[Core] Split SourceDescriptor out from ValueDescriptor

ValueDescriptor ended up being used for overlapping purposes:
binding/resources for the source but then for some values like objects
and predfined values it had extra information not contained in the
Value. SourceDescriptor was added to hold things like bindings or
resources that describe where the value came from, while ValueDescriptor
will continue to hold extra details about the value.

There were two possible approaches to this, to add the object property
as-is in this commit, or to add bespoke properties for a Resource and
Binding. Ultimately both approaches have fairly similar level pros and
cons. What it ended up coming down to for me is that these properties
are mutually exclusive. While we can guard against this in a few ways
(target the same backing property, throw an exception), both of these
cases require extra code to be mindful of in some usage where it does
not with a single property, or worse it is silently unexpected behavior.

While it is cleaner to not have to cast, the risk of introducing extra
code complexity for the sake of structure tips the scales.

4 files changed, 23 insertions, 15 deletions

diff --git a/Xamarin.PropertyEditing.Tests/CombinablePredefinedViewModelTests.cs b/Xamarin.PropertyEditing.Tests/CombinablePredefinedViewModelTests.cs
index 4cd84ce..ac00575 100644
--- a/Xamarin.PropertyEditing.Tests/CombinablePredefinedViewModelTests.cs
+++ b/Xamarin.PropertyEditing.Tests/CombinablePredefinedViewModelTests.cs
@@ -552,6 +552,7 @@ namespace Xamarin.PropertyEditing.Tests
 				return new ValueInfo<IReadOnlyList<int>> {
 					Value = setValues,
 					Source = valueInfo.Source,
+					SourceDescriptor = valueInfo.SourceDescriptor,
 					CustomExpression = valueInfo.CustomExpression,
 					ValueDescriptor = valueInfo.ValueDescriptor
 				};
diff --git a/Xamarin.PropertyEditing.Tests/MaterialDesignColorViewModelTests.cs b/Xamarin.PropertyEditing.Tests/MaterialDesignColorViewModelTests.cs
index d211599..2611002 100644
--- a/Xamarin.PropertyEditing.Tests/MaterialDesignColorViewModelTests.cs
+++ b/Xamarin.PropertyEditing.Tests/MaterialDesignColorViewModelTests.cs
@@ -261,7 +261,7 @@ namespace Xamarin.PropertyEditing.Tests
 			await mockEditor.SetValueAsync (mockProperty.Object, new ValueInfo<CommonSolidBrush> {
 				Source = ValueSource.Resource,
 				Value = resource.Value,
-				ValueDescriptor = resource
+				SourceDescriptor = resource
 			});
 
 			var vm = new BrushPropertyViewModel (platform, mockProperty.Object, new[] { mockEditor });
diff --git a/Xamarin.PropertyEditing.Tests/MockObjectEditor.cs b/Xamarin.PropertyEditing.Tests/MockObjectEditor.cs
index 8c0f6a5..14ea035 100644
--- a/Xamarin.PropertyEditing.Tests/MockObjectEditor.cs
+++ b/Xamarin.PropertyEditing.Tests/MockObjectEditor.cs
@@ -69,7 +69,7 @@ namespace Xamarin.PropertyEditing.Tests
 
 		public event EventHandler<EditorPropertyChangedEventArgs> PropertyChanged;
 
-		public Func<IPropertyInfo, object, object> ValueEvaluator
+		public Func<IPropertyInfo, object, object, object> ValueEvaluator
 		{
 			get;
 			set;
@@ -150,11 +150,12 @@ namespace Xamarin.PropertyEditing.Tests
 				CustomExpression = value.CustomExpression,
 				Source = value.Source,
 				ValueDescriptor = value.ValueDescriptor,
+				SourceDescriptor = value.SourceDescriptor,
 				Value = value.Value
 			};
 
 			if (value.Source != ValueSource.Local && ValueEvaluator != null) {
-				value.Value = (T)ValueEvaluator (property, value.ValueDescriptor);
+				value.Value = (T)ValueEvaluator (property, value.ValueDescriptor, value.SourceDescriptor);
 			} else if (value.Source == ValueSource.Unset || (property.ValueSources.HasFlag (ValueSources.Default) && Equals (value.Value, default(T))) && value.ValueDescriptor == null) {
 				this.values.Remove (property);
 				PropertyChanged?.Invoke (this, new EditorPropertyChangedEventArgs (property));
@@ -171,7 +172,9 @@ namespace Xamarin.PropertyEditing.Tests
 					var softType = typeof(ValueInfo<>).MakeGenericType (property.Type);
 					softValue = Activator.CreateInstance (softType);
 					softType.GetProperty ("Value").SetValue (softValue, v);
+					softType.GetProperty ("ValueDescriptor").SetValue (softValue, value.ValueDescriptor);
 					softType.GetProperty ("Source").SetValue (softValue, value.Source);
+					softType.GetProperty ("SourceDescriptor").SetValue (softValue, value.SourceDescriptor);
 				}
 
 				if (typeof(T).Name == "IReadOnlyList`1") {
@@ -189,18 +192,18 @@ namespace Xamarin.PropertyEditing.Tests
 			}
 
 			// Set to resource won't pass values so we will store it on the info since we just pass it back in GetValue
-			if (value.Source == ValueSource.Resource && value.ValueDescriptor is Resource) {
-				Type rt = value.ValueDescriptor.GetType();
+			if (value.Source == ValueSource.Resource && value.SourceDescriptor is Resource) {
+				Type rt = value.SourceDescriptor.GetType();
 				if (rt.IsGenericType) {
 					Type ta = rt.GetGenericArguments ()[0];
 					if (typeof (T).IsAssignableFrom (ta)) {
 						PropertyInfo pi = rt.GetProperty ("Value");
-						value.Value = (T)pi.GetValue (value.ValueDescriptor);
+						value.Value = (T)pi.GetValue (value.SourceDescriptor);
 					} else {
 						TypeConverter converter = TypeDescriptor.GetConverter (ta);
 						if (converter != null && converter.CanConvertTo(typeof(T))) {
 							PropertyInfo pi = rt.GetProperty ("Value");
-							value.Value = (T)converter.ConvertTo (pi.GetValue (value.ValueDescriptor), typeof (T));
+							value.Value = (T)converter.ConvertTo (pi.GetValue (value.SourceDescriptor), typeof (T));
 						}
 					}
 				}
@@ -225,6 +228,7 @@ namespace Xamarin.PropertyEditing.Tests
 						CustomExpression = info.CustomExpression,
 						Source = info.Source,
 						ValueDescriptor = info.ValueDescriptor,
+						SourceDescriptor = info.SourceDescriptor,
 						Value = info.Value
 					};
 				} else if (value == null || value is T) {
@@ -258,12 +262,13 @@ namespace Xamarin.PropertyEditing.Tests
 						Source = underlyingInfo?.Source ?? ValueSource.Local
 					}, typeof(ValueInfo<T>));
 				} else {
-					object descriptor = null;
+					object sourceDescriptor = null, valueDescriptor = null;
 					ValueSource source = ValueSource.Local;
 					Type valueType = value.GetType ();
 					if (valueType.IsConstructedGenericType && valueType.GetGenericTypeDefinition () == typeof(ValueInfo<>)) {
 						source = (ValueSource)valueType.GetProperty ("Source").GetValue (value);
-						descriptor = valueType.GetProperty (nameof (ValueInfo<T>.ValueDescriptor)).GetValue (value);
+						sourceDescriptor = valueType.GetProperty (nameof (ValueInfo<T>.SourceDescriptor)).GetValue (value);
+						valueDescriptor = valueType.GetProperty (nameof (ValueInfo<T>.ValueDescriptor)).GetValue (value);
 						value = valueType.GetProperty ("Value").GetValue (value);
 						valueType = valueType.GetGenericArguments ()[0];
 					}
@@ -274,13 +279,15 @@ namespace Xamarin.PropertyEditing.Tests
 						return new ValueInfo<T> {
 							Source = source,
 							Value = (T)newValue,
-							ValueDescriptor = descriptor
+							ValueDescriptor = valueDescriptor,
+							SourceDescriptor = sourceDescriptor
 						};
 					} else if (typeof(T).IsAssignableFrom (valueType)) {
 						return new ValueInfo<T> {
 							Source = source,
 							Value = (T)value,
-							ValueDescriptor = descriptor
+							ValueDescriptor = valueDescriptor,
+							SourceDescriptor = sourceDescriptor
 						};
 					}
 				}
diff --git a/Xamarin.PropertyEditing.Tests/PropertyViewModelTests.cs b/Xamarin.PropertyEditing.Tests/PropertyViewModelTests.cs
index 04a23af..a24b2ee 100644
--- a/Xamarin.PropertyEditing.Tests/PropertyViewModelTests.cs
+++ b/Xamarin.PropertyEditing.Tests/PropertyViewModelTests.cs
@@ -434,8 +434,8 @@ namespace Xamarin.PropertyEditing.Tests
 			var resourcesMock = new Mock<IResourceProvider> ();
 			resourcesMock.Setup (rp => rp.GetResourcesAsync (editor.Target, mockProperty.Object, It.IsAny<CancellationToken> ())).ReturnsAsync (new[] { resource });
 
-			editor.ValueEvaluator = (info, o) => {
-				if (o == resource)
+			editor.ValueEvaluator = (info, val, source) => {
+				if (source == resource)
 					return value;
 
 				return default(TValue);
@@ -464,7 +464,7 @@ namespace Xamarin.PropertyEditing.Tests
 			var editor = new MockObjectEditor (mockProperty.Object);
 			await editor.SetValueAsync (mockProperty.Object, new ValueInfo<TValue> {
 				Source = ValueSource.Resource,
-				ValueDescriptor = resource,
+				SourceDescriptor = resource,
 				Value = value
 			});
 
@@ -490,7 +490,7 @@ namespace Xamarin.PropertyEditing.Tests
 			var editor = new MockObjectEditor (mockProperty.Object);
 			await editor.SetValueAsync (mockProperty.Object, new ValueInfo<TValue> {
 				Source = ValueSource.Resource,
-				ValueDescriptor = resource,
+				SourceDescriptor = resource,
 				Value = value
 			});