WIP: Trying to use Style priority for ControlTemplate values.

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs

@@ -65,6 +65,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 new AvaloniaXamlIlCompiledBindingsMetadataRemover()
                 );
 
+            Transformers.Add(new AvaloniaXamlIlControlTemplatePriorityTransformer());
             Transformers.Add(new AvaloniaXamlIlMetadataRemover());
             Transformers.Add(new AvaloniaXamlIlRootObjectScope());
 

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlControlTemplatePriorityTransformer.cs

@@ -0,0 +1,37 @@
+using System.Linq;
+using XamlX.Ast;
+using XamlX.Transform;
+
+namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
+{
+    /// <summary>
+    /// Transforms property assignments within ControlTemplates to use Style priority where possible.
+    /// </summary>
+    class AvaloniaXamlIlControlTemplatePriorityTransformer : IXamlAstTransformer
+    {
+        public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
+        {
+            var bindingPriorityType = context.GetAvaloniaTypes().BindingPriority;
+
+            if (node is XamlPropertyAssignmentNode prop &&
+                prop.Property is XamlIlAvaloniaProperty avaloniaProperty &&
+                context.ParentNodes().Any(IsControlTemplate))
+            {
+                // If there are any setters which accept a binding priority, then add the Style
+                // binding priority as a value.
+                if (avaloniaProperty.Setters.Any(x => x.Parameters[0] == bindingPriorityType))
+                {
+                    prop.Values.Insert(0, new XamlConstantNode(node, bindingPriorityType, 1));
+                }
+            }
+
+            return node;
+        }
+
+        private static bool IsControlTemplate(IXamlAstNode node)
+        {
+            return node is AvaloniaXamlIlTargetTypeMetadataNode tt &&
+                tt.ScopeType == AvaloniaXamlIlTargetTypeMetadataNode.ScopeTypes.ControlTemplate;
+        }
+    }
+}

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs

@@ -15,6 +15,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlType AvaloniaObjectExtensions { get; }
         public IXamlType AvaloniaProperty { get; }
         public IXamlType AvaloniaPropertyT { get; }
+        public IXamlType StyledPropertyT { get; }
         public IXamlType AvaloniaAttachedPropertyT { get; }
         public IXamlType IBinding { get; }
         public IXamlMethod AvaloniaObjectBindMethod { get; }
@@ -94,6 +95,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             AvaloniaObjectExtensions = cfg.TypeSystem.GetType("Avalonia.AvaloniaObjectExtensions");
             AvaloniaProperty = cfg.TypeSystem.GetType("Avalonia.AvaloniaProperty");
             AvaloniaPropertyT = cfg.TypeSystem.GetType("Avalonia.AvaloniaProperty`1");
+            StyledPropertyT = cfg.TypeSystem.GetType("Avalonia.StyledProperty`1");
             AvaloniaAttachedPropertyT = cfg.TypeSystem.GetType("Avalonia.AttachedProperty`1");
             BindingPriority = cfg.TypeSystem.GetType("Avalonia.Data.BindingPriority");
             IBinding = cfg.TypeSystem.GetType("Avalonia.Data.IBinding");

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/XamlIlAvaloniaPropertyHelper.cs

@@ -189,7 +189,13 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             CustomAttributes = original.CustomAttributes;
             if (!original.CustomAttributes.Any(ca => ca.Type.Equals(types.AssignBindingAttribute)))
                 Setters.Insert(0, new BindingSetter(types, original.DeclaringType, field));
-            
+
+            if (field.FieldType.GenericTypeDefinition == types.StyledPropertyT)
+            {
+                var propertyType = field.FieldType.GenericArguments[0];
+                Setters.Insert(1, new SetValueWithPrioritySetter(types, original.DeclaringType, field, propertyType));
+            }
+
             Setters.Insert(0, new UnsetValueSetter(types, original.DeclaringType, field));
         }
 
@@ -240,6 +246,39 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             }
         }
 
+        class SetValueWithPrioritySetter : AvaloniaPropertyCustomSetter
+        {
+            public SetValueWithPrioritySetter(AvaloniaXamlIlWellKnownTypes types, IXamlType declaringType, IXamlField avaloniaProperty,
+                IXamlType propertyType)
+                : base(types, declaringType, avaloniaProperty)
+            {
+                Parameters = new[] { types.BindingPriority, propertyType };
+            }
+
+            public override void Emit(IXamlILEmitter emitter)
+            {
+                var method = Types.AvaloniaObject
+                    .FindMethod(m => m.IsPublic && !m.IsStatic && m.Name == "SetValue"
+                                     && m.Parameters.Count == 3
+                                     && m.Parameters[0].Name == "StyledPropertyBase`1"
+                                     && m.Parameters[2].Equals(Types.BindingPriority));
+
+                if (method == null)
+                    throw new XamlTypeSystemException(
+                        $"Unable to find AvaloniaObject.SetValue<T>(StyledPropertyBase<T>, T, BindingPriority).");
+
+                method = method.MakeGenericMethod(new[] { Parameters[1] });
+
+                using (var bloc = emitter.LocalsPool.GetLocal(Parameters[1]))
+                    emitter
+                        .Stloc(bloc.Local)
+                        .Ldsfld(AvaloniaProperty)
+                        .Ldloc(bloc.Local)
+                        .Ldc_I4(0)
+                        .EmitCall(method, true);
+            }
+        }
+
         class UnsetValueSetter : AvaloniaPropertyCustomSetter
         {
             public UnsetValueSetter(AvaloniaXamlIlWellKnownTypes types, IXamlType declaringType, IXamlField avaloniaProperty)