[tests] expanded testing around design-time builds

Really need to make sure design-time builds work, as intellisense would break!

Author: jonathanpeppers

Xamarin.Forms.Xaml.UnitTests/MSBuild/MSBuildTests.cs

@@ -288,6 +288,7 @@ public void LinkedFile ([Values (false, true)] bool sdkStyle)
 		}
 
 		//https://github.com/dotnet/project-system/blob/master/docs/design-time-builds.md
+		//https://daveaglick.com/posts/running-a-design-time-build-with-msbuild-apis
 		[Test]
 		public void DesignTimeBuild ([Values (false, true)] bool sdkStyle)
 		{
@@ -296,13 +297,33 @@ public void DesignTimeBuild ([Values (false, true)] bool sdkStyle)
 			var projectFile = Path.Combine (tempDirectory, "test.csproj");
 			project.Save (projectFile);
 			RestoreIfNeeded (projectFile, sdkStyle);
-			Build (projectFile, "Compile", "/p:DesignTimeBuild=True /p:BuildingInsideVisualStudio=True /p:BuildingProject=False");
+			Build (projectFile, "CompileDesignTime", "/p:DesignTimeBuild=True /p:BuildingInsideVisualStudio=True /p:SkipCompilerExecution=True /p:ProvideCommandLineArgs=True");
 
-			AssertExists (Path.Combine (intermediateDirectory, "test.dll"), nonEmpty: true);
-			AssertExists (Path.Combine (intermediateDirectory, "Pages", "MainPage.xaml.g.cs"), nonEmpty: true);
-			AssertExists (Path.Combine (intermediateDirectory, "XamlC.stamp"));
+			var assembly = Path.Combine (intermediateDirectory, "test.dll");
+			var mainPageXamlG = Path.Combine (intermediateDirectory, "Pages", "MainPage.xaml.g.cs");
+			var xamlCStamp = Path.Combine (intermediateDirectory, "XamlC.stamp");
+
+			//The assembly should not be compiled
+			AssertDoesNotExist (assembly);
+			AssertExists (mainPageXamlG, nonEmpty: true);
+			AssertExists (xamlCStamp);
+
+			var expectedXamlG = new FileInfo (mainPageXamlG).LastWriteTimeUtc;
+			var expectedXamlC = new FileInfo (xamlCStamp).LastWriteTimeUtc;
+
+			//Build again, a full build
+			Build (projectFile);
+			AssertExists (assembly, nonEmpty: true);
+			AssertExists (mainPageXamlG, nonEmpty: true);
+			AssertExists (xamlCStamp);
+
+			var actualXamlG = new FileInfo (mainPageXamlG).LastWriteTimeUtc;
+			var actualXamlC = new FileInfo (xamlCStamp).LastWriteTimeUtc;
+			Assert.AreEqual (expectedXamlG, actualXamlG, $"Timestamps should match for {mainPageXamlG}.");
+			Assert.AreNotEqual (expectedXamlC, actualXamlC, $"Timestamps should *not* match for {xamlCStamp}.");
 		}
 
+		//I believe the designer might invoke this target manually
 		[Test]
 		public void UpdateDesignTimeXaml ([Values (false, true)] bool sdkStyle)
 		{
@@ -314,6 +335,7 @@ public void UpdateDesignTimeXaml ([Values (false, true)] bool sdkStyle)
 			Build (projectFile, "UpdateDesignTimeXaml");
 
 			AssertExists (Path.Combine (intermediateDirectory, "Pages", "MainPage.xaml.g.cs"), nonEmpty: true);
+			AssertDoesNotExist (Path.Combine (intermediateDirectory, "XamlC.stamp"));
 		}
 
 		[Test]