[XamlG] builds incrementally

Context: xamarin#2230

The main performance problem with the collection of MSBuild targets in
`Xamarin.Forms.targets` is they don't build incrementally. I addressed
this with `XamlC` using a "stamp" file; however, it is not quite so
easy to setup the same thing with `XamlG`.

They way "incremental" builds are setup in MSBuild, is by specifying
the `Inputs` and `Outputs` of a `<Target />`. MSBuild will partially
build a target when some outputs are not up to date, and skip it
entirely if they are all up to date.

The best docs I can find on MSBuild incremental builds:
https://msdn.microsoft.com/en-us/library/ms171483.aspx

Unfortunately a few things had to happen to make this work for
`XamlG`:
- Define a new target `_FindXamlGFiles` that is invoked before `XamlG`
- `_FindXamlGFiles` defines the `_XamlGInputs` and `_XamlGOutputs`
  `<ItemGroup />`'s
- `_FindXamlGFiles` must also define `<Compile />` and `<FileWrites />`,
  in case the `XamlG` target is skipped
- `XamlGTask` now needs to get passed in a list of `OutputFiles`,
  since we have computed these paths ahead of time
- `XamlGTask` should validate the lengths of `XamlFiles` and
  `OutputFiles` match, used error message from MSBuild proper:
  https://github.com/Microsoft/msbuild/blob/a691a44f0e515e9a03ede8df0bff22185681c8b9/src/Tasks/Copy.cs#L505

`XamlG` now builds incrementally!

Other changes:
- I reordered my past `<Output />` element in XamlC to match others in
  this file
- `FilesWrite` is actually supposed to be `FileWrites`, see canonical
  source of how `Clean` works and what `FileWrites` is here:
  dotnet/msbuild#2408 (comment)

Unfortunately, there is more to do to get this PR "feature complete":
- TESTS? This refactoring is complex and has a bit of nuance to it. I
  can't imagine we can get this improvement right (at all), without
  some new testing infrastructure in place that invokes MSBuild.
- There were some checks for `%(TargetPath)` being blank in the C#
  code of `XamlGTask`. I presume this is from the designer and/or
  design-time builds. We probably need to figure out the proper way to
  do this instead of using `%(TargetPath)` at all. After testing the
  current implementation I had all kinds of crazy temporary files in
  my `$(IntermediateOutputPath)`. This probably isn't the right way to
  handle this issue.
- CssG needs the exact same setup, as it was patterned after `XamlG`

.nuspec/Xamarin.Forms.targets

@@ -64,18 +64,21 @@
 		</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'" />
+			<_XamlGOutputs Include="@(_XamlGInputs->'$(IntermediateOutputPath)%(TargetPath).g.cs')" />
+			<FileWrites Include="@(_XamlGOutputs)" />
+			<Compile Include="@(_XamlGOutputs)" />
+		</ItemGroup>
+	</Target>
+
+	<Target Name="XamlG" BeforeTargets="BeforeCompile" DependsOnTargets="_FindXamlGFiles" Inputs="@(_XamlGInputs)" Outputs="@(_XamlGOutputs)">
 		<XamlGTask
-			XamlFiles="@(EmbeddedResource)" Condition="'%(Extension)' == '.xaml' AND '$(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)" />
 	</Target>
 
 	<!-- XamlC -->
@@ -95,7 +98,7 @@
 			DebugType = "$(DebugType)"
 			KeepXamlResources = "$(XFKeepXamlResources)" />
 		<Touch Files="$(IntermediateOutputPath)XamlC.stamp" AlwaysCreate="True">
-			<Output TaskParameter="TouchedFiles" ItemName="FileWrites"/>
+			<Output ItemName="FileWrites" TaskParameter="TouchedFiles" />
 		</Touch>
 	</Target>
 
@@ -115,7 +118,7 @@
 			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

@@ -10,17 +10,14 @@ 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()
 		{
@@ -32,22 +29,20 @@ public override bool Execute()
 				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.LogErrorWithCodeFromResources("General.TwoVectorsMustHaveSameLength", XamlFiles.Length, OutputFiles?.Length, "XamlFiles", "OutputFiles");
+				return false;
+			}
 
-				var outputFile = Path.Combine(OutputPath, $"{targetPath}.g.cs");
-				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);
+			for (int i = 0; i < XamlFiles.Length; i++) {
+				var xamlFile = XamlFiles [i];
+				var outputFile = OutputFiles [i];
+				var generator = new XamlGenerator(xamlFile, Language, AssemblyName, outputFile.ItemSpec, 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.ItemSpec, 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.Xamlg/Xamlg.cs

@@ -80,7 +80,7 @@ public static void Main(string[] args)
 					AssemblyName = "test",
 					Language = "C#",
 					XamlFiles = new[] { item },
-					OutputPath = Path.GetDirectoryName(f),
+					OutputFiles = new[] { new TaskItem(Path.Combine(Path.GetDirectoryName(f), item.ItemSpec)) }
 				};