[XamlC] correctly resolve override of virtual event handlers (#247)

Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs

@@ -316,7 +316,12 @@ internal static string GetContentProperty(TypeReference typeRef)
 			//If the target is an event, connect
 			//			IL_0007:  ldloc.0 
 			//			IL_0008:  ldarg.0 
+			//
 			//			IL_0009:  ldftn instance void class Xamarin.Forms.Xaml.XamlcTests.MyPage::OnButtonClicked(object, class [mscorlib]System.EventArgs)
+			//OR, if the handler is virtual
+			//			    IL_000x:  ldarg.0 
+			//			    IL_0009:  ldvirtftn instance void class Xamarin.Forms.Xaml.XamlcTests.MyPage::OnButtonClicked(object, class [mscorlib]System.EventArgs)
+			//
 			//			IL_000f:  newobj instance void class [mscorlib]System.EventHandler::'.ctor'(object, native int)
 			//			IL_0014:  callvirt instance void class [Xamarin.Forms.Core]Xamarin.Forms.Button::add_Clicked(class [mscorlib]System.EventHandler)
 
@@ -345,7 +350,14 @@ internal static string GetContentProperty(TypeReference typeRef)
 						string.Format("EventHandler \"{0}\" not found in type \"{1}\"", value, context.Body.Method.DeclaringType.FullName),
 						iXmlLineInfo);
 				}
-				context.IL.Emit(OpCodes.Ldftn, handler);
+				if (handler.IsVirtual)
+				{
+					context.IL.Emit(OpCodes.Ldarg_0);
+					context.IL.Emit(OpCodes.Ldvirtftn, handler);
+				}
+				else
+					context.IL.Emit(OpCodes.Ldftn, handler);
+
 				//FIXME: eventually get the right ctor instead fo the First() one, just in case another one could exists (not even sure it's possible).
 				var ctor = module.Import(eventinfo.EventType.Resolve().GetConstructors().First());
 				ctor = ctor.ResolveGenericParameters(eventinfo.EventType, module);

Xamarin.Forms.Xaml.UnitTests/EventsConnection.xaml

@@ -12,5 +12,7 @@
 								Clicked="HandleClickedOnBase" />
 		<local:ElementWithEvent x:Name="elementwithAsyncprivateHandler"
 								Clicked="HandleClickedPrivateAsync" />
+		<local:ElementWithEvent x:Name="elementWithVirtualHandler"
+								Clicked="HandleVirtualClicked" />
 	</StackLayout>
 </local:BaseForEvents>

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

@@ -71,6 +71,12 @@ async void HandleClickedPrivateAsync (object sender, EventArgs e)
 			asyncPrivateClicked++;
 		}
 
+		int baseForVirtualClicked;
+		protected virtual void HandleVirtualClicked(object sender, EventArgs e)
+		{
+			baseForVirtualClicked++;
+		}
+
 		[TestFixture]
 		public class Tests
 		{
@@ -113,6 +119,37 @@ public void TestAsyncPrivateHandler (bool useCompiledXaml)
 				layout.elementwithAsyncprivateHandler.SendClicked ();
 				Assert.AreEqual (1, layout.asyncPrivateClicked);
 			}
+
+			[TestCase(false)]
+			[TestCase(true)]
+			public void TestVirtualHandler(bool useCompiledXaml)
+			{
+				var layout = new SubForEvents(useCompiledXaml);
+				Assert.AreEqual(0, layout.baseForVirtualClicked);
+				Assert.AreEqual(0, layout.overrideClicked);
+				layout.elementWithVirtualHandler.SendClicked();
+				Assert.AreEqual(0, layout.baseForVirtualClicked);
+				Assert.AreEqual(1, layout.overrideClicked);
+			}
+		}
+	}
+
+	public class SubForEvents : EventsConnection
+	{
+		public SubForEvents(bool useCompiledXaml) : base(useCompiledXaml)
+		{
+		}
+
+		public int overrideClicked;
+		protected override void HandleVirtualClicked(object sender, EventArgs e)
+		{
+			overrideClicked++;
+		}
+
+#pragma warning disable 1998 // considered for removal
+		async void HandleClickedPrivateAsync(object sender, EventArgs e)
+#pragma warning restore 1998
+		{
 		}
 	}
 }