Always include common files in package

When the '$(IncludeContentInPack)' property is false, files specified via '@(None)', '@(Content)' items
are excluded from the NuGet package. Adding '@(PackageFile)' items directly to '%(_PackageFiles)' item
via the following target ensures that they will always be included in the package.

So, Use '%(PackageFile)' item to always include files in the package. By default, they are included in
the root of the package but can be overridden via '%(PackageFile.TargetPath)' metadata.

Author: Nirmal4G

CommunityToolkit.Mvvm/CommunityToolkit.Mvvm.csproj

@@ -30,11 +30,9 @@
       Include a custom targets file to check the included source generator.
       Including it in .NET 6+ targets is not needed as it guarantees Roslyn 4.x.
   -->
-  <ItemGroup>
-    <None Include="CommunityToolkit.Mvvm.targets" Pack="True" PackagePath="build\netstandard2.0" />
-    <None Include="CommunityToolkit.Mvvm.targets" Pack="True" PackagePath="build\netstandard2.1" />
-    <None Include="CommunityToolkit.Mvvm.targets" Pack="True" PackagePath="buildTransitive\netstandard2.0" />
-    <None Include="CommunityToolkit.Mvvm.targets" Pack="True" PackagePath="buildTransitive\netstandard2.1" />
+  <ItemGroup Condition="'$(TargetFramework)' != '' AND '$(TargetFramework)' != 'net6.0'">
+    <PackageFile Include="CommunityToolkit.Mvvm.targets" IsSpecific="True" TargetPath="build\$(TargetFramework)" />
+    <PackageFile Include="CommunityToolkit.Mvvm.targets" IsSpecific="True" TargetPath="buildTransitive\$(TargetFramework)" />
   </ItemGroup>
 
   <!-- .NET Standard 2.0 doesn't have the Span<T> and IAsyncEnumerable<T> types -->
@@ -66,9 +64,8 @@
             output path and building it first is the only way to ensure they are included.
   -->
   <ItemGroup>
-    <None Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll"
-          PackagePath="analyzers\dotnet\roslyn4.0\cs"
-          Pack="true" Visible="false" />
+    <PackageFile Include="..\CommunityToolkit.Mvvm.SourceGenerators\bin\$(Configuration)\netstandard2.0\CommunityToolkit.Mvvm.SourceGenerators.dll"
+                 TargetPath="analyzers\dotnet\roslyn4.0\cs" />
   </ItemGroup>
 
 </Project>

build/Community.Toolkit.Common.props

@@ -24,4 +24,11 @@
     <ContinuousIntegrationBuild>$(TF_BUILD)</ContinuousIntegrationBuild>
   </PropertyGroup>
 
+  <ItemDefinitionGroup>
+    <PackageFile>
+      <Visible>False</Visible>
+      <TargetPath>\</TargetPath>
+    </PackageFile>
+  </ItemDefinitionGroup>
+
 </Project>

build/Community.Toolkit.Common.targets

@@ -20,9 +20,52 @@
   </PropertyGroup>
 
   <ItemGroup Condition="$(IsPackable)">
-    <None Pack="true" PackagePath="\" Visible="False" Include="$(BuildToolsDirectory)Icon.png" />
-    <None Pack="true" PackagePath="\" Visible="False" Include="$(RepositoryDirectory)License.md" />
-    <None Pack="true" PackagePath="\" Visible="False" Include="$(RepositoryDirectory)ThirdPartyNotices.txt" />
+    <PackageFile Include="$(BuildToolsDirectory)Icon.png" />
+    <PackageFile Include="$(RepositoryDirectory)License.md" />
+    <PackageFile Include="$(RepositoryDirectory)ThirdPartyNotices.txt" />
   </ItemGroup>
 
+  <!--
+      Use 'GenerateNuSpecDependsOn' extensibility point to include custom global assets in the package.
+      Use 'TargetsForTfmSpecificContentInPackage' extensibility point to include custom TFM-specific assets in the package.
+  -->
+  <PropertyGroup>
+    <GenerateNuSpecDependsOn>_AddGlobalPackageFilesToNuGetPack</GenerateNuSpecDependsOn>
+    <TargetsForTfmSpecificContentInPackage>_AddPackageFilesPerTargetFrameworkToNuGetPack</TargetsForTfmSpecificContentInPackage>
+  </PropertyGroup>
+
+  <!--
+      When the '$(IncludeContentInPack)' property is false, files specified via '@(None)', '@(Content)' items
+      are excluded from the NuGet package. Adding '@(PackageFile)' items directly to '%(_PackageFiles)' item
+      via the following target ensures that they will always be included in the package.
+
+      So, Use '%(PackageFile)' item to always include files in the package. By default, they are included in
+      the root of the package but can be overridden via '%(PackageFile.TargetPath)' metadata. Since, 'TargetPath'
+      is just an alias of the 'PackagePath' metadata, multiple paths and different file names can also be specified.
+  -->
+  <Target Name="_AddGlobalPackageFilesToNuGetPack"
+          AfterTargets="_CalculateInputsOutputsForPack">
+    <ItemGroup>
+      <_PackageFiles Include="@(PackageFile)" Exclude="@(PackageFile->WithMetadataValue('IsSpecific', 'true'))" Condition="'%(PackageFile.Pack)' != 'false'">
+        <PackagePath Condition="'%(PackageFile.TargetPath)' != ''">%(PackageFile.TargetPath)</PackagePath>
+      </_PackageFiles>
+    </ItemGroup>
+  </Target>
+
+  <!--
+      Same 'PackageFile' as above but processed per target framework when 'IsSpecific: True' metadata is specified.
+      Since it is target specific, it also validates whether the package path contains the current framework alias.
+  -->
+  <Target Name="_AddPackageFilesPerTargetFrameworkToNuGetPack">
+    <ItemGroup>
+      <TFMSpecificPackageFile Include="@(PackageFile->WithMetadataValue('IsSpecific', 'true'))" Condition="'%(PackageFile.Pack)' != 'false'">
+        <PackagePath Condition="'%(PackageFile.TargetPath)' != ''">%(PackageFile.TargetPath)</PackagePath>
+      </TFMSpecificPackageFile>
+    </ItemGroup>
+
+    <!-- Error out when the target path of the package file doesn't contain target framework to differentiate itself within the package -->
+    <Error Code="NCTDEV02" Condition="!$([System.String]::new('%(PackagePath)').Contains('$(TargetFramework)')) AND '@(TFMSpecificPackageFile)' != ''"
+             Text="The package file ('%(Identity)') is 'TargetFramework' specific and should include the value ('$(TargetFramework)') somewhere in the target path ('%(PackagePath)')." />
+  </Target>
+
 </Project>