[X] add callback for previewer on failing ctor (#5101)

Add a callback when object instantiation or creation fails, so the
previewer can replace it by an educated guess

Xamarin.Forms.Xaml.UnitTests/DesignTimeLoaderTests.cs

@@ -22,6 +22,7 @@ namespace Xamarin.Forms.Xaml.UnitTests
 			Device.PlatformServices = null;
 			XamlLoader.FallbackTypeResolver = null;
 			XamlLoader.ValueCreatedCallback = null;
+			XamlLoader.InstantiationFailedCallback = null;
 			Xamarin.Forms.Internals.ResourceLoader.ExceptionHandler = null;
 		}
 
@@ -450,5 +451,41 @@ namespace Xamarin.Forms.Xaml.UnitTests
 
 			Assert.That(myButton.BackgroundColor, Is.Not.EqualTo(Color.Blue));
 		}
+
+		[Test]
+		public void CanReplaceTypeWhenInstantiationThrows()
+		{
+			var xaml = @"
+					<ContentPage xmlns=""http://xamarin.com/schemas/2014/forms""
+						xmlns:x=""http://schemas.microsoft.com/winfx/2009/xaml""
+						xmlns:local=""clr-namespace:Xamarin.Forms.Xaml.UnitTests;assembly=Xamarin.Forms.Xaml.UnitTests""
+						xmlns:missing=""clr-namespace:MissingNamespace;assembly=MissingAssembly"">
+						<StackLayout>
+							<local:InstantiateThrows />
+							<local:InstantiateThrows x:FactoryMethod=""CreateInstance"" />
+							<local:InstantiateThrows>
+								<x:Arguments>
+									<x:Int32>1</x:Int32>
+								</x:Arguments>
+							</local:InstantiateThrows>
+							<missing:Test>
+								<x:Arguments>
+									<x:Int32>1</x:Int32>
+								</x:Arguments>
+							</missing:Test>
+						</StackLayout>
+					</ContentPage>";
+			XamlLoader.FallbackTypeResolver = (p, type) => type ?? typeof(Button);
+			XamlLoader.InstantiationFailedCallback = (type) => new Button();
+			var o = XamlLoader.Create(xaml, true);
+			Assert.DoesNotThrow(() => XamlLoader.Create(xaml, true));
+		}
+	}
+
+	public class InstantiateThrows
+	{
+		public InstantiateThrows() => throw new InvalidOperationException();
+		public static InstantiateThrows CreateInstance() => throw new InvalidOperationException();
+		public InstantiateThrows(int value) => throw new InvalidOperationException();
 	}
 }

Xamarin.Forms.Xaml.UnitTests/FactoryMethodMissingCtor.xaml.cs

@@ -7,11 +7,7 @@ namespace Xamarin.Forms.Xaml.UnitTests
 	[XamlCompilation(XamlCompilationOptions.Skip)]
 	public partial class FactoryMethodMissingCtor : MockView
 	{
-		public FactoryMethodMissingCtor()
-		{
-			InitializeComponent();
-		}
-
+		public FactoryMethodMissingCtor() => InitializeComponent();
 		public FactoryMethodMissingCtor(bool useCompiledXaml)
 		{
 			//this stub will be replaced at compile time
@@ -20,15 +16,11 @@ namespace Xamarin.Forms.Xaml.UnitTests
 		[TestFixture]
 		public class Tests
 		{
-			[SetUp]
-			public void SetUp()
-			{
-				Device.PlatformServices = new MockPlatformServices();
-			}
+			[SetUp] public void Setup() => Device.PlatformServices = new MockPlatformServices();
+			[TearDown] public void TearDown() => Device.PlatformServices = null;
 
-			[TestCase(false)]
-			[TestCase(true)]
-			public void Throw(bool useCompiledXaml)
+			[Test]
+			public void Throw([Values(false, true)]bool useCompiledXaml)
 			{
 				if (useCompiledXaml)
 					Assert.Throws(new XamlParseExceptionConstraint(7, 4), () => MockCompiler.Compile(typeof(FactoryMethodMissingCtor)));

Xamarin.Forms.Xaml/CreateValuesVisitor.cs

@@ -80,8 +80,14 @@ namespace Xamarin.Forms.Xaml
 						if (converted != null && converted.GetType() == type)
 							value = converted;
 					}
-					if (value == null)
-						value = Activator.CreateInstance(type);
+					if (value == null) {
+						try {
+							value = Activator.CreateInstance(type);
+						}
+						catch (TargetInvocationException tie) {
+							value = XamlLoader.InstantiationFailedCallback?.Invoke(new XamlLoader.CallbackTypeInfo { XmlNamespace = node.XmlType.NamespaceUri, XmlTypeName = node.XmlType.Name }) ?? throw tie;
+						}
+					}
 				}
 				catch (TargetInvocationException e) when (e.InnerException is XamlParseException || e.InnerException is XmlException) {
 					throw e.InnerException;
@@ -181,7 +187,13 @@ namespace Xamarin.Forms.Xaml
 							ci.GetParameters().Length != 0 && ci.IsPublic &&
 							ci.GetParameters().All(pi => pi.CustomAttributes.Any(attr => attr.AttributeType == typeof (ParameterAttribute))));
 			object[] arguments = CreateArgumentsArray(node, ctorInfo);
-			return ctorInfo.Invoke(arguments);
+			try {
+				return ctorInfo.Invoke(arguments);
+			}
+			catch (Exception e) when (e is TargetInvocationException || e is MissingMemberException) {
+				return XamlLoader.InstantiationFailedCallback?.Invoke(new XamlLoader.CallbackTypeInfo { XmlNamespace = node.XmlType.NamespaceUri, XmlTypeName = node.XmlType.Name }) ?? throw e;
+			}
+
 		}
 
 		public object CreateFromFactory(Type nodeType, IElementNode node)
@@ -191,7 +203,12 @@ namespace Xamarin.Forms.Xaml
 			if (!node.Properties.ContainsKey(XmlName.xFactoryMethod))
 			{
 				//non-default ctor
-				return Activator.CreateInstance(nodeType, arguments);
+				try {
+					return Activator.CreateInstance(nodeType, arguments);
+				}
+				catch (Exception e) when (e is TargetInvocationException || e is MissingMemberException) {
+					return XamlLoader.InstantiationFailedCallback?.Invoke(new XamlLoader.CallbackTypeInfo { XmlNamespace = node.XmlType.NamespaceUri, XmlTypeName = node.XmlType.Name }) ?? throw e;
+				}
 			}
 
 			var factoryMethod = ((string)((ValueNode)node.Properties[XmlName.xFactoryMethod]).Value);
@@ -219,7 +236,12 @@ namespace Xamarin.Forms.Xaml
 			var mi = nodeType.GetRuntimeMethods().FirstOrDefault(isMatch);
 			if (mi == null)
 				throw new MissingMemberException($"No static method found for {nodeType.FullName}::{factoryMethod} ({string.Join(", ", types.Select(t => t.FullName))})");
-			return mi.Invoke(null, arguments);
+			try {
+				return mi.Invoke(null, arguments);
+			}
+			catch (TargetInvocationException tie) {
+				return XamlLoader.InstantiationFailedCallback?.Invoke(new XamlLoader.CallbackTypeInfo { XmlNamespace = node.XmlType.NamespaceUri, XmlTypeName = node.XmlType.Name}) ?? throw tie;
+			}
 		}
 
 		public object[] CreateArgumentsArray(IElementNode enode)

Xamarin.Forms.Xaml/XamlLoader.cs

@@ -317,5 +317,6 @@ namespace Xamarin.Forms.Xaml
 
 		internal static Func<IList<FallbackTypeInfo>, Type, Type> FallbackTypeResolver { get; set; }
 		internal static Action<CallbackTypeInfo, object> ValueCreatedCallback  { get; set; }
+		internal static Func<CallbackTypeInfo, object> InstantiationFailedCallback { get; set; }
 	}
 }