Dynamically Pack MVVM SourceGen project outputs

Previously, static output path to the MVVM SourceGen assembly was used to pack the MVVM project.
This leads to error when OutputPath was updated dynamically when testing or in forks.

So, here, we'll update the build so that the SourceGen build outputs will be dynamically packed.

CommunityToolkit.Mvvm.SourceGenerators/CommunityToolkit.Mvvm.SourceGenerators.csproj

@@ -26,4 +26,6 @@
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" PrivateAssets="All" Pack="false" />
   </ItemGroup>
 
+  <Import Project="$(BuildToolsDirectory)Community.Toolkit.GetBuildOutputs.targets" />
+
 </Project>

CommunityToolkit.Mvvm/CommunityToolkit.Mvvm.csproj

@@ -50,20 +50,33 @@
   </ItemGroup>
 
   <!-- Source generator project reference for packing -->
-  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+  <ItemGroup>
     <ProjectReference Include="..\CommunityToolkit.Mvvm.SourceGenerators\CommunityToolkit.Mvvm.SourceGenerators.csproj"
                       ReferenceOutputAssembly="false" />
   </ItemGroup>
 
+  <!-- Use 'GenerateNuspecDependsOn' extensibility point to include build assets into our package -->
+  <PropertyGroup>
+    <GenerateNuspecDependsOn>GetSourceGenBuildOutputs</GenerateNuspecDependsOn>
+  </PropertyGroup>
+
   <!--
-      Pack the source generator to the right package folder.
-      TODO: Find a way to get the analyzer project's build output path,
-            and include using NuGet Pack's extensibility target points.
+      Pack the source generator build outputs to the correct package folder for it to be used as an analyzer.
+      The following target uses 'GenerateNuspecDependsOn' extensibility point to pack the build assets
+      by using our custom 'GetBuildOutputs' target imported within the target project.
   -->
-  <ItemGroup Label="Package">
-    <None Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll"
-          PackagePath="analyzers\dotnet\roslyn4.0\cs"
-          Pack="true" Visible="false" />
-  </ItemGroup>
+  <Target Name="GetSourceGenBuildOutputs" AfterTargets="_CalculateInputsOutputsForPack">
+    <MSBuild
+      Projects="@(ProjectReference->WithMetadataValue('ReferenceOutputAssembly', 'false'))"
+      Targets="GetBuildOutputs">
+      <Output TaskParameter="TargetOutputs" ItemName="SourceGenBuildOutput" />
+    </MSBuild>
+    <ItemGroup>
+      <!-- Include SourceGen Build Outputs in the package -->
+      <_PackageFiles Include="@(SourceGenBuildOutput)" PackagePath="analyzers\dotnet\roslyn4.0\cs" />
+      <!-- Use the 'FinalOutputPath' metadata to mark the inputs thus enabling incremental builds -->
+      <NuGetPackInput Include="@(SourceGenBuildOutput->'%(FinalOutputPath)')" />
+    </ItemGroup>
+  </Target>
 
 </Project>

build/Community.Toolkit.GetBuildOutputs.targets

@@ -0,0 +1,47 @@
+<Project>
+
+  <!-- We depend on these Common Targets that gathers various Build Outputs of the included project -->
+  <PropertyGroup>
+    <_GetBuildOutputsDependsOn>BuiltProjectOutputGroup;DocumentationProjectOutputGroup</_GetBuildOutputsDependsOn>
+    <_GetBuildOutputsDependsOn Condition="'$(IncludeSymbols)' != 'false'">$(_GetBuildOutputsDependsOn);DebugSymbolsProjectOutputGroup</_GetBuildOutputsDependsOn>
+    <_GetBuildOutputsDependsOn Condition="'$(IncludeSatelliteAssemblies)' != 'false'">$(_GetBuildOutputsDependsOn);SatelliteDllsProjectOutputGroup</_GetBuildOutputsDependsOn>
+  </PropertyGroup>
+
+  <!-- Check for 'TargetFramework' input before proceeding. -->
+  <Target Name="_CheckTargetFrameworkInput"
+          DependsOnTargets="_CheckForInvalidConfigurationAndPlatform"
+          Condition="'$(TargetFramework)' == ''">
+    <Error Code="NCTDEV01" Text="The 'GetBuildOutputsPerTarget' target is meant to output the build items per 'TargetFramework'. Please call the target with a non-empty 'TargetFramework'."/>
+  </Target>
+
+  <!--
+      Gathers various Build Outputs from included Project per 'TargetFramework'.
+      This target is called in by the 'GetBuildOutputs' target to combine build outputs from all 'TargetFrameworks'.
+  -->
+  <Target Name="GetBuildOutputsPerTarget"
+          DependsOnTargets="_CheckTargetFrameworkInput;$(_GetBuildOutputsDependsOn)"
+          Returns="@(BuildOutputPerTarget)">
+    <ItemGroup>
+      <BuildOutputPerTarget Include="@(BuiltProjectOutputGroupOutput);@(DocumentationProjectOutputGroupOutput)"/>
+      <BuildOutputPerTarget Include="@(DebugSymbolsProjectOutputGroupOutput)" Condition="'$(IncludeSymbols)' != 'false'"/>
+      <BuildOutputPerTarget Include="@(SatelliteDllsProjectOutputGroupOutput)" Condition="'$(IncludeSatelliteAssemblies)' != 'false'"/>
+      <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" TargetFramework="$(TargetFramework)"/>
+    </ItemGroup>
+  </Target>
+
+  <!--
+      Gathers various Build Outputs from this Project across 'TargetFrameworks'.
+      This target is called in the parent project to pack into a package.
+  -->
+  <Target Name="GetBuildOutputs"
+          DependsOnTargets="_GetTargetFrameworksOutput"
+          Returns="@(BuildOutput)">
+    <MSBuild
+      Projects="$(MSBuildProjectFullPath)"
+      Targets="GetBuildOutputsPerTarget"
+      Properties="TargetFramework=%(_TargetFrameworks.Identity)">
+      <Output TaskParameter="TargetOutputs" ItemName="BuildOutput" />
+    </MSBuild>
+  </Target>
+
+</Project>