Implemented MultiBindingExpression

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -375,7 +375,7 @@ public BindingAdaptor(IObservable<object?> source)
                 return new InstancedBinding(expression, BindingMode.OneWay, BindingPriority.LocalValue);
             }
 
-            BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty property, object? anchor)
+            BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty? property, object? anchor)
             {
                 return new UntypedObservableBindingExpression(_source, BindingPriority.LocalValue);
             }

src/Avalonia.Base/Data/Core/IBinding2.cs

@@ -15,6 +15,6 @@ internal interface IBinding2 : IBinding
 {
     BindingExpressionBase Instance(
         AvaloniaObject target,
-        AvaloniaProperty targetProperty,
+        AvaloniaProperty? targetProperty,
         object? anchor);
 }

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

@@ -0,0 +1,133 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace Avalonia.Data.Core;
+
+internal class MultiBindingExpression : UntypedBindingExpressionBase, IBindingExpressionSink
+{
+    private static readonly object s_uninitialized = new object();
+    private readonly IBinding[] _bindings;
+    private readonly IMultiValueConverter? _converter;
+    private readonly CultureInfo? _converterCulture;
+    private readonly object? _converterParameter;
+    private readonly UntypedBindingExpressionBase?[] _expressions;
+    private readonly object? _fallbackValue;
+    private readonly object? _targetNullValue;
+    private readonly object?[] _values;
+    private readonly ReadOnlyCollection<object?> _valuesView;
+
+    public MultiBindingExpression(
+        BindingPriority priority,
+        IList<IBinding> bindings,
+        IMultiValueConverter? converter,
+        CultureInfo? converterCulture,
+        object? converterParameter,
+        object? fallbackValue,
+        object? targetNullValue)
+            : base(priority)
+    {
+        _bindings = [.. bindings];
+        _converter = converter;
+        _converterCulture = converterCulture;
+        _converterParameter = converterParameter;
+        _expressions = new UntypedBindingExpressionBase[_bindings.Length];
+        _fallbackValue = fallbackValue;
+        _targetNullValue = targetNullValue;
+        _values = new object?[_bindings.Length];
+        _valuesView = new(_values);
+
+#if NETSTANDARD2_0
+        for (var i = 0; i < _bindings.Length; ++i)
+            _values[i] = s_uninitialized;
+#else
+        Array.Fill(_values, s_uninitialized);
+#endif
+    }
+
+    public override string Description => "MultiBinding";
+
+    protected override void StartCore()
+    {
+        if (!TryGetTarget(out var target))
+            throw new AvaloniaInternalException("MultiBindingExpression has no target.");
+
+        for (var i = 0; i < _bindings.Length; ++i)
+        {
+            var binding = _bindings[i]; 
+
+            if (binding is not IBinding2 b)
+                throw new NotSupportedException($"Unsupported IBinding implementation '{binding}'.");
+
+            var expression = b.Instance(target, null, null);
+
+            if (expression is not UntypedBindingExpressionBase e)
+                throw new NotSupportedException($"Unsupported BindingExpressionBase implementation '{expression}'.");
+
+            _expressions[i] = e;
+            e.AttachAndStart(this, target, null, Priority);
+        }
+    }
+
+    protected override void StopCore()
+    {
+        for (var i = 0; i < _expressions.Length; ++i)
+        {
+            _expressions[i]?.Dispose();
+            _expressions[i] = null;
+            _values[i] = s_uninitialized;
+        }
+    }
+
+    void IBindingExpressionSink.OnChanged(
+        UntypedBindingExpressionBase instance,
+        bool hasValueChanged,
+        bool hasErrorChanged,
+        object? value,
+        BindingError? error)
+    {
+        var i = Array.IndexOf(_expressions, instance);
+        Debug.Assert(i != -1);
+
+        _values[i] = BindingNotification.ExtractValue(value);
+        PublishValue();
+    }
+
+    void IBindingExpressionSink.OnCompleted(UntypedBindingExpressionBase instance)
+    {
+        // Nothing to do here.
+    }
+
+    private void PublishValue()
+    {
+        foreach (var v in _values)
+        {
+            if (v == s_uninitialized)
+                return;
+        }
+
+        if (_converter is not null)
+        {
+            var culture = _converterCulture ?? CultureInfo.CurrentCulture;
+            var converted = _converter.Convert(_valuesView, TargetType, _converterParameter, culture);
+
+            converted = BindingNotification.ExtractValue(converted);
+
+            if (converted != BindingOperations.DoNothing)
+            {
+                if (converted == null)
+                    converted = _targetNullValue;
+                if (converted == AvaloniaProperty.UnsetValue)
+                    converted = _fallbackValue;
+                PublishValue(converted);
+            }
+        }
+        else
+        {
+            PublishValue(_valuesView);
+        }
+    }
+}

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

@@ -204,7 +204,7 @@ internal override void Attach(
     internal void AttachAndStart(
         IBindingExpressionSink subscriber,
         AvaloniaObject target,
-        AvaloniaProperty targetProperty,
+        AvaloniaProperty? targetProperty,
         BindingPriority priority)
     {
         AttachCore(subscriber, null, target, targetProperty, priority);
@@ -261,7 +261,7 @@ private void AttachCore(
         IBindingExpressionSink sink,
         ImmediateValueFrame? frame,
         AvaloniaObject target,
-        AvaloniaProperty targetProperty,
+        AvaloniaProperty? targetProperty,
         BindingPriority priority)
     {
         if (_sink is not null)
@@ -273,7 +273,7 @@ private void AttachCore(
         _frame = frame;
         _target = new(target);
         TargetProperty = targetProperty;
-        TargetType = targetProperty.PropertyType;
+        TargetType = targetProperty?.PropertyType ?? typeof(object);
         Priority = priority;
     }
 
@@ -409,6 +409,9 @@ protected void Log(AvaloniaObject target, string error, LogEventLevel level = Lo
     /// <param name="error">The new binding or data validation error.</param>
     private protected void PublishValue(object? value, BindingError? error = null)
     {
+        Debug.Assert(value is not BindingNotification);
+        Debug.Assert(value != BindingOperations.DoNothing);
+
         if (!IsRunning)
             return;
 

src/Avalonia.Base/Data/IndexerBinding.cs

@@ -31,7 +31,7 @@ public IndexerBinding(
             return new InstancedBinding(expression, Mode, BindingPriority.LocalValue);
         }
 
-        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty targetProperty, object? anchor)
+        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty? targetProperty, object? anchor)
         {
             return new IndexerBindingExpression(Source, Property, target, targetProperty, Mode);
         }

src/Avalonia.Base/Data/TemplateBinding.cs

@@ -21,6 +21,7 @@ public partial class TemplateBinding : UntypedBindingExpressionBase,
         IDisposable
     {
         private bool _isSetterValue;
+        private bool _hasPublishedValue;
 
         public TemplateBinding()
             : base(BindingPriority.Template)
@@ -82,7 +83,7 @@ public TemplateBinding([InheritDataTypeFrom(InheritDataTypeFromScopeKind.Control
             return new(target, InstanceCore(), Mode, BindingPriority.Template);
         }
 
-        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty property, object? anchor)
+        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty? property, object? anchor)
         {
             return InstanceCore();
         }
@@ -108,6 +109,7 @@ internal override bool WriteValueToSource(object? value)
 
         protected override void StartCore()
         {
+            _hasPublishedValue = false;
             OnTemplatedParentChanged();
             if (TryGetTarget(out var target))
                 target.PropertyChanged += OnTargetPropertyChanged;
@@ -199,11 +201,12 @@ private void PublishValue()
 
                 value = ConvertToTargetType(value);
                 PublishValue(value, error);
+                _hasPublishedValue = true;
 
                 if (Mode == BindingMode.OneTime)
                     Stop();
             }
-            else
+            else if (_hasPublishedValue)
             {
                 PublishValue(AvaloniaProperty.UnsetValue);
             }

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

@@ -59,11 +59,11 @@ public CompiledBindingExtension ProvideValue(IServiceProvider provider)
         }
 
         private protected override BindingExpressionBase Instance(
-            AvaloniaProperty targetProperty,
             AvaloniaObject target,
+            AvaloniaProperty? targetProperty,
             object? anchor)
         {
-            var enableDataValidation = targetProperty.GetMetadata(target).EnableDataValidation ?? false;
+            var enableDataValidation = targetProperty?.GetMetadata(target).EnableDataValidation ?? false;
             return InstanceCore(target, targetProperty, anchor, enableDataValidation);
         }
 

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

@@ -56,7 +56,7 @@ public IBinding ProvideValue(IServiceProvider serviceProvider)
             return new InstancedBinding(target, expression, BindingMode.OneWay, _priority);
         }
 
-        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty targetProperty, object? anchor)
+        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty? targetProperty, object? anchor)
         {
             if (ResourceKey is null)
                 throw new InvalidOperationException("DynamicResource must have a ResourceKey.");

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

@@ -73,11 +73,11 @@ public Binding(string path, BindingMode mode = BindingMode.Default)
         }
 
         private protected override BindingExpressionBase Instance(
-            AvaloniaProperty targetProperty,
             AvaloniaObject target,
+            AvaloniaProperty? targetProperty,
             object? anchor)
         {
-            var enableDataValidation = targetProperty.GetMetadata(target).EnableDataValidation ?? false;
+            var enableDataValidation = targetProperty?.GetMetadata(target).EnableDataValidation ?? false;
             return InstanceCore(targetProperty, target, anchor, enableDataValidation);
         }
 

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

@@ -95,8 +95,8 @@ public BindingBase(BindingMode mode = BindingMode.Default)
             bool enableDataValidation = false);
 
         private protected abstract BindingExpressionBase Instance(
-            AvaloniaProperty targetProperty,
             AvaloniaObject target,
+            AvaloniaProperty? targetProperty,
             object? anchor);
 
         private protected (BindingMode, UpdateSourceTrigger) ResolveDefaultsFromMetadata(
@@ -118,9 +118,9 @@ private protected (BindingMode, UpdateSourceTrigger) ResolveDefaultsFromMetadata
             return (mode, trigger);
         }
 
-        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty property, object? anchor)
+        BindingExpressionBase IBinding2.Instance(AvaloniaObject target, AvaloniaProperty? property, object? anchor)
         {
-            return Instance(property, target, anchor);
+            return Instance(target, property, anchor);
         }
     }
 }