Add DefaultLanguage fallback policy for published values (#13814)

* Add DefaultLanguage fallback

* Implement DefaultLanguage fallback

* Add DefaultLanguage fallback test

src/Umbraco.Core/Models/PublishedContent/Fallback.cs

@@ -3,56 +3,82 @@ using System.Collections;
 namespace Umbraco.Cms.Core.Models.PublishedContent;
 
 /// <summary>
-///     Manages the built-in fallback policies.
+/// Manages the built-in fallback policies.
 /// </summary>
 public struct Fallback : IEnumerable<int>
 {
     /// <summary>
-    ///     Do not fallback.
+    /// Do not fallback.
     /// </summary>
     public const int None = 0;
 
     private readonly int[] _values;
 
     /// <summary>
-    ///     Initializes a new instance of the <see cref="Fallback" /> struct with values.
+    /// Initializes a new instance of the <see cref="Fallback" /> struct with values.
     /// </summary>
+    /// <param name="values">The values.</param>
     private Fallback(int[] values) => _values = values;
 
     /// <summary>
-    ///     Gets an ordered set of fallback policies.
+    /// Gets an ordered set of fallback policies.
     /// </summary>
-    /// <param name="values"></param>
+    /// <param name="values">The values.</param>
+    /// <returns>
+    /// The fallback policy.
+    /// </returns>
     public static Fallback To(params int[] values) => new(values);
 
     /// <summary>
-    ///     Fallback to default value.
+    /// Fallback to the default value.
     /// </summary>
     public const int DefaultValue = 1;
 
     /// <summary>
-    ///     Fallback to other languages.
+    /// Fallback to other languages.
     /// </summary>
     public const int Language = 2;
 
     /// <summary>
-    ///     Fallback to tree ancestors.
+    /// Fallback to tree ancestors.
     /// </summary>
     public const int Ancestors = 3;
 
     /// <summary>
-    ///     Gets the fallback to default value policy.
+    /// Fallback to the default language.
     /// </summary>
+    public const int DefaultLanguage = 4;
+
+    /// <summary>
+    /// Gets the fallback to the default language policy.
+    /// </summary>
+    /// <value>
+    /// The default language fallback policy.
+    /// </value>
+    public static Fallback ToDefaultLanguage => new Fallback(new[] { DefaultLanguage });
+
+    /// <summary>
+    /// Gets the fallback to the default value policy.
+    /// </summary>
+    /// <value>
+    /// The default value fallback policy.
+    /// </value>
     public static Fallback ToDefaultValue => new(new[] { DefaultValue });
 
     /// <summary>
-    ///     Gets the fallback to language policy.
+    /// Gets the fallback to language policy.
     /// </summary>
+    /// <value>
+    /// The language fallback policy.
+    /// </value>
     public static Fallback ToLanguage => new(new[] { Language });
 
     /// <summary>
-    ///     Gets the fallback to tree ancestors policy.
+    /// Gets the fallback to tree ancestors policy.
     /// </summary>
+    /// <value>
+    /// The tree ancestors fallback policy.
+    /// </value>
     public static Fallback ToAncestors => new(new[] { Ancestors });
 
     /// <inheritdoc />

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

@@ -44,6 +44,13 @@ public class PublishedValueFallback : IPublishedValueFallback
                         return true;
                     }
 
+                    break;
+                case Fallback.DefaultLanguage:
+                    if (TryGetValueWithDefaultLanguageFallback(property, culture, segment, out value))
+                    {
+                        return true;
+                    }
+
                     break;
                 default:
                     throw NotSupportedFallbackMethod(f, "property");
@@ -85,6 +92,13 @@ public class PublishedValueFallback : IPublishedValueFallback
                         return true;
                     }
 
+                    break;
+                case Fallback.DefaultLanguage:
+                    if (TryGetValueWithDefaultLanguageFallback(content, alias, culture, segment, out value))
+                    {
+                        return true;
+                    }
+
                     break;
                 default:
                     throw NotSupportedFallbackMethod(f, "element");
@@ -141,6 +155,13 @@ public class PublishedValueFallback : IPublishedValueFallback
                         return true;
                     }
 
+                    break;
+                case Fallback.DefaultLanguage:
+                    if (TryGetValueWithDefaultLanguageFallback(content, alias, culture, segment, out value))
+                    {
+                        return true;
+                    }
+
                     break;
                 default:
                     throw NotSupportedFallbackMethod(f, "content");
@@ -347,4 +368,42 @@ public class PublishedValueFallback : IPublishedValueFallback
             language = language2;
         }
     }
+
+    private bool TryGetValueWithDefaultLanguageFallback<T>(IPublishedProperty property, string? culture, string? segment, out T? value)
+    {
+        value = default;
+
+        if (culture.IsNullOrWhiteSpace())
+        {
+            return false;
+        }
+
+        string? defaultCulture = _localizationService?.GetDefaultLanguageIsoCode();
+        if (culture.InvariantEquals(defaultCulture) == false && property.HasValue(defaultCulture, segment))
+        {
+            value = property.Value<T>(this, defaultCulture, segment);
+            return true;
+        }
+
+        return false;
+    }
+
+    private bool TryGetValueWithDefaultLanguageFallback<T>(IPublishedElement element, string alias, string? culture, string? segment, out T? value)
+    {
+        value = default;
+
+        if (culture.IsNullOrWhiteSpace())
+        {
+            return false;
+        }
+
+        string? defaultCulture = _localizationService?.GetDefaultLanguageIsoCode();
+        if (culture.InvariantEquals(defaultCulture) == false && element.HasValue(alias, defaultCulture, segment))
+        {
+            value = element.Value<T>(this, alias, defaultCulture, segment);
+            return true;
+        }
+
+        return false;
+    }
 }

tests/Umbraco.Tests.UnitTests/Umbraco.Infrastructure/PublishedCache/PublishedContentLanguageVariantTests.cs

@@ -242,6 +242,15 @@ public class PublishedContentLanguageVariantTests : PublishedSnapshotServiceTest
         Assert.IsNull(value);
     }
 
+    [Test]
+    public void Can_Get_Content_For_Unpopulated_Requested_DefaultLanguage_With_Fallback()
+    {
+        var snapshot = GetPublishedSnapshot();
+        var content = snapshot.Content.GetAtRoot().First();
+        var value = content.Value(PublishedValueFallback, "welcomeText", "fr", fallback: Fallback.ToDefaultLanguage);
+        Assert.AreEqual("Welcome", value);
+    }
+
     [Test]
     public void Do_Not_Get_Content_Recursively_Unless_Requested()
     {