fix: Allow multiple default value providers

Author: MartinZikmund

src/Uno.UI/UI/Xaml/DependencyPropertyDetails.cs

@@ -184,7 +184,7 @@ private bool SetValueFast(object? value, DependencyPropertyValuePrecedences prec
 			return false;
 		}
 
-		internal void SetDefaultValue(object defaultValue)
+		internal void SetDefaultValue(object? defaultValue)
 		{
 			_defaultValue = defaultValue;
 			_flags |= Flags.DefaultValueSet;

src/Uno.UI/UI/Xaml/DependencyPropertyDetailsCollection.cs

@@ -39,7 +39,7 @@ partial class DependencyPropertyDetailsCollection : IDisposable
 		private int _entriesLength;
 		private int _minId;
 		private int _maxId;
-		private DependencyObjectStore.DefaultValueProvider? _defaultValueProvider;
+		private List<DependencyObjectStore.DefaultValueProvider>? _defaultValueProviders = null;
 
 		private object? Owner => _hardOwnerReference ?? _ownerReference.Target;
 
@@ -141,9 +141,9 @@ public DependencyPropertyDetails FindPropertyDetails(DependencyProperty property
 				{
 					propertyEntry = new DependencyPropertyDetails(property, _ownerType);
 
-					if(_defaultValueProvider != null && _defaultValueProvider(property, out var v))
+					if (TryResolveDefaultValueFromProviders(property, out var value))
 					{
-						propertyEntry.SetDefaultValue(v);
+						propertyEntry.SetDefaultValue(value);
 					}
 				}
 
@@ -178,9 +178,9 @@ public DependencyPropertyDetails FindPropertyDetails(DependencyProperty property
 
 					ref var propertyEntry = ref Entries![property.UniqueId - _minId];
 					propertyEntry = new DependencyPropertyDetails(property, _ownerType);
-					if (_defaultValueProvider != null && _defaultValueProvider(property, out var v))
+					if (TryResolveDefaultValueFromProviders(property, out var value))
 					{
-						propertyEntry.SetValue(v, DependencyPropertyValuePrecedences.DefaultValue);
+						propertyEntry.SetValue(value, DependencyPropertyValuePrecedences.DefaultValue);
 					}
 
 					return propertyEntry;
@@ -192,6 +192,24 @@ public DependencyPropertyDetails FindPropertyDetails(DependencyProperty property
 			}
 		}
 
+		private bool TryResolveDefaultValueFromProviders(DependencyProperty property, out object? value)
+		{
+			if (_defaultValueProviders != null)
+			{
+				for (int i = _defaultValueProviders.Count - 1; i >= 0; i--)
+				{
+					var provider = _defaultValueProviders[i];
+					if (provider.Invoke(property, out var resolvedValue))
+					{
+						value = resolvedValue;
+						return true;
+					}
+				}
+			}
+			value = null;
+			return false;
+		}
+
 		private void AssignEntries(DependencyPropertyDetails[] newEntries, int newSize)
 		{
 			ReturnEntriesToPool();
@@ -214,9 +232,24 @@ private void ReturnEntriesToPool()
 
 		internal IEnumerable<DependencyPropertyDetails> GetAllDetails() => Entries.Trim();
 
+		/// <summary>
+		/// Adds a default value provider.
+		/// </summary>
+		/// <param name="provider">Default value provider.</param>
+		/// <remarks>
+		/// Providers which are registered later have higher precedence.
+		/// E.g. when both a derived and base class register their own default
+		/// value provider in the constructor for the same property, the derived
+		/// class value is used.
+		/// </remarks>
 		public void RegisterDefaultValueProvider(DependencyObjectStore.DefaultValueProvider provider)
 		{
-			_defaultValueProvider = provider;
+			if (_defaultValueProviders == null)
+			{
+				_defaultValueProviders = new List<DependencyObjectStore.DefaultValueProvider>();
+			}
+
+			_defaultValueProviders.Add(provider);
 		}
 
 		internal void TryEnableHardReferences()