flutter/packages/flutter_tools/gradle/aar_init_script.gradle

// This script is used to initialize the build in a module or plugin project.
// During this phase, the script applies the Maven plugin and configures the
// destination of the local repository.
// The local repository will contain the AAR and POM files.

import java.nio.file.Paths
import org.gradle.api.Project
import org.gradle.api.artifacts.Configuration
import org.gradle.api.artifacts.maven.MavenDeployer
import org.gradle.api.plugins.MavenPlugin
import org.gradle.api.tasks.Upload

void configureProject(Project project, String outputDir) {
    if (!project.hasProperty("android")) {
        throw new GradleException("Android property not found.")
    }
    if (!project.android.hasProperty("libraryVariants")) {
        throw new GradleException("Can't generate AAR on a non Android library project.");
    }

    project.apply plugin: "maven"

    project.android.libraryVariants.all { variant ->
        addAarTask(project, variant)
    }
    // Snapshot versions include the timestamp in the artifact name.
    // Therefore, remove the snapshot part, so new runs of `flutter build aar` overrides existing artifacts.
    // This version isn't relevant in Flutter since the pub version is used
    // to resolve dependencies.
    project.version = project.version.replace("-SNAPSHOT", "")

    project.uploadArchives {
        repositories {
            mavenDeployer {
                repository(url: "file://${outputDir}/outputs/repo")
            }
        }
    }
    if (!project.property("is-plugin").toBoolean()) {
        return
    }
    if (project.hasProperty('localEngineOut')) {
        // TODO(egarciad): Support local engine.
        // This most likely requires refactoring `flutter.gradle`, so the logic can be reused.
        throw new GradleException(
            "Local engine isn't supported when building the plugins as AAR. " +
            "See: https://github.com/flutter/flutter/issues/40866")
    }

    // This is a Flutter plugin project. Plugin projects don't apply the Flutter Gradle plugin,
    // as a result, add the dependency on the embedding.
    project.repositories {
        maven {
            url "http://download.flutter.io"
        }
    }
    String engineVersion = Paths.get(getFlutterRoot(project), "bin", "internal", "engine.version")
        .toFile().text.trim()
    project.dependencies {
        // Add the embedding dependency.
        compileOnly ("io.flutter:flutter_embedding_release:1.0.0-$engineVersion") {
            // We only need to expose io.flutter.plugin.*
            // No need for the embedding transitive dependencies.
            transitive = false
        }
    }
}

String getFlutterRoot(Project project) {
    if (!project.hasProperty("flutter-root")) {
        throw new GradleException("The `-Pflutter-root` flag must be specified.")
    }
    return project.property("flutter-root")
}

void addAarTask(Project project, variant) {
    String variantName = variant.name.capitalize()
    String taskName = "assembleAar$variantName"
    project.tasks.create(name: taskName) {
        // This check is required to be able to configure the archives before `uploadArchives` runs.
        if (!project.gradle.startParameter.taskNames.contains(taskName)) {
            return
        }
        project.uploadArchives.repositories.mavenDeployer {
            pom {
                artifactId = "${project.name}_${variant.name.toLowerCase()}"
            }
        }
        overrideDefaultPublishConfig(project, variant)
        // Generate the Maven artifacts.
        finalizedBy "uploadArchives"
    }
}

// This method mimics the logic in AGP when `android.defaultPublishConfig` is set in `build.gradle`:
// https://android.googlesource.com/platform/tools/base/+/studio-master-dev/build-system/gradle-core/src/main/java/com/android/build/gradle/internal/variant/VariantHelper.java
//
// Unfortunately, `android.defaultPublishConfig` cannot be overriden at this point since
// AGP already run this code.
void overrideDefaultPublishConfig(Project project, variant) {
    String variantName = variant.name.capitalize()
    Task bundle = project.tasks.findByName("bundle${variantName}Aar") // gradle:3.2.0
    if (bundle == null) {
        bundle = project.tasks.findByName("bundle${variantName}") // gradle:3.1.0
    }
    if (bundle == null) {
        throw new GradleException("Can't generate AAR for variant ${variantName}.");
    }
    // Clear the current archive artifacts since the artifacts are based on `android.defaultPublishConfig`.
    project.configurations["archives"].artifacts.clear()
    // Add the artifact that will be published.
    project.artifacts.add("archives", bundle)

    def scopeMappings = project.uploadArchives.repositories.mavenDeployer.pom.scopeMappings
    // Clear the scope mappings added by AGP since they are based on the current `android.defaultPublishConfig`.
    scopeMappings.mappings.clear()
    // Add the new mappings.
    for (Configuration configuration : flattenConfiguration(variant.runtimeConfiguration)) {
        scopeMappings.addMapping(/* priority = */ 300, configuration, "compile")
    }
}

Set<Configuration> flattenConfiguration(Configuration configuration) {
    Set<Configuration> configs = [configuration]
    for (Configuration extend : configuration.extendsFrom) {
        configs.addAll(flattenConfiguration(extend))
    }
    return configs
}

projectsEvaluated {
    assert rootProject.hasProperty("is-plugin")
    if (rootProject.property("is-plugin").toBoolean()) {
        assert rootProject.hasProperty("output-dir")
        // In plugin projects, the root project is the plugin.
        configureProject(rootProject, rootProject.property("output-dir"))
        return
    }
    // The module project is the `:flutter` subproject.
    Project moduleProject = rootProject.subprojects.find { it.name == "flutter" }
    assert moduleProject != null
    assert moduleProject.hasProperty("output-dir")
    configureProject(moduleProject, moduleProject.property("output-dir"))

    // Gets the plugin subprojects.
    Set<Project> modulePlugins = rootProject.subprojects.findAll {
        it.name != "flutter" && it.name != "app"
    }
    // When a module is built as a Maven artifacts, plugins must also be built this way
    // because the module POM's file will include a dependency on the plugin Maven artifact.
    // This is due to the Android Gradle Plugin expecting all library subprojects to be published
    // as Maven artifacts.
    modulePlugins.each { pluginProject ->
        configureProject(pluginProject, moduleProject.property("output-dir"))
        moduleProject.android.libraryVariants.all { variant ->
            // Configure the `assembleAar<variantName>` task for each plugin's projects and make
            // the module's equivalent task depend on the plugin's task.
            String variantName = variant.name.capitalize()
            moduleProject.tasks.findByPath("assembleAar$variantName")
                .dependsOn(pluginProject.tasks.findByPath("assembleAar$variantName"))
        }
    }
}