Add StringFormat option to text cells.

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

@@ -1,4 +1,6 @@
-using Avalonia.Media;
+using System.Globalization;
+
+using Avalonia.Media;
 
 namespace Avalonia.Controls.Models.TreeDataGrid
 {
@@ -21,7 +23,8 @@ namespace Avalonia.Controls.Models.TreeDataGrid
         /// Gets the cell's text wrapping mode.
         /// </summary>
         TextWrapping TextWrapping { get; }
-        
+
+        /// <summary>
         /// Gets the cell's text alignment mode.
         /// </summary>
         TextAlignment TextAlignment { get; }

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

@@ -1,9 +1,21 @@
-using Avalonia.Media;
+using System.Globalization;
+
+using Avalonia.Media;
 
 namespace Avalonia.Controls.Models.TreeDataGrid
 {
     public interface ITextCellOptions : ICellOptions
     {
+        /// <summary>
+        /// Gets the format string to be used to format the cell value.
+        /// </summary>
+        string StringFormat { get; }
+
+        /// <summary>
+        /// Gets the culture to be used in conjunction with <see cref="StringFormat"/>.
+        /// </summary>
+        CultureInfo Culture { get; }
+
         /// <summary>
         /// Gets the text trimming mode for the cell.
         /// </summary>

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

@@ -1,7 +1,6 @@
 using System;
-using System.Collections.Generic;
 using System.ComponentModel;
-using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
 using System.Reactive.Subjects;
 using Avalonia.Data;
 using Avalonia.Media;
@@ -11,11 +10,11 @@ namespace Avalonia.Controls.Models.TreeDataGrid
     public class TextCell<T> : NotifyingBase, ITextCell, IDisposable, IEditableObject
     {
         private readonly ISubject<BindingValue<T>>? _binding;
+        private readonly ITextCellOptions? _options;
         private readonly IDisposable? _subscription;
-        [AllowNull] private T? _value;
-        [AllowNull] private T? _cancelValue;
+        private string? _editText;
+        private T? _value;
         private bool _isEditing;
-        private ITextCellOptions? _options;
 
         public TextCell(T? value)
         {
@@ -48,15 +47,31 @@ namespace Avalonia.Controls.Models.TreeDataGrid
 
         public string? Text
         {
-            get => _value?.ToString();
-            set{
-                if (string.IsNullOrEmpty(value))
+            get
+            {
+                if (_isEditing)
+                    return _editText;
+                else if (_options?.StringFormat is { } format)
+                    return string.Format(_options.Culture ?? CultureInfo.CurrentCulture, format, _value);
+                else
+                    return _value?.ToString();
+            }
+            set
+            {
+                if (_isEditing)
                 {
-                    Value = default(T?);
+                    _editText = value;
                 }
                 else
                 {
-                    Value = (T?)Convert.ChangeType(value, typeof(T));
+                    try
+                    {
+                        Value = (T?)Convert.ChangeType(value, typeof(T));
+                    }
+                    catch
+                    {
+                        // TODO: Data validation errors.
+                    }
                 }
             }
         }
@@ -78,7 +93,7 @@ namespace Avalonia.Controls.Models.TreeDataGrid
             if (!_isEditing && !IsReadOnly)
             {
                 _isEditing = true;
-                _cancelValue = Value;
+                _editText = Text;
             }
         }
 
@@ -86,19 +101,19 @@ namespace Avalonia.Controls.Models.TreeDataGrid
         {
             if (_isEditing)
             {
-                Value = _cancelValue;
                 _isEditing = false;
-                _cancelValue = default;
+                _editText = null;
             }
         }
 
         public void EndEdit()
         {
-            if (_isEditing && !EqualityComparer<T>.Default.Equals(_value, _cancelValue))
+            if (_isEditing)
             {
+                var text = _editText;
                 _isEditing = false;
-                _cancelValue = default;
-                _binding!.OnNext(_value!);
+                _editText = null;
+                Text = text;
             }
         }
 

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

@@ -1,4 +1,6 @@
-using Avalonia.Media;
+using System.Globalization;
+
+using Avalonia.Media;
 
 namespace Avalonia.Controls.Models.TreeDataGrid
 {
@@ -13,6 +15,16 @@ namespace Avalonia.Controls.Models.TreeDataGrid
         /// </summary>
         public bool IsTextSearchEnabled { get; set; }
 
+        /// <summary>
+        /// Gets or sets the format string for the cells in the column.
+        /// </summary>
+        public string StringFormat { get; set; } = "{0}";
+
+        /// <summary>
+        /// Culture info used in conjunction with <see cref="StringFormat"/>
+        /// </summary>
+        public CultureInfo Culture { get; set; } = CultureInfo.CurrentCulture;
+
         /// <summary>
         /// Gets or sets the text trimming mode for the cells in the column.
         /// </summary>

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridCell.cs

@@ -93,7 +93,10 @@ namespace Avalonia.Controls.Primitives
         protected void EndEdit()
         {
             if (EndEditCore() && Model is IEditableObject editable)
+            {
                 editable.EndEdit();
+                UpdateValue();
+            }
         }
 
         protected void SubscribeToModelChanges()
@@ -108,6 +111,10 @@ namespace Avalonia.Controls.Primitives
                 inpc.PropertyChanged -= OnModelPropertyChanged;
         }
 
+        protected virtual void UpdateValue()
+        {
+        }
+
         protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
             _treeDataGrid = this.FindLogicalAncestorOfType<TreeDataGrid>();

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridTextCell.cs

@@ -1,8 +1,8 @@
 using System.ComponentModel;
+using System.Globalization;
+using System.Reflection;
 using Avalonia.Controls.Models.TreeDataGrid;
 using Avalonia.Controls.Selection;
-using Avalonia.Input;
-using Avalonia.Interactivity;
 using Avalonia.Media;
 
 namespace Avalonia.Controls.Primitives
@@ -64,6 +64,7 @@ namespace Avalonia.Controls.Primitives
             get => _textAlignment;
             set => SetAndRaise(TextAlignmentProperty, ref _textAlignment, value);
         }
+
         public override void Realize(
             TreeDataGridElementFactory factory,
             ITreeDataGridSelectionInteraction? selection,
@@ -71,7 +72,7 @@ namespace Avalonia.Controls.Primitives
             int columnIndex,
             int rowIndex)
         {
-            Value = model.Value?.ToString();
+            Value = (model as ITextCell)?.Text;
             TextTrimming = (model as ITextCell)?.TextTrimming ?? TextTrimming.CharacterEllipsis;
             TextWrapping = (model as ITextCell)?.TextWrapping ?? TextWrapping.NoWrap;
             TextAlignment = (model as ITextCell)?.TextAlignment ?? TextAlignment.Left;
@@ -85,6 +86,11 @@ namespace Avalonia.Controls.Primitives
             base.Unrealize();
         }
 
+        protected override void UpdateValue()
+        {
+            Value = (Model as ITextCell)?.Text;
+        }
+
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);

tests/Avalonia.Controls.TreeDataGrid.Tests/Models/TextCellTests.cs

@@ -63,7 +63,7 @@ namespace Avalonia.Controls.TreeDataGridTests.Models
             target.Text = "new";
 
             Assert.Equal("new", target.Text);
-            Assert.Equal("new", target.Value);
+            Assert.Equal("initial", target.Value);
             Assert.Equal(new[] { "initial"}, result);
 
             target.EndEdit();
@@ -86,7 +86,7 @@ namespace Avalonia.Controls.TreeDataGridTests.Models
             target.Text = "new";
 
             Assert.Equal("new", target.Text);
-            Assert.Equal("new", target.Value);
+            Assert.Equal("initial", target.Value);
             Assert.Equal(new[] { "initial" }, result);
 
             target.CancelEdit();
@@ -95,5 +95,46 @@ namespace Avalonia.Controls.TreeDataGridTests.Models
             Assert.Equal("initial", target.Value);
             Assert.Equal(new[] { "initial" }, result);
         }
+
+        public class StringFormat
+        {
+            [AvaloniaFact(Timeout = 10000)]
+            public void Initial_Int_Value_Is_Formatted()
+            {
+                var binding = new BehaviorSubject<BindingValue<int>>(42);
+                var target = new TextCell<int>(binding, true, GetOptions());
+
+                Assert.Equal("42.00", target.Text);
+                Assert.Equal(42, target.Value);
+            }
+
+            [AvaloniaFact(Timeout = 10000)]
+            public void Int_Value_Is_Formatted_After_Editing()
+            {
+                var binding = new BehaviorSubject<BindingValue<int>>(42);
+                var target = new TextCell<int>(binding, false, GetOptions());
+                var result = new List<int>();
+
+                binding.Subscribe(x => result.Add(x.Value));
+
+                target.BeginEdit();
+                target.Text = "43";
+
+                Assert.Equal("43", target.Text);
+                Assert.Equal(42, target.Value);
+                Assert.Equal(new[] { 42 }, result);
+
+                target.EndEdit();
+
+                Assert.Equal("43.00", target.Text);
+                Assert.Equal(43, target.Value);
+                Assert.Equal(new[] { 42, 43 }, result);
+            }
+
+            private ITextCellOptions? GetOptions(string format = "{0:n2}")
+            {
+                return new TextColumnOptions<int> { StringFormat = format };
+            }
+        }
     }
 }