List<ApkData> splitsToGenerate =
        getApksToGenerate(outputScope, supportedAbis, buildTargetAbi, buildTargetDensity);

android {
    splits {
        // Configures multiple APKs based on screen density.
        density {
            // Configures multiple APKs based on screen density.
            enable true
            // Specifies a list of screen densities Gradle should not create multiple APKs for.
            exclude "ldpi", "xxhdpi", "xxxhdpi"
            // Specifies a list of compatible screen size settings for the manifest.
            compatibleScreens ‘small‘, ‘normal‘, ‘large‘, ‘xlarge‘
        }
    }
}

// ProcessAndroidResources.doFullTaskAction
List<ApkData> apkDataList = new ArrayList<>(splitsToGenerate);
for (ApkData apkData : splitsToGenerate) {
    if (apkData.requiresAapt()) {
        // 这里只处理 main 和不依赖 density 的资源
        boolean codeGen =
                (apkData.getType() == OutputFile.OutputType.MAIN
                        || apkData.getFilter(OutputFile.FilterType.DENSITY) == null);
        if (codeGen) {
            apkDataList.remove(apkData);
            invokeAaptForSplit(
                    manifestsOutputs,
                    libraryInfoList,
                    packageIdFileSet,
                    splitList,
                    featureResourcePackages,
                    apkData,
                    codeGen,
                    aapt);
            break;
        }
    }
}

// ProcessAndroidResources.invokeAaptForSplit
void invokeAaptForSplit(...) {
    // ...
    String packageForR = null;
    File srcOut = null;
    File symbolOutputDir = null;
    File proguardOutputFile = null;
    File mainDexListProguardOutputFile = null;
    // 如果传了 generateCode 参数，会生成 R.java 
    if (generateCode) {
        packageForR = originalApplicationId;

        // we have to clean the source folder output in case the package name changed.
        srcOut = getSourceOutputDir();
        if (srcOut != null) {
            FileUtils.cleanOutputDir(srcOut);
        }

        symbolOutputDir = textSymbolOutputDir.get();
        proguardOutputFile = getProguardOutputFile();
        mainDexListProguardOutputFile = getMainDexListProguardOutputFile();
    }
    // ...
    getBuilder().processResources(aapt, config);
}

performSystemPropertiesInjection(mergingReportBuilder, xmlDocumentOptional.get());
// ManifestMerger2.performSystemPropertiesInjection
protected void performSystemPropertiesInjection(
        @NonNull MergingReport.Builder mergingReport,
        @NonNull XmlDocument xmlDocument) {
    for (ManifestSystemProperty manifestSystemProperty : ManifestSystemProperty.values()) {
        String propertyOverride = mSystemPropertyResolver.getValue(manifestSystemProperty);
        if (propertyOverride != null) {
            manifestSystemProperty.addTo(
                    mergingReport.getActionRecorder(), xmlDocument, propertyOverride);
        }
    }
}

for (File inputFile : mFlavorsAndBuildTypeFiles) {
    LoadedManifestInfo overlayDocument = load(
            new ManifestInfo(null, inputFile, XmlDocument.Type.OVERLAY,
                    Optional.of(mainPackageAttribute.get().getValue())),
            selectors,
            mergingReportBuilder);

    // 检查 package 定义
    Optional<XmlAttribute> packageAttribute =
            overlayDocument.getXmlDocument().getPackage();
    if (loadedMainManifestInfo.getOriginalPackageName().isPresent() &&
            packageAttribute.isPresent()
            && !loadedMainManifestInfo.getOriginalPackageName().get().equals(
            packageAttribute.get().getValue())) {
        // 如果 package 定义重复的话，会输出下面信息，我们平时应该或多或少见过类似的错误
        String message = mMergeType == MergeType.APPLICATION
                ? String.format(
                        "Overlay manifest:package attribute declared at %1$s value=(%2$s)\n"
                                + "\thas a different value=(%3$s) "
                                + "declared in main manifest at %4$s\n"
                                + "\tSuggestion: remove the overlay declaration at %5$s "
                                + "\tand place it in the build.gradle:\n"
                                + "\t\tflavorName {\n"
                                + "\t\t\tapplicationId = \"%2$s\"\n"
                                + "\t\t}",
                        packageAttribute.get().printPosition(),
                        packageAttribute.get().getValue(),
                        mainPackageAttribute.get().getValue(),
                        mainPackageAttribute.get().printPosition(),
                        packageAttribute.get().getSourceFile().print(true))
                : String.format(
                        "Overlay manifest:package attribute declared at %1$s value=(%2$s)\n"
                                + "\thas a different value=(%3$s) "
                                + "declared in main manifest at %4$s",
                        packageAttribute.get().printPosition(),
                        packageAttribute.get().getValue(),
                        mainPackageAttribute.get().getValue(),
                        mainPackageAttribute.get().printPosition());
        // ...
        return mergingReportBuilder.build();
    }
}

DexArchiveBuilderTransform.transform -> DexArchiveBuilderTransform.convertJarToDexArchive -> DexArchiveBuilderTransform.convertToDexArchive -> DexArchiveBuilderTransform.launchProcessing -> DxDexArchiveBuilder.convert