Initial implementation of UpdateSourceTrigger.

src/Avalonia.Base/Data/BindingExpressionBase.cs

@@ -17,6 +17,21 @@ public abstract class BindingExpressionBase : IDisposable, ISetterInstance
         GC.SuppressFinalize(this);
     }
 
+    /// <summary>
+    /// Sends the current binding target value to the binding source property in 
+    /// <see cref="BindingMode.TwoWay"/> or <see cref="BindingMode.OneWayToSource"/> bindings.
+    /// </summary>
+    /// <remarks>
+    /// This method does nothing when the Mode of the binding is not 
+    /// <see cref="BindingMode.TwoWay"/> or <see cref="BindingMode.OneWayToSource"/>.
+    /// 
+    /// If the UpdateSourceTrigger value of your binding is set to
+    /// <see cref="UpdateSourceTrigger.Explicit"/>, you must call the 
+    /// <see cref="UpdateSource"/> method or the changes will not propagate back to the
+    /// source.
+    /// </remarks>
+    public virtual void UpdateSource() { }
+
     /// <summary>
     /// Forces a data transfer from the binding source to the binding target.
     /// </summary>

src/Avalonia.Base/Data/Core/BindingExpression.cs

@@ -8,6 +8,8 @@ using System.Text;
 using Avalonia.Data.Converters;
 using Avalonia.Data.Core.ExpressionNodes;
 using Avalonia.Data.Core.Parsers;
+using Avalonia.Input;
+using Avalonia.Interactivity;
 using Avalonia.Logging;
 using Avalonia.Utilities;
 
@@ -49,6 +51,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     /// <param name="targetTypeConverter">
     /// A final type converter to be run on the produced value.
     /// </param>
+    /// <param name="updateSourceTrigger">The trigger for updating the source value.</param>
     public BindingExpression(
         object? source,
         IReadOnlyList<ExpressionNode> nodes,
@@ -61,11 +64,14 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         BindingPriority priority = BindingPriority.LocalValue,
         string? stringFormat = null,
         object? targetNullValue = null,
-        TargetTypeConverter? targetTypeConverter = null)
+        TargetTypeConverter? targetTypeConverter = null,
+        UpdateSourceTrigger updateSourceTrigger = UpdateSourceTrigger.PropertyChanged)
             : base(priority, enableDataValidation)
     {
         if (mode == BindingMode.Default)
             throw new ArgumentException("Binding mode cannot be Default.", nameof(mode));
+        if (updateSourceTrigger == UpdateSourceTrigger.Default)
+            throw new ArgumentException("UpdateSourceTrigger cannot be Default.", nameof(updateSourceTrigger));
 
         if (source == AvaloniaProperty.UnsetValue)
             source = null;
@@ -79,8 +85,9 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
             converterCulture is not null ||
             converterParameter is not null ||
             fallbackValue != AvaloniaProperty.UnsetValue ||
+            !string.IsNullOrWhiteSpace(stringFormat) ||
             (targetNullValue is not null && targetNullValue != AvaloniaProperty.UnsetValue) ||
-            !string.IsNullOrWhiteSpace(stringFormat))
+            updateSourceTrigger is not UpdateSourceTrigger.PropertyChanged)
         {
             _uncommon = new()
             {
@@ -88,13 +95,14 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
                 _converterCulture = converterCulture,
                 _converterParameter = converterParameter,
                 _fallbackValue = fallbackValue,
-                _targetNullValue = targetNullValue ?? AvaloniaProperty.UnsetValue,
                 _stringFormat = stringFormat switch
                 {
                     string s when string.IsNullOrWhiteSpace(s) => null,
                     string s when !s.Contains('{') => $"{{0:{stringFormat}}}",
                     _ => stringFormat,
                 },
+                _targetNullValue = targetNullValue ?? AvaloniaProperty.UnsetValue,
+                _updateSourceTrigger = updateSourceTrigger,
             };
         }
 
@@ -127,9 +135,16 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     public CultureInfo ConverterCulture => _uncommon?._converterCulture ?? CultureInfo.CurrentCulture;
     public object? ConverterParameter => _uncommon?._converterParameter;
     public object? FallbackValue => _uncommon is not null ? _uncommon._fallbackValue : AvaloniaProperty.UnsetValue;
-    public object? TargetNullValue => _uncommon?._targetNullValue ?? AvaloniaProperty.UnsetValue;
     public ExpressionNode LeafNode => _nodes[_nodes.Count - 1];
     public string? StringFormat => _uncommon?._stringFormat;
+    public object? TargetNullValue => _uncommon?._targetNullValue ?? AvaloniaProperty.UnsetValue;
+    public UpdateSourceTrigger UpdateSourceTrigger => _uncommon?._updateSourceTrigger ?? UpdateSourceTrigger.PropertyChanged;
+
+    public override void UpdateSource()
+    {
+        if (_mode is BindingMode.TwoWay or BindingMode.OneWayToSource)
+            WriteTargetValueToSource();
+    }
 
     public override void UpdateTarget()
     {
@@ -353,7 +368,12 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
                 if (_mode is BindingMode.OneWayToSource)
                     PublishValue(target.GetValue(TargetProperty));
 
-                target.PropertyChanged += OnTargetPropertyChanged;
+                var trigger = UpdateSourceTrigger;
+
+                if (trigger is UpdateSourceTrigger.PropertyChanged)
+                    target.PropertyChanged += OnTargetPropertyChanged;
+                else if (trigger is UpdateSourceTrigger.LostFocus && target is IInputElement ie)
+                    ie.LostFocus += OnTargetLostFocus;
             }
         }
         else
@@ -370,7 +390,12 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         if (_mode is BindingMode.TwoWay or BindingMode.OneWayToSource &&
             TryGetTarget(out var target))
         {
-            target.PropertyChanged -= OnTargetPropertyChanged;
+            var trigger = UpdateSourceTrigger;
+
+            if (trigger is UpdateSourceTrigger.PropertyChanged)
+                target.PropertyChanged += OnTargetPropertyChanged;
+            else if (trigger is UpdateSourceTrigger.LostFocus && target is IInputElement ie)
+                ie.LostFocus += OnTargetLostFocus;
         }
     }
 
@@ -450,7 +475,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
 
     private void WriteTargetValueToSource()
     {
-        Debug.Assert(_mode == BindingMode.OneWayToSource);
+        Debug.Assert(_mode is BindingMode.TwoWay or BindingMode.OneWayToSource);
 
         if (TryGetTarget(out var target) &&
             TargetProperty is not null &&
@@ -461,14 +486,20 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         }
     }
 
+    private void OnTargetLostFocus(object? sender, RoutedEventArgs e)
+    {
+        Debug.Assert(UpdateSourceTrigger is UpdateSourceTrigger.LostFocus);
+
+        WriteTargetValueToSource();
+    }
+
     private void OnTargetPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
     {
         Debug.Assert(_mode is BindingMode.TwoWay or BindingMode.OneWayToSource);
+        Debug.Assert(UpdateSourceTrigger is UpdateSourceTrigger.PropertyChanged);
 
         if (e.Property == TargetProperty)
-        {
             WriteValueToSource(e.NewValue);
-        }
     }
 
     private object? ConvertFallback(object? fallback, string fallbackName)
@@ -512,5 +543,6 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         public object? _fallbackValue;
         public string? _stringFormat;
         public object? _targetNullValue;
+        public UpdateSourceTrigger _updateSourceTrigger;
     }
 }

src/Avalonia.Base/Data/UpdateSourceTrigger.cs

@@ -0,0 +1,29 @@
+namespace Avalonia.Data;
+
+/// <summary>
+/// Describes the timing of binding source updates.
+/// </summary>
+public enum UpdateSourceTrigger
+{
+    /// <summary>
+    /// The default <see cref="UpdateSourceTrigger"/> value of the binding target property.
+    /// This currently defaults to <see cref="PropertyChanged"/>.
+    /// </summary>
+    Default,
+
+    /// <summary>
+    /// Updates the binding source immediately whenever the binding target property changes.
+    /// </summary>
+    PropertyChanged,
+
+    /// <summary>
+    /// Updates the binding source whenever the binding target element loses focus.
+    /// </summary>
+    LostFocus,
+
+    /// <summary>
+    /// Updates the binding source only when you call the 
+    /// <see cref="BindingExpressionBase.UpdateSource()"/> method.
+    /// </summary>
+    Explicit,
+}

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindingExtension.cs

@@ -114,7 +114,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                 ? sn.SelectSource(Source, target, DefaultAnchor?.Target)
                 : Source != AvaloniaProperty.UnsetValue ? Source : target;
 
-           return new BindingExpression(
+            var (mode, trigger) = ResolveDefaultsFromMetadata(target, targetProperty);
+
+            return new BindingExpression(
                 source,
                 nodes,
                 FallbackValue,
@@ -122,10 +124,11 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                 converterCulture: ConverterCulture,
                 converterParameter: ConverterParameter,
                 enableDataValidation: enableDataValidation,
-                mode: ResolveBindingMode(target, targetProperty),
+                mode: mode,
                 stringFormat: StringFormat,
                 targetNullValue: TargetNullValue,
-                targetTypeConverter: TargetTypeConverter.GetDefaultConverter());
+                targetTypeConverter: TargetTypeConverter.GetDefaultConverter(),
+                updateSourceTrigger: trigger);
         }
     }
 }

src/Markup/Avalonia.Markup/Data/Binding.cs

@@ -156,6 +156,8 @@ namespace Avalonia.Data
                 sn.SelectSource(Source, target, DefaultAnchor?.Target) :
                 Source != AvaloniaProperty.UnsetValue ? Source : target;
 
+            var (mode, trigger) = ResolveDefaultsFromMetadata(target, targetProperty);
+
             return new BindingExpression(
                 source,
                 nodes,
@@ -164,10 +166,11 @@ namespace Avalonia.Data
                 converterCulture: ConverterCulture,
                 converterParameter: ConverterParameter,
                 enableDataValidation: enableDataValidation,
-                mode: ResolveBindingMode(target, targetProperty),
+                mode: mode,
                 stringFormat: StringFormat,
                 targetNullValue: TargetNullValue,
-                targetTypeConverter: TargetTypeConverter.GetReflectionConverter());
+                targetTypeConverter: TargetTypeConverter.GetReflectionConverter(),
+                updateSourceTrigger: trigger);
         }
 
         private INameScope? GetNameScope()

src/Markup/Avalonia.Markup/Data/BindingBase.cs

@@ -75,6 +75,12 @@ namespace Avalonia.Data
         /// </summary>
         public string? StringFormat { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value that determines the timing of binding source updates for
+        /// <see cref="BindingMode.TwoWay"/> and <see cref="BindingMode.OneWayToSource"/> bindings.
+        /// </summary>
+        public UpdateSourceTrigger UpdateSourceTrigger { get; set; }
+
         public WeakReference? DefaultAnchor { get; set; }
 
         public WeakReference<INameScope?>? NameScope { get; set; }
@@ -90,13 +96,23 @@ namespace Avalonia.Data
 
         private protected abstract BindingExpressionBase Instance(AvaloniaProperty targetProperty, AvaloniaObject target);
 
-        private protected BindingMode ResolveBindingMode(AvaloniaObject target, AvaloniaProperty? targetProperty)
+        private protected (BindingMode, UpdateSourceTrigger) ResolveDefaultsFromMetadata(
+            AvaloniaObject target,
+            AvaloniaProperty? targetProperty)
         {
-            if (Mode != BindingMode.Default)
-                return Mode;
-            if (targetProperty is null)
-                return BindingMode.OneWay;
-            return targetProperty.GetMetadata(target.GetType()).DefaultBindingMode;
+            var mode = Mode;
+            var trigger = UpdateSourceTrigger == UpdateSourceTrigger.Default ?
+                UpdateSourceTrigger.PropertyChanged : UpdateSourceTrigger;
+
+            if (mode == BindingMode.Default)
+            {
+                if (targetProperty?.GetMetadata(target.GetType()) is { } metadata)
+                    mode = metadata.DefaultBindingMode;
+                else
+                    mode = BindingMode.OneWay;
+            }
+
+            return (mode, trigger);
         }
 
         BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty property) => Instance(property, target);

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests_UpdateSourceTrigger.cs

@@ -0,0 +1,150 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Data;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Data.Core
+{
+    [InvariantCulture]
+    public class BindingExpressionTests_UpdateSourceTrigger
+    {
+        [Fact]
+        public void TwoWay_PropertyChanged_Should_Update_Source_On_Property_Changed()
+        {
+            using var app = Start();
+            var data = new ViewModel();
+            var binding = new Binding
+            {
+                Path = "Foo",
+                UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged,
+            };
+
+            var target = new TextBox { DataContext = data };
+            target.Bind(TextBox.TextProperty, binding);
+
+            var root = new TestRoot(target);
+
+            Assert.Equal("foo", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            target.Text = "bar";
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("bar", data.Foo);
+        }
+
+        [Fact]
+        public void TwoWay_LostFocus_Should_Update_Source_On_Lost_Focus()
+        {
+            using var app = Start();
+            var data = new ViewModel();
+            var binding = new Binding
+            {
+                Path = "Foo",
+                UpdateSourceTrigger = UpdateSourceTrigger.LostFocus,
+            };
+
+            var target = new TextBox { DataContext = data };
+            target.Bind(TextBox.TextProperty, binding);
+
+            var root = new TestRoot(target);
+            root.Focusable = true;
+            target.Focus();
+
+            Assert.Equal("foo", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            target.Text = "bar";
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            root.Focus();
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("bar", data.Foo);
+        }
+
+        [Fact]
+        public void OneWayToSource_LostFocus_Should_Update_Source_On_Lost_Focus()
+        {
+            using var app = Start();
+            var data = new ViewModel();
+            var binding = new Binding
+            {
+                Path = "Foo",
+                Mode = BindingMode.OneWayToSource,
+                UpdateSourceTrigger = UpdateSourceTrigger.LostFocus,
+            };
+
+            var target = new TextBox { DataContext = data };
+            target.Bind(TextBox.TextProperty, binding);
+
+            var root = new TestRoot(target);
+            root.Focusable = true;
+            target.Focus();
+
+            Assert.Null(target.Text);
+            Assert.Null(data.Foo);
+
+            target.Text = "bar";
+
+            Assert.Equal("bar", target.Text);
+            Assert.Null(data.Foo);
+
+            root.Focus();
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("bar", data.Foo);
+        }
+
+        [Fact]
+        public void TwoWay_Explicit_Should_Update_Source_On_Call_To_UpdateSource()
+        {
+            using var app = Start();
+            var data = new ViewModel();
+            var binding = new Binding
+            {
+                Path = "Foo",
+                UpdateSourceTrigger = UpdateSourceTrigger.Explicit,
+            };
+
+            var target = new TextBox { DataContext = data };
+            var expression = target.Bind(TextBox.TextProperty, binding);
+
+            var root = new TestRoot(target);
+            root.Focusable = true;
+            target.Focus();
+
+            Assert.Equal("foo", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            target.Text = "bar";
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            root.Focus();
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("foo", data.Foo);
+
+            expression.UpdateSource();
+
+            Assert.Equal("bar", target.Text);
+            Assert.Equal("bar", data.Foo);
+        }
+
+        private static IDisposable Start()
+        {
+            return UnitTestApplication.Start(TestServices.RealFocus);
+        }
+
+        private class ViewModel
+        {
+            public ViewModel(string foo = "foo") => Foo = foo;
+            public string Foo { get; set; }
+        }
+    }
+}