Implement basic fallback plumbing (in progress)

src/Umbraco.Web/Models/PublishedContent/PublishedValueFallback.cs

@@ -0,0 +1,92 @@
+using Umbraco.Core.Models.PublishedContent;
+
+namespace Umbraco.Web.Models.PublishedContent
+{
+    /// <summary>
+    /// Provides a default implementation for <see cref="IPublishedValueFallback"/>.
+    /// </summary>
+    public class PublishedValueFallback : IPublishedValueFallback
+    {
+        // this is our default implementation
+        // kinda reproducing what was available in v7
+
+        /// <inheritdoc />
+        public object GetValue(IPublishedProperty property, string culture, string segment, object defaultValue)
+        {
+            // no fallback here
+            return defaultValue;
+        }
+
+        /// <inheritdoc />
+        public T GetValue<T>(IPublishedProperty property, string culture, string segment, T defaultValue)
+        {
+            // no fallback here
+            return defaultValue;
+        }
+
+        /// <inheritdoc />
+        public object GetValue(IPublishedElement content, string alias, string culture, string segment, object defaultValue)
+        {
+            // no fallback here
+            return defaultValue;
+        }
+
+        /// <inheritdoc />
+        public T GetValue<T>(IPublishedElement content, string alias, string culture, string segment, T defaultValue)
+        {
+            // no fallback here
+            return defaultValue;
+        }
+
+        /// <inheritdoc />
+        public object GetValue(IPublishedContent content, string alias, string culture, string segment, object defaultValue, bool recurse)
+        {
+            // no fallback here
+            if (!recurse) return defaultValue;
+
+            // is that ok?
+            return GetValue<object>(content, alias, culture, segment, defaultValue, recurse);
+        }
+
+        /// <inheritdoc />
+        public T GetValue<T>(IPublishedContent content, string alias, string culture, string segment, T defaultValue, bool recurse)
+        {
+            // no fallback here
+            if (!recurse) return defaultValue;
+
+            // otherwise, implement recursion as it was implemented in PublishedContentBase
+
+            // fixme caching?
+            //
+            // all caches were using PublishedContentBase.GetProperty(alias, recurse) to get the property,
+            // then,
+            // NuCache.PublishedContent was storing the property in GetAppropriateCache() with key "NuCache.Property.Recurse[" + DraftOrPub(previewing) + contentUid + ":" + typeAlias + "]";
+            // XmlPublishedContent was storing the property in _cacheProvider with key $"XmlPublishedCache.PublishedContentCache:RecursiveProperty-{Id}-{alias.ToLowerInvariant()}";
+            // DictionaryPublishedContent was storing the property in _cacheProvider with key $"XmlPublishedCache.PublishedMediaCache:RecursiveProperty-{Id}-{alias.ToLowerInvariant()}";
+            //
+            // at the moment, caching has been entirely removed, until we better understand caching + fallback
+
+            IPublishedProperty property = null; // if we are here, content's property has no value
+            IPublishedProperty noValueProperty = null;
+            do
+            {
+                content = content.Parent;
+                property = content?.GetProperty(alias);
+                if (property != null) noValueProperty = property;
+            } while (content != null && (property == null || property.HasValue(culture, segment) == false));
+
+            // if we found a content with the property having a value, return that property value
+            if (property != null && property.HasValue(culture, segment))
+                return property.Value<T>(culture, segment);
+
+            // if we found a property, even though with no value, return that property value
+            // because the converter may want to handle the missing value. ie if defaultValue is default,
+            // either specified or by default, the converter may want to substitute something else.
+            if (noValueProperty != null)
+                return noValueProperty.Value<T>(culture, segment, defaultValue: defaultValue);
+
+            // else return default
+            return defaultValue;
+        }
+    }
+}