每日一问 | 玩转 Gradle，可不能不熟悉 Transform，那么，我要开始问了。

不知道

什么是Task

其实核心还是要先从什么是Task讲起了。

一个Task代表一个构建工作的原子操作，例如编译calsses或者生成javadoc。

Gradle中，每一个待编译的工程都叫一个Project。每一个Project在构建的时候都包含一系列的Task。比如一个Android APK的编译可能包含：Java源码编译Task、资源编译Task、JNI编译Task、lint检查Task、打包生成APK的Task、签名Task等。插件本身就是包含了若干Task的。

简单的说我们的项目编译以assembleDebug为例子，会顺序执行非常多的gradle task任务，举个例子比如说aapt，javac，kotlinc等等，他们都是作为一个task存在的。

AGP中的Transform

   AppExtension appExtension = project.getExtensions().getByType(AppExtension.class);
   appExtension.registerTransform(new DoubleTabTransform(project));

当我们在编写一个Transform的plugin的时候，其实是对安卓的AppExtension进行了一个注册Transform的操作而已，那么Transform的本质到底是什么呢？

高能预警，下面的源代码比较长，可以考虑直接跳过看结论，但是看得懂的同学最好可以学习下。

public class LibraryTaskManager extends TaskManager {

    @Override
    public void createTasksForVariantScope(@NonNull final VariantScope variantScope) {
        ...
         // ----- External Transforms -----
        // apply all the external transforms.
        List<Transform> customTransforms = extension.getTransforms();
        List<List<Object>> customTransformsDependencies = extension.getTransformsDependencies();

        for (int i = 0, count = customTransforms.size(); i < count; i++) {
            Transform transform = customTransforms.get(i);

            // Check the transform only applies to supported scopes for libraries:
            // We cannot transform scopes that are not packaged in the library
            // itself.
            Sets.SetView<? super Scope> difference =
                    Sets.difference(transform.getScopes(), TransformManager.PROJECT_ONLY);
            if (!difference.isEmpty()) {
                String scopes = difference.toString();
                globalScope
                        .getAndroidBuilder()
                        .getIssueReporter()
                        .reportError(
                                Type.GENERIC,
                                new EvalIssueException(
                                        String.format(
                                                "Transforms with scopes '%s' cannot be applied to library projects.",
                                                scopes)));
            }

            List<Object> deps = customTransformsDependencies.get(i);
            transformManager.addTransform(
                    taskFactory,
                    variantScope,
                    transform,
                    null,
                    task -> {
                        if (!deps.isEmpty()) {
                            task.dependsOn(deps);
                        }
                    },
                    taskProvider -> {
                        // if the task is a no-op then we make assemble task
                        // depend on it.
                        if (transform.getScopes().isEmpty()) {
                            TaskFactoryUtils.dependsOn(
                                    variantScope.getTaskContainer().getAssembleTask(),
                                    taskProvider);
                        }
                    });
        }

        // Now add transforms for intermediate publishing (projects to projects).
        File jarOutputFolder = variantScope.getIntermediateJarOutputFolder();
        File mainClassJar = new File(jarOutputFolder, FN_CLASSES_JAR);
        File mainResJar = new File(jarOutputFolder, FN_INTERMEDIATE_RES_JAR);
        LibraryIntermediateJarsTransform intermediateTransform =
                new LibraryIntermediateJarsTransform(
                        mainClassJar,
                        mainResJar,
                        variantConfig::getPackageFromManifest,
                        extension.getPackageBuildConfig());
        excludeDataBindingClassesIfNecessary(variantScope, intermediateTransform);

        BuildArtifactsHolder artifacts = variantScope.getArtifacts();
        transformManager.addTransform(
                taskFactory,
                variantScope,
                intermediateTransform,
                taskName -> {
                    // publish the intermediate classes.jar
                    artifacts.appendArtifact(
                            InternalArtifactType.LIBRARY_CLASSES,
                            ImmutableList.of(mainClassJar),
                            taskName);
                    // publish the res jar
                    artifacts.appendArtifact(
                            InternalArtifactType.LIBRARY_JAVA_RES,
                            ImmutableList.of(mainResJar),
                            taskName);
                },
                null,
                null);

        taskFactory.register(new LibraryDexingTask.CreationAction(variantScope));

        // Create a jar with both classes and java resources.  This artifact is not
        // used by the Android application plugin and the task usually don't need to
        // be executed.  The artifact is useful for other Gradle users who needs the
        // 'jar' artifact as API dependency.
        taskFactory.register(new ZipMergingTask.CreationAction(variantScope));

        // now add a transform that will take all the native libs and package
        // them into an intermediary folder. This processes only the PROJECT
        // scope.
        final File intermediateJniLibsFolder = new File(jarOutputFolder, FD_JNI);

        LibraryJniLibsTransform intermediateJniTransform =
                new LibraryJniLibsTransform(
                        "intermediateJniLibs",
                        intermediateJniLibsFolder,
                        TransformManager.PROJECT_ONLY);
        transformManager.addTransform(
                taskFactory,
                variantScope,
                intermediateJniTransform,
                taskName -> {
                    // publish the jni folder as intermediate
                    variantScope
                            .getArtifacts()
                            .appendArtifact(
                                    InternalArtifactType.LIBRARY_JNI,
                                    ImmutableList.of(intermediateJniLibsFolder),
                                    taskName);
                },
                null,
                null);

        // Now go back to fill the pipeline with transforms used when
        // publishing the AAR

        // first merge the resources. This takes the PROJECT and LOCAL_DEPS
        // and merges them together.
        createMergeJavaResTransform(variantScope);

        // ----- Minify next -----
        maybeCreateJavaCodeShrinkerTransform(variantScope);
        maybeCreateResourcesShrinkerTransform(variantScope);

        // now add a transform that will take all the class/res and package them
        // into the main and secondary jar files that goes in the AAR.
        // This transform technically does not use its transform output, but that's
        // ok. We use the transform mechanism to get incremental data from
        // the streams.
        // This is used for building the AAR.

        File classesJar = variantScope.getAarClassesJar();
        File libsDirectory = variantScope.getAarLibsDirectory();

        LibraryAarJarsTransform transform =
                new LibraryAarJarsTransform(
                        classesJar,
                        libsDirectory,
                        artifacts.hasArtifact(InternalArtifactType.ANNOTATIONS_TYPEDEF_FILE)
                                ? artifacts.getFinalArtifactFiles(
                                        InternalArtifactType.ANNOTATIONS_TYPEDEF_FILE)
                                : null,
                        variantConfig::getPackageFromManifest,
                        extension.getPackageBuildConfig());

        excludeDataBindingClassesIfNecessary(variantScope, transform);

        transformManager.addTransform(
                taskFactory,
                variantScope,
                transform,
                taskName -> {
                    variantScope
                            .getArtifacts()
                            .appendArtifact(
                                    InternalArtifactType.AAR_MAIN_JAR,
                                    ImmutableList.of(classesJar),
                                    taskName);
                    variantScope
                            .getArtifacts()
                            .appendArtifact(
                                    InternalArtifactType.AAR_LIBS_DIRECTORY,
                                    ImmutableList.of(libsDirectory),
                                    taskName);
                },
                null,
                null);

        // now add a transform that will take all the native libs and package
        // them into the libs folder of the bundle. This processes both the PROJECT
        // and the LOCAL_PROJECT scopes
        final File jniLibsFolder =
                variantScope.getIntermediateDir(InternalArtifactType.LIBRARY_AND_LOCAL_JARS_JNI);
        LibraryJniLibsTransform jniTransform =
                new LibraryJniLibsTransform(
                        "syncJniLibs",
                        jniLibsFolder,
                        TransformManager.SCOPE_FULL_LIBRARY_WITH_LOCAL_JARS);
        transformManager.addTransform(
                taskFactory,
                variantScope,
                jniTransform,
                taskName ->
                        variantScope
                                .getArtifacts()
                                .appendArtifact(
                                        InternalArtifactType.LIBRARY_AND_LOCAL_JARS_JNI,
                                        ImmutableList.of(jniLibsFolder),
                                        taskName),
                null,
                null);

        createLintTasks(variantScope);
        createBundleTask(variantScope);
    }
}

自定义Transform和其他系统Transform执行的顺序

而且上述方法我们可以看出，任务还是会根据DAG（有向无环图）生成Task，其中会包含一些系统的Transform，其顺序有可能会被插入到自定义的Transform之前，而有一些则会被放置在所有的Tranform执行之后。比如LibraryJniLibsTransform。

Transform和Task的关系

从这部分源代码其实我们就可以看出，我们注册到AppExtension里面的Transform,最后会createTasksForVariantScope方法调用到。

@NonNull
 public <T extends Transform> Optional<TaskProvider<TransformTask>> addTransform(
         @NonNull TaskFactory taskFactory,
         @NonNull TransformVariantScope scope,
         @NonNull T transform,
         @Nullable PreConfigAction preConfigAction,
         @Nullable TaskConfigAction<TransformTask> configAction,
         @Nullable TaskProviderCallback<TransformTask> providerCallback) {

     if (!validateTransform(transform)) {
         // validate either throws an exception, or records the problem during sync
         // so it's safe to just return null here.
         return Optional.empty();
     }

     List<TransformStream> inputStreams = Lists.newArrayList();
     String taskName = scope.getTaskName(getTaskNamePrefix(transform));

     // get referenced-only streams
     List<TransformStream> referencedStreams = grabReferencedStreams(transform);

     // find input streams, and compute output streams for the transform.
     IntermediateStream outputStream = findTransformStreams(
             transform,
             scope,
             inputStreams,
             taskName,
             scope.getGlobalScope().getBuildDir());

     if (inputStreams.isEmpty() && referencedStreams.isEmpty()) {
         // didn't find any match. Means there is a broken order somewhere in the streams.
         issueReporter.reportError(
                 Type.GENERIC,
                 new EvalIssueException(
                         String.format(
                                 "Unable to add Transform '%s' on variant '%s': requested streams not available: %s+%s / %s",
                                 transform.getName(),
                                 scope.getFullVariantName(),
                                 transform.getScopes(),
                                 transform.getReferencedScopes(),
                                 transform.getInputTypes())));
         return Optional.empty();
     }

     //noinspection PointlessBooleanExpression
     if (DEBUG && logger.isEnabled(LogLevel.DEBUG)) {
         logger.debug("ADDED TRANSFORM(" + scope.getFullVariantName() + "):");
         logger.debug("\tName: " + transform.getName());
         logger.debug("\tTask: " + taskName);
         for (TransformStream sd : inputStreams) {
             logger.debug("\tInputStream: " + sd);
         }
         for (TransformStream sd : referencedStreams) {
             logger.debug("\tRef'edStream: " + sd);
         }
         if (outputStream != null) {
             logger.debug("\tOutputStream: " + outputStream);
         }
     }

     transforms.add(transform);

     // create the task...
     return Optional.of(
             taskFactory.register(
                     new TransformTask.CreationAction<>(
                             scope.getFullVariantName(),
                             taskName,
                             transform,
                             inputStreams,
                             referencedStreams,
                             outputStream,
                             recorder),
                     preConfigAction,
                     configAction,
                     providerCallback));
 }

然后我们看transformManager.addTransform,直接观察最后一行代码，发现我们会往taskFactory中直接注册一个TransformTask，那么回答第一个问题，一个Transform就是gradle plugin中的一个原子的Task，所以Transform==Task。

Transform是如何被执行的

当我们知道Transform的本质是Task之后就知道了，他会和一个普通的Task一样被执行，而且会按照特定的依赖顺序执行。

看看