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作者：xiaoyang、逮虾户
问题
每当大家有在编译期间有修改字节码的需求，第一反应应该就是使用 Transform 吧，我们知道每个 Transform 都有它的输入、输出，问几个问题：
- 在编译过程中，有哪些“系统Transform”执行？
- 自定义 Transform和其他系统Transform执行的顺序是怎么样的？
- Transform和 gradle task的关系是怎么样的？
参考答案：
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。
最后
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