Compiled path support for cast

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

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Avalonia.Data.Core
+{
+    public class TypeCastNode : ExpressionNode
+    {
+        public override string Description => $"as {TargetType.FullName}";
+
+        public Type TargetType { get; }
+
+        public TypeCastNode(Type type)
+        {
+            TargetType = type;
+        }
+
+        protected virtual object Cast(object value)
+        {
+            return TargetType.IsInstanceOfType(value) ? value : null;
+        }
+
+        protected override void StartListeningCore(WeakReference<object> reference)
+        {
+            if (reference.TryGetTarget(out object target))
+            {
+                target = Cast(target);
+                reference = target == null ? NullReference : new WeakReference<object>(target);
+            }
+
+            base.StartListeningCore(reference);
+        }
+    }
+}

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -23,6 +23,7 @@
         <Compile Include="MarkupExtensions\CompiledBindings\ObservableStreamPlugin.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\PropertyInfoAccessorFactory.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\PropertyInfoAccessorPlugin.cs" />
+        <Compile Include="MarkupExtensions\CompiledBindings\StrongTypeCastNode.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\TaskStreamPlugin.cs" />
         <Compile Include="MarkupExtensions\DynamicResourceExtension.cs" />
         <Compile Include="MarkupExtensions\ResolveByNameExtension.cs" />

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Data.Core;
 using Avalonia.Data.Core.Plugins;
@@ -53,6 +54,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
                     case IStronglyTypedStreamElement stream:
                         node = new StreamNode(stream.CreatePlugin());
                         break;
+                    case ITypeCastElement typeCast:
+                        node = new StrongTypeCastNode(typeCast.Type, typeCast.Cast);
+                        break;
                     default:
                         throw new InvalidOperationException($"Unknown binding path element type {element.GetType().FullName}");
                 }
@@ -66,6 +70,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         internal SourceMode SourceMode => _elements.Count > 0 && _elements[0] is IControlSourceBindingPathElement ? SourceMode.Control : SourceMode.Data;
 
         internal object RawSource { get; }
+
+        public override string ToString()
+            => string.Concat(_elements.Select(e => e.ToString()));
     }
 
     public class CompiledBindingPathBuilder
@@ -126,6 +133,12 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
             return this;
         }
 
+        public CompiledBindingPathBuilder TypeCast<T>()
+        {
+            _elements.Add(new TypeCastPathElement<T>());
+            return this;
+        }
+
         public CompiledBindingPathBuilder SetRawSource(object rawSource)
         {
             _rawSource = rawSource;
@@ -157,6 +170,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         public IPropertyInfo Property { get; }
 
         public Func<WeakReference<object>, IPropertyInfo, IPropertyAccessor> AccessorFactory { get; }
+
+        public override string ToString()
+            => $".{Property.Name}";
     }
 
     internal interface IStronglyTypedStreamElement : ICompiledBindingPathElement
@@ -164,6 +180,13 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         IStreamPlugin CreatePlugin();
     }
 
+    internal interface ITypeCastElement : ICompiledBindingPathElement
+    {
+        Type Type { get; }
+
+        Func<object, object> Cast { get; }
+    }
+
     internal class TaskStreamPathElement<T> : IStronglyTypedStreamElement
     {
         public static readonly TaskStreamPathElement<T> Instance = new TaskStreamPathElement<T>();
@@ -181,6 +204,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
     internal class SelfPathElement : ICompiledBindingPathElement, IControlSourceBindingPathElement
     {
         public static readonly SelfPathElement Instance = new SelfPathElement();
+
+        public override string ToString()
+            => "$self";
     }
 
     internal class AncestorPathElement : ICompiledBindingPathElement, IControlSourceBindingPathElement
@@ -193,6 +219,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
         public Type AncestorType { get; }
         public int Level { get; }
+
+        public override string ToString()
+           => $"$parent[{AncestorType?.Name},{Level}]";
     }
 
     internal class VisualAncestorPathElement : ICompiledBindingPathElement, IControlSourceBindingPathElement
@@ -217,6 +246,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
         public INameScope NameScope { get; }
         public string Name { get; }
+
+        public override string ToString()
+            => $"#{Name}";
     }
 
     internal class ArrayElementPathElement : ICompiledBindingPathElement
@@ -229,5 +261,24 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
         public int[] Indices { get; }
         public Type ElementType { get; }
+        public override string ToString()
+            => $"[{string.Join(",", Indices)}]";
+    }
+
+    internal class TypeCastPathElement<T> : ITypeCastElement
+    {
+        private static object TryCast(object obj)
+        {
+            if (obj is T result)
+                return result;
+            return null;
+        }
+
+        public Type Type => typeof(T);
+
+        public Func<object, object> Cast => TryCast;
+
+        public override string ToString()
+            => $"({Type.FullName})";
     }
 }

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/StrongTypeCastNode.cs

@@ -0,0 +1,18 @@
+using System;
+using Avalonia.Data.Core;
+
+namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
+{
+    public class StrongTypeCastNode : TypeCastNode
+    {
+        private Func<object, object> _cast;
+
+        public StrongTypeCastNode(Type type, Func<object, object> cast) : base(type)
+        {
+            _cast = cast;
+        }
+
+        protected override object Cast(object value)
+            => _cast(value);
+    }
+}