[r8] Include build metadata in app bundles (#12646)

## Summary

- pass `--build-metadata-output` to R8
- include the generated metadata at `BUNDLE-METADATA/com.android.tools/r8.json` in app bundles
- verify both R8 metadata files are packaged and incremental builds remain up to date

This metadata lets Google Play report R8 optimization, shrinking, and obfuscation coverage for the upcoming app-quality requirements announced in [App quality: Memory optimization and secure onboarding](https://android-developers.googleblog.com/2026/08/app-quality-memory-optimization-secure-onboarding.html).

Related to #12639.
Related to #12535.

## What .NET 10 customers can try now

Save the following as `Directory.Build.targets` beside the project. It enables R8 for Release builds, downloads R8 9.0.32, generates `r8.json`, and includes it in the app bundle.

The response file is required for this workaround because the shipping .NET 10 task passes `$(AndroidR8ExtraArguments)` directly on the Java command line, with platform-dependent handling of multiple arguments.

```xml
<Project>
  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
    <AndroidLinkTool>r8</AndroidLinkTool>
  </PropertyGroup>

  <Target
      Name="IncludeR8BuildMetadata"
      BeforeTargets="_CompileToDalvik"
      Condition=" '$(AndroidLinkTool)' == 'r8' ">
    <PropertyGroup>
      <_R8BuildMetadataFile>$(IntermediateOutputPath)r8.json</_R8BuildMetadataFile>
      <_R8JarFile>$([System.IO.Path]::GetFullPath('$(IntermediateOutputPath)r8-9.0.32.jar'))</_R8JarFile>
      <_R8MetadataRsp>$(IntermediateOutputPath)r8-build-metadata.rsp</_R8MetadataRsp>
      <AndroidR8JarPath>$(_R8JarFile)</AndroidR8JarPath>
      <AndroidR8ExtraArguments>@$(_R8MetadataRsp)</AndroidR8ExtraArguments>
    </PropertyGroup>

    <DownloadFile
        Condition=" !Exists('$(_R8JarFile)') "
        SourceUrl="https://storage.googleapis.com/r8-releases/raw/9.0.32/r8.jar"
        DestinationFolder="$(IntermediateOutputPath)"
        DestinationFileName="r8-9.0.32.jar"
    />
    <WriteLinesToFile
        File="$(_R8MetadataRsp)"
        Overwrite="true"
        Lines="--build-metadata-output;$(_R8BuildMetadataFile)"
    />

    <ItemGroup>
      <AndroidAppBundleMetaDataFile Include="com.android.tools/r8.json:$(_R8BuildMetadataFile)" />
    </ItemGroup>
  </Target>
</Project>
```

Build the app bundle with:

```console
dotnet build -c Release -p:AndroidPackageFormat=aab
```

The resulting signed AAB should contain `BUNDLE-METADATA/com.android.tools/r8.json`. This workaround was verified on macOS by @yuseok-kim-edushare and end-to-end on Windows with the .NET 10 Android workload: the metadata reports R8 9.0.32 and its DEX SHA-256 matches the packaged `classes.dex`.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tasks/R8.cs

@@ -34,6 +34,7 @@ public class R8 : D8
 		public string? ProguardGeneratedApplicationConfiguration { get; set; }
 		public string? ProguardCommonXamarinConfiguration { get; set; }
 		public string? ProguardMappingFileOutput { get; set; }
+		public string? BuildMetadataFileOutput { get; set; }
 		public ITaskItem []? ProguardConfigurationFiles { get; set; }
 		public bool UseTrimmableNativeAotProguardConfiguration { get; set; }
 
@@ -122,6 +123,11 @@ protected override string CreateResponseFile ()
 			// Now append R8-specific arguments to the response file
 			using var response = new StreamWriter (responseFile, append: true, encoding: Files.UTF8withoutBOM);
 
+			if (!BuildMetadataFileOutput.IsNullOrEmpty ()) {
+				WriteArg (response, "--build-metadata-output");
+				WriteArg (response, Path.GetFullPath (BuildMetadataFileOutput));
+			}
+
 			if (EnableMultiDex) {
 				if (MinSdkVersion >= 21) {
 					if (CustomMainDexListFiles?.Length > 0) {

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/PackagingTest.cs

@@ -17,7 +17,7 @@ namespace Xamarin.Android.Build.Tests
 	public class PackagingTest : BaseTest
 	{
 		[Test]
-		public void CheckProguardMappingFileExists ([Values (AndroidRuntime.CoreCLR, AndroidRuntime.NativeAOT)] AndroidRuntime runtime)
+		public void CheckR8MetadataFilesExist ([Values (AndroidRuntime.CoreCLR, AndroidRuntime.NativeAOT)] AndroidRuntime runtime)
 		{
 			const bool isRelease = true;
 			if (IgnoreUnsupportedConfiguration (runtime, release: isRelease)) {
@@ -31,11 +31,23 @@ public void CheckProguardMappingFileExists ([Values (AndroidRuntime.CoreCLR, And
 			proj.SetProperty (proj.ReleaseProperties, KnownProperties.AndroidLinkTool, "r8");
 			// Projects must set $(AndroidCreateProguardMappingFile) to true to opt in
 			proj.SetProperty (proj.ReleaseProperties, "AndroidCreateProguardMappingFile", true);
+			proj.SetProperty ("AndroidPackageFormat", "aab");
 
 			using (var b = CreateApkBuilder ()) {
 				string mappingFile = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, "mapping.txt");
 				Assert.IsTrue (b.Build (proj), "build should have succeeded.");
 				FileAssert.Exists (mappingFile, $"'{mappingFile}' should have been generated.");
+				var aab = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, $"{proj.PackageName}-Signed.aab");
+				FileAssert.Exists (aab, $"'{aab}' should have been generated.");
+				using (var zip = ZipHelper.OpenZip (aab)) {
+					Assert.IsTrue (zip.Any (e => e.FullName == "BUNDLE-METADATA/com.android.tools.build.obfuscation/proguard.map"), $"AAB file `{aab}` should contain the ProGuard mapping.");
+					Assert.IsTrue (zip.Any (e => e.FullName == "BUNDLE-METADATA/com.android.tools/r8.json"), $"AAB file `{aab}` should contain the R8 build metadata.");
+				}
+
+				Assert.IsTrue (b.Build (proj), "second build should have succeeded.");
+				foreach (var target in new [] { "_CompileToDalvik", "_BuildApkEmbed" }) {
+					Assert.IsTrue (b.Output.IsTargetSkipped (target), $"`{target}` should be skipped!");
+				}
 			}
 		}
 

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -230,6 +230,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<_AndroidLibraryFlatArchivesDirectory>$(IntermediateOutputPath)flata\</_AndroidLibraryFlatArchivesDirectory>
 	<_AndroidLibraryFlatFilesDirectory>$(IntermediateOutputPath)flat\</_AndroidLibraryFlatFilesDirectory>
 	<_AndroidStampDirectory>$(IntermediateOutputPath)stamp\</_AndroidStampDirectory>
+	<_AndroidR8BuildMetadataFile>$(IntermediateOutputPath)r8.json</_AndroidR8BuildMetadataFile>
         <_AndroidApplicationSharedLibraryPath>$(IntermediateOutputPath)app_shared_libraries\</_AndroidApplicationSharedLibraryPath>
 	<_ResolvedUserAssembliesHashFile>$(IntermediateOutputPath)resolvedassemblies.hash</_ResolvedUserAssembliesHashFile>
   <_AndroidResolvedResourcesHashFile>$(IntermediateOutputPath)_AndroidResolvedResources.hash</_AndroidResolvedResourcesHashFile>
@@ -2167,6 +2168,10 @@ because xbuild doesn't support framework reference assemblies.
         Condition=" Exists('$(AndroidProguardMappingFile)') "
         Include="com.android.tools.build.obfuscation/proguard.map:$(AndroidProguardMappingFile)"
     />
+    <AndroidAppBundleMetaDataFile
+        Condition=" (('$(AndroidLinkTool)' == 'r8' And '$(_ProguardProjectConfiguration)' != '') Or '$(AndroidEnableMultiDex)' == 'True') And Exists('$(_AndroidR8BuildMetadataFile)') "
+        Include="com.android.tools/r8.json:$(_AndroidR8BuildMetadataFile)"
+    />
   </ItemGroup>
 </Target>
 

src/Xamarin.Android.Build.Tasks/Xamarin.Android.D8.targets

@@ -78,6 +78,7 @@ Copyright (C) 2018 Xamarin. All rights reserved.
         ProguardGeneratedReferenceConfiguration="$(_ProguardProjectConfiguration)"
         ProguardGeneratedApplicationConfiguration="$(IntermediateOutputPath)proguard\proguard_project_primary.cfg"
         ProguardMappingFileOutput="$(AndroidProguardMappingFile)"
+        BuildMetadataFileOutput="$(_AndroidR8BuildMetadataFile)"
         ProguardConfigurationFiles="@(_ProguardConfiguration)"
         UseTrimmableNativeAotProguardConfiguration="$(_UseTrimmableNativeAotProguardConfiguration)"
         EnableShrinking="$(_R8EnableShrinking)"
@@ -115,6 +116,7 @@ Copyright (C) 2018 Xamarin. All rights reserved.
     <ItemGroup>
       <FileWrites Include="$(_AndroidIntermediateDexOutputDirectory)*.dex" />
       <FileWrites Include="$(AndroidProguardMappingFile)" />
+      <FileWrites Include="$(_AndroidR8BuildMetadataFile)" Condition=" Exists('$(_AndroidR8BuildMetadataFile)') " />
     </ItemGroup>
 
   </Target>