[XamlG] builds incrementally, add MSBuild integration tests

.nuspec/Xamarin.Forms.targets

@@ -64,18 +64,23 @@
 		</CoreCompileDependsOn>
 	</PropertyGroup>
 
-	<Target Name="XamlG" BeforeTargets="BeforeCompile" DependsOnTargets="PrepareResourceNames" Condition="'$(_XamlGAlreadyExecuted)'!='true'">
-		<PropertyGroup>
-			<_XamlGAlreadyExecuted>true</_XamlGAlreadyExecuted>
-		</PropertyGroup>
+	<Target Name="_FindXamlGFiles" DependsOnTargets="PrepareResourceNames">
+		<ItemGroup>
+			<_XamlGInputs Include="@(EmbeddedResource)" Condition="'%(Extension)' == '.xaml' AND '$(DefaultLanguageSourceExtension)' == '.cs' AND '%(TargetPath)' != ''" />
+			<_XamlGOutputs Include="@(_XamlGInputs->'$(IntermediateOutputPath)%(TargetPath).g.cs')" />
+		</ItemGroup>
+	</Target>
+
+	<Target Name="XamlG" BeforeTargets="BeforeCompile" DependsOnTargets="_FindXamlGFiles" Inputs="@(_XamlGInputs)" Outputs="@(_XamlGOutputs)">
 		<XamlGTask
-			XamlFiles="@(EmbeddedResource)" Condition="'%(Extension)' == '.xaml' AND '$(DefaultLanguageSourceExtension)' == '.cs'"
-			Language = "$(Language)"
-			AssemblyName = "$(AssemblyName)"
-			OutputPath = "$(IntermediateOutputPath)">
-			<Output ItemName="FilesWrite" TaskParameter="GeneratedCodeFiles" />
-			<Output ItemName="Compile" TaskParameter="GeneratedCodeFiles" />
-		</XamlGTask>
+			XamlFiles="@(_XamlGInputs)"
+			OutputFiles="@(_XamlGOutputs)"
+			Language="$(Language)"
+			AssemblyName="$(AssemblyName)" />
+		<ItemGroup>
+			<FileWrites Include="@(_XamlGOutputs)" />
+			<Compile Include="@(_XamlGOutputs)" />
+		</ItemGroup>
 	</Target>
 
 	<!-- XamlC -->
@@ -86,17 +91,18 @@
 		</CompileDependsOn>
 	</PropertyGroup>
 
-	<Target Name="XamlC" AfterTargets="AfterCompile" Condition="'$(_XamlCAlreadyExecuted)'!='true'">
-		<PropertyGroup>
-			<_XamlCAlreadyExecuted>true</_XamlCAlreadyExecuted>
-		</PropertyGroup>
+	<Target Name="XamlC" AfterTargets="AfterCompile" Inputs="$(IntermediateOutputPath)$(TargetFileName)" Outputs="$(IntermediateOutputPath)XamlC.stamp">
 		<XamlCTask
 			Assembly = "$(IntermediateOutputPath)$(TargetFileName)"
 			ReferencePath = "@(ReferencePath)"
 			OptimizeIL = "true"
 			DebugSymbols = "$(DebugSymbols)"
 			DebugType = "$(DebugType)"
 			KeepXamlResources = "$(XFKeepXamlResources)" />
+		<Touch Files="$(IntermediateOutputPath)XamlC.stamp" AlwaysCreate="True" />
+		<ItemGroup>
+			<FileWrites Include="$(IntermediateOutputPath)XamlC.stamp" />
+		</ItemGroup>
 	</Target>
 
 	<!-- CssG -->
@@ -115,7 +121,7 @@
 			Language = "$(Language)"
 			AssemblyName = "$(AssemblyName)"
 			OutputPath = "$(IntermediateOutputPath)">
-			<Output ItemName="FilesWrite" TaskParameter="GeneratedCodeFiles" />
+			<Output ItemName="FileWrites" TaskParameter="GeneratedCodeFiles" />
 			<Output ItemName="Compile" TaskParameter="GeneratedCodeFiles" />
 		</CssGTask>
 	</Target>

Xamarin.Forms.Build.Tasks/XamlGTask.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.IO;
 using System.Xml;
 
@@ -10,44 +9,45 @@ namespace Xamarin.Forms.Build.Tasks
 {
 	public class XamlGTask : Task
 	{
-		List<ITaskItem> _generatedCodeFiles = new List<ITaskItem>();
-
 		[Required]
 		public ITaskItem[] XamlFiles { get; set; }
 
-		[Output]
-		public ITaskItem[] GeneratedCodeFiles => _generatedCodeFiles.ToArray();
+		[Required]
+		public ITaskItem[] OutputFiles { get; set; }
 
 		public string Language { get; set; }
 		public string AssemblyName { get; set; }
-		public string OutputPath { get; set; }
 
 		public override bool Execute()
 		{
 			bool success = true;
 			Log.LogMessage(MessageImportance.Normal, "Generating code behind for XAML files");
 
-			if (XamlFiles == null) {
+			//NOTE: should not happen due to [Required], but there appears to be a place this is class is called directly
+			if (XamlFiles == null || OutputFiles == null) {
 				Log.LogMessage("Skipping XamlG");
 				return true;
 			}
 
-			foreach (var xamlFile in XamlFiles) {
-				//when invoked from `UpdateDesigntimeXaml` target, the `TargetPath` isn't set, use a random one instead
-				var targetPath = xamlFile.GetMetadata("TargetPath");
-				if (string.IsNullOrWhiteSpace(targetPath))
-					targetPath = $".{Path.GetRandomFileName()}";
+			if (XamlFiles.Length != OutputFiles.Length) {
+				Log.LogError("\"{2}\" refers to {0} item(s), and \"{3}\" refers to {1} item(s). They must have the same number of items.", XamlFiles.Length, OutputFiles.Length, "XamlFiles", "OutputFiles");
+				return false;
+			}
 
-				var outputFile = Path.Combine(OutputPath, $"{targetPath}.g.cs");
+			for (int i = 0; i < XamlFiles.Length; i++) {
+				var xamlFile = XamlFiles[i];
+				var outputFile = OutputFiles[i].ItemSpec;
 				if (Path.DirectorySeparatorChar == '/' && outputFile.Contains(@"\"))
 					outputFile = outputFile.Replace('\\','/');
 				else if (Path.DirectorySeparatorChar == '\\' && outputFile.Contains(@"/"))
 					outputFile = outputFile.Replace('/', '\\');
-	
+
 				var generator = new XamlGenerator(xamlFile, Language, AssemblyName, outputFile, Log);
 				try {
-					if (generator.Execute())
-						_generatedCodeFiles.Add(new TaskItem(Microsoft.Build.Evaluation.ProjectCollection.Escape(outputFile)));
+					if (!generator.Execute()) {
+						//If Execute() fails, the file still needs to exist because it is added to the <Compile/> ItemGroup
+						File.WriteAllText (outputFile, string.Empty);
+					}
 				}
 				catch (XmlException xe) {
 					Log.LogError(null, null, null, xamlFile.ItemSpec, xe.LineNumber, xe.LinePosition, 0, 0, xe.Message, xe.HelpLink, xe.Source);

Xamarin.Forms.Xaml.UnitTests/DummyBuildEngine.cs → Xamarin.Forms.Xaml.UnitTests/MSBuild/DummyBuildEngine.cs

@@ -1,21 +1,31 @@
 using System;
 using System.Collections;
+using System.Collections.Generic;
 using Microsoft.Build.Framework;
 
 namespace Xamarin.Forms.Xaml.UnitTests
 {
 	public class DummyBuildEngine : IBuildEngine
 	{
+		public List<BuildErrorEventArgs> Errors { get; } = new List<BuildErrorEventArgs> ();
+
+		public List<BuildWarningEventArgs> Warnings { get; } = new List<BuildWarningEventArgs> ();
+
+		public List<BuildMessageEventArgs> Messages { get; } = new List<BuildMessageEventArgs> ();
+
 		public void LogErrorEvent (BuildErrorEventArgs e)
 		{
+			Errors.Add (e);
 		}
 
 		public void LogWarningEvent (BuildWarningEventArgs e)
 		{
+			Warnings.Add (e);
 		}
 
 		public void LogMessageEvent (BuildMessageEventArgs e)
 		{
+			Messages.Add (e);
 		}
 
 		public void LogCustomEvent (CustomBuildEventArgs e)