            _ignoreCollectionChanges = true;
+
+            try { Collapse(_roots); }
+            finally { _ignoreCollectionChanges = false; }
+
+            _flattenedRows.Clear();
+            InitializeRows();
+            CollectionChanged?.Invoke(this, CollectionExtensions.ResetEvent);
+        }
+
         public (int index, double y) GetRowAt(double y)
         {
             if (MathUtilities.IsZero(y))

tests/Avalonia.Controls.TreeDataGrid.Tests/HierarchicalTreeDataGridSourceTests.cs

@@ -467,6 +467,28 @@ public void ExpandAll_Expands_All_Rows(bool sorted)
 
                 Assert.Equal(65, target.Rows.Count);
             }
+
+            [AvaloniaTheory(Timeout = 10000)]
+            [InlineData(false)]
+            [InlineData(true)]
+            public void CollapseAll_Collapses_All_Rows(bool sorted)
+            {
+                var data = CreateData(5, 3, 3);
+                var target = CreateTarget(data, sorted);
+
+                // We need to expand before we can collapse.
+                target.ExpandAll();
+                Assert.Equal(65, target.Rows.Count);
+
+                // Now we can test collapsing.
+                target.CollapseAll();
+                Assert.Equal(5, target.Rows.Count);
+
+                // Ensure that nested rows were collapsed, i.e. only the first level of rows is
+                // visible after expanding now.
+                target.Expand(0);
+                Assert.Equal(8, target.Rows.Count);
+            }
         }
 
         public class ExpansionBinding