Implement diagnostic API.

Doesn't actually fill in the "details" property for styled properties because this API is going to change shortly with the new devtools.

src/Avalonia.Base/AvaloniaObject.cs

@@ -587,6 +587,33 @@ namespace Avalonia
             }
         }
 
+        internal AvaloniaPropertyValue GetDiagnosticInternal(AvaloniaProperty property)
+        {
+            if (property.IsDirect)
+            {
+                return new AvaloniaPropertyValue(
+                    property,
+                    GetValue(property),
+                    BindingPriority.Unset,
+                    "Local Value");
+            }
+            else if (_values != null)
+            {
+                var result = _values.GetDiagnostic(property);
+
+                if (result != null)
+                {
+                    return result;
+                }
+            }
+
+            return new AvaloniaPropertyValue(
+                property,
+                GetValue(property),
+                BindingPriority.Unset,
+                "Unset");
+        }
+
         /// <summary>
         /// Logs a binding error for a property.
         /// </summary>

src/Avalonia.Base/Diagnostics/AvaloniaObjectExtensions.cs

@@ -22,36 +22,7 @@ namespace Avalonia.Diagnostics
         /// </returns>
         public static AvaloniaPropertyValue GetDiagnostic(this AvaloniaObject o, AvaloniaProperty property)
         {
-            throw new NotImplementedException();
-            ////var set = o.GetSetValues();
-
-            ////if (set.TryGetValue(property, out var obj))
-            ////{
-            ////    if (obj is PriorityValue value)
-            ////    {
-            ////        return new AvaloniaPropertyValue(
-            ////            property,
-            ////            o.GetValue(property),
-            ////            (BindingPriority)value.ValuePriority,
-            ////            value.GetDiagnostic());
-            ////    }
-            ////    else
-            ////    {
-            ////        return new AvaloniaPropertyValue(
-            ////            property,
-            ////            obj,
-            ////            BindingPriority.LocalValue,
-            ////            "Local value");
-            ////    }
-            ////}
-            ////else
-            ////{
-            ////    return new AvaloniaPropertyValue(
-            ////        property,
-            ////        o.GetValue(property),
-            ////        BindingPriority.Unset,
-            ////        "Unset");
-            ////}
+            return o.GetDiagnosticInternal(property);
         }
     }
 }

src/Avalonia.Base/ValueStore.cs

@@ -129,6 +129,23 @@ namespace Avalonia
             }
         }
 
+        public Diagnostics.AvaloniaPropertyValue? GetDiagnostic(AvaloniaProperty property)
+        {
+            if (_values.TryGetValue(property, out var slot))
+            {
+                if (slot is IValue value)
+                {
+                    return new Diagnostics.AvaloniaPropertyValue(
+                        property,
+                        value.Value.HasValue ? (object)value.Value : AvaloniaProperty.UnsetValue,
+                        value.ValuePriority,
+                        null);
+                }
+            }
+
+            return null;
+        }
+
         void IValueSink.ValueChanged<T>(
             StyledPropertyBase<T> property,
             BindingPriority priority,