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Android WorkManager浅谈

2019-08-26 15:11 3115 查看

一、原文翻译

WorkManager API 可以很容易的指定可延迟的异步任务。允许你创建任务,并把它交给WorkManager来立即运行或在适当的时间运行。WorkManager根据设备API的级别和应用程序状态等因素来选择适当的方式运行任务。如果WorkManager在应用程序运行时执行你的任务,它会在应用程序进程的新线程中执行。如果应用程序没有运行,WorkManager会根据设备API级别和包含的依赖项选择适当的方式安排后台任务,可能会使用JobScheduler、Firebase JobDispatcher或AlarmManager。你不需要编写设备逻辑来确定设备有哪些功能和选择适当的API;相反,你只要把它交给WorkManager让它选择最佳的方式。

Note:WorkManager适用于需要保证即使应用程序退出系统也能运行任务,比如上传应用数据到服务器。不适用于当应用程序退出后台进程能安全终止工作,这种情况推荐使用ThreadPools。

功能:

基础功能

  • 使用WorkManager创建运行在你选择的环境下的单个任务或指定间隔的重复任务
  • WorkManager API使用几个不同的类,有时,你需要继承一些类。
  • Worker 指定需要执行的任务。有一个抽象类Worker,你需要继承并在此处工作。在后台线程同步工作的类。WorkManager在运行时实例化Worker类,并在预先指定的线程调用doWork方法(见Configuration.getExecutor())。此方法同步处理你的工作,意味着一旦方法返回,Worker被视为已经完成并被销毁。如果你需要异步执行或调用异步API,应使用ListenableWorker。如果因为某种原因工作没抢占,相同的Worker实例不会被重用。即每个Worker实例只会调用一次doWork()方法,如果需要重新运行工作单元,需要创建新的Worker。Worker最大10分钟完成执行并ListenableWorker.Result。如果过期,则会被发出信号停止。(Worker的doWork()方法是同步的,方法执行完则结束,不会重复执行,且默认超时时间是10分钟,超过则被停止。)
  • WorkRequest 代表一个独立的任务。一个WorkRequest对象至少指定哪个Worker类应该执行该任务。但是,你还可以给WorkRequest添加详细信息,比如任务运行时的环境。每个WorkRequest有一个自动生成的唯一ID,你可以使用ID来取消排队的任务或获取任务的状态。WorkRequest是一个抽象类,你需要使用它一个子类,OneTimeWorkRequest或PeriodicWorkRequest。 WorkRequest.Builder 创建WorkRequest对象的帮助类,你需要使用子类OneTimeWorkRequest.Builder或PeriodicWorkRequest.Builder。
  • Constraints(约束) 指定任务执行时的限制(如只有网络连接时)。使用Constraints.Builder创建Constraints对象,并在创建WorkRequest对象前传递给WorkRequest.Builder。
  • WorkManager 排队和管理WorkRequest。将WorkRequest对象传递给WorkManager来将任务添加到队列。WorkManager 使用分散加载系统资源的方式安排任务,同时遵守你指定的约束。
      WorkManager使用一种底层作业调度服务基于下面的标注
    • 使用JobScheduler API23+
    • 使用AlarmManager + BroadcastReceiver API14-22
  • WorkInfo 包含有关特定任务的信息。WorkManager为每个WorkRequest对象提供一个LiveData。LiveData持有WorkInfo对象,通过观察LiveData,你可以确定任务的当前状态,并在任务完成后获取任何返回的值。
  • 二、源码简单分析

    android.arch.work:work-runtime-1.0.0-beta03

    WorkerManager的具体实现类是WorkManagerImpl。

    WorkManager不同的方法,会创建不同的***Runnable类来执行。

    下面是整体的包结构

    以EnqueueRunnable为例

    @Override
    public void run() {
    try {
    if (mWorkContinuation.hasCycles()) {
    throw new IllegalStateException(
    String.format("WorkContinuation has cycles (%s)", mWorkContinuation));
    }
    boolean needsScheduling = addToDatabase();
    if (needsScheduling) {
    
    final Context context =
    mWorkContinuation.getWorkManagerImpl().getApplicationContext();
    PackageManagerHelper.setComponentEnabled(context, RescheduleReceiver.class, true);
    scheduleWorkInBackground();
    }
    mOperation.setState(Operation.SUCCESS);
    } catch (Throwable exception) {
    mOperation.setState(new Operation.State.FAILURE(exception));
    }
    }
    /**
    * Schedules work on the background scheduler.
    */
    @VisibleForTesting
    public void scheduleWorkInBackground() {
    WorkManagerImpl workManager = mWorkContinuation.getWorkManagerImpl();
    Schedulers.schedule(
    workManager.getConfiguration(),
    workManager.getWorkDatabase(),
    workManager.getSchedulers());
    }

    主要执行在Schedulers类中

    /**
    * Schedules {@link WorkSpec}s while honoring the {@link Scheduler#MAX_SCHEDULER_LIMIT}.
    *
    * @param workDatabase The {@link WorkDatabase}.
    * @param schedulers  The {@link List} of {@link Scheduler}s to delegate to.
    */
    public static void schedule(
    @NonNull Configuration configuration,
    @NonNull WorkDatabase workDatabase,
    List<Scheduler> schedulers) {
    if (schedulers == null || schedulers.size() == 0) {
    return;
    }
    
    ...
    
    if (eligibleWorkSpecs != null && eligibleWorkSpecs.size() > 0) {
    WorkSpec[] eligibleWorkSpecsArray = eligibleWorkSpecs.toArray(new WorkSpec[0]);
    // Delegate to the underlying scheduler.
    for (Scheduler scheduler : schedulers) {
    scheduler.schedule(eligibleWorkSpecsArray);
    }
    }
    }

    下面看下Scheduler的子类

    最后会创建WorkerWrapper包装类,来执行我们定义的Worker类。

    @WorkerThread
    @Override
    public void run() {
    mTags = mWorkTagDao.getTagsForWorkSpecId(mWorkSpecId);
    mWorkDescription = createWorkDescription(mTags);
    runWorker();
    }
    
    private void runWorker() {
    if (tryCheckForInterruptionAndResolve()) {
    return;
    }
    
    mWorkDatabase.beginTransaction();
    try {
    mWorkSpec = mWorkSpecDao.getWorkSpec(mWorkSpecId);
    if (mWorkSpec == null) {
    Logger.get().error(
    TAG,
    String.format("Didn't find WorkSpec for id %s", mWorkSpecId));
    resolve(false);
    return;
    }
    
    // running, finished, or is blocked.
    if (mWorkSpec.state != ENQUEUED) {
    resolveIncorrectStatus();
    mWorkDatabase.setTransactionSuccessful();
    return;
    }
    
    // Case 1:
    // Ensure that Workers that are backed off are only executed when they are supposed to.
    // GreedyScheduler can schedule WorkSpecs that have already been backed off because
    // it is holding on to snapshots of WorkSpecs. So WorkerWrapper needs to determine
    // if the ListenableWorker is actually eligible to execute at this point in time.
    
    // Case 2:
    // On API 23, we double scheduler Workers because JobScheduler prefers batching.
    // So is the Work is periodic, we only need to execute it once per interval.
    // Also potential bugs in the platform may cause a Job to run more than once.
    
    if (mWorkSpec.isPeriodic() || mWorkSpec.isBackedOff()) {
    long now = System.currentTimeMillis();
    if (now < mWorkSpec.calculateNextRunTime()) {
    resolve(false);
    return;
    }
    }
    mWorkDatabase.setTransactionSuccessful();
    } finally {
    mWorkDatabase.endTransaction();
    }
    
    // Merge inputs. This can be potentially expensive code, so this should not be done inside
    // a database transaction.
    Data input;
    if (mWorkSpec.isPeriodic()) {
    input = mWorkSpec.input;
    } else {
    InputMerger inputMerger = InputMerger.fromClassName(mWorkSpec.inputMergerClassName);
    if (inputMerger == null) {
    Logger.get().error(TAG, String.format("Could not create Input Merger %s",
    mWorkSpec.inputMergerClassName));
    setFailedAndResolve();
    return;
    }
    List<Data> inputs = new ArrayList<>();
    inputs.add(mWorkSpec.input);
    inputs.addAll(mWorkSpecDao.getInputsFromPrerequisites(mWorkSpecId));
    input = inputMerger.merge(inputs);
    }
    
    WorkerParameters params = new WorkerParameters(
    UUID.fromString(mWorkSpecId),
    input,
    mTags,
    mRuntimeExtras,
    mWorkSpec.runAttemptCount,
    mConfiguration.getExecutor(),
    mWorkTaskExecutor,
    mConfiguration.getWorkerFactory());
    
    // Not always creating a worker here, as the WorkerWrapper.Builder can set a worker override
    // in test mode.
    if (mWorker == null) {
    mWorker = mConfiguration.getWorkerFactory().createWorkerWithDefaultFallback(
    mAppContext,
    mWorkSpec.workerClassName,
    params);
    }
    
    if (mWorker == null) {
    Logger.get().error(TAG,
    String.format("Could not create Worker %s", mWorkSpec.workerClassName));
    setFailedAndResolve();
    return;
    }
    
    if (mWorker.isUsed()) {
    Logger.get().error(TAG,
    String.format("Received an already-used Worker %s; WorkerFactory should return "
    + "new instances",
    mWorkSpec.workerClassName));
    setFailedAndResolve();
    return;
    }
    mWorker.setUsed();
    
    // Try to set the work to the running state. Note that this may fail because another thread
    // may have modified the DB since we checked last at the top of this function.
    if (trySetRunning()) {
    if (tryCheckForInterruptionAndResolve()) {
    return;
    }
    
    final SettableFuture<ListenableWorker.Result> future = SettableFuture.create();
    // Call mWorker.startWork() on the main thread.
    mWorkTaskExecutor.getMainThreadExecutor()
    .execute(new Runnable() {
    @Override
    public void run() {
    try {
    mInnerFuture = mWorker.startWork();
    future.setFuture(mInnerFuture);
    } catch (Throwable e) {
    future.setException(e);
    }
    
    }
    });
    
    // Avoid synthetic accessors.
    final String workDescription = mWorkDescription;future.addListener(new Runnable() {
    @Override
    @SuppressLint("SyntheticAccessor")
    public void run() {
    try {
    // If the ListenableWorker returns a null result treat it as a failure.
    ListenableWorker.Result result = future.get();
    if (result == null) {
    Logger.get().error(TAG, String.format(
    "%s returned a null result. Treating it as a failure.",
    mWorkSpec.workerClassName));
    } else {
    mResult = result;
    }
    } catch (CancellationException exception) {
    // Cancellations need to be treated with care here because innerFuture
    // cancellations will bubble up, and we need to gracefully handle that.
    Logger.get().info(TAG, String.format("%s was cancelled", workDescription),
    exception);
    } catch (InterruptedException | ExecutionException exception) {
    Logger.get().error(TAG,
    String.format("%s failed because it threw an exception/error",
    workDescription), exception);
    } finally {
    onWorkFinished();
    }
    }
    }, mWorkTaskExecutor.getBackgroundExecutor());} else {
    resolveIncorrectStatus();
    }
    }

    这里使用了androidx.work.impl.utils.futures.SettableFuture,并调用了addListener方法,该回调方法会在调用set时执行。

    future.addListener(new Runnable() {
    @Override
    @SuppressLint("SyntheticAccessor")
    public void run() {
    try {
    // If the ListenableWorker returns a null result treat it as a failure.
    ListenableWorker.Result result = future.get();
    if (result == null) {
    Logger.get().error(TAG, String.format(
    "%s returned a null result. Treating it as a failure.",
    mWorkSpec.workerClassName));
    } else {
    mResult = result;
    }
    } catch (CancellationException exception) {
    // Cancellations need to be treated with care here because innerFuture
    // cancellations will bubble up, and we need to gracefully handle that.
    Logger.get().info(TAG, String.format("%s was cancelled", workDescription),
    exception);
    } catch (InterruptedException | ExecutionException exception) {
    Logger.get().error(TAG,
    String.format("%s failed because it threw an exception/error",
    workDescription), exception);
    } finally {
    onWorkFinished();
    }
    }
    }, mWorkTaskExecutor.getBackgroundExecutor());

    下面看下核心的Worker类

    @Override
    public final @NonNull ListenableFuture<Result> startWork() {
    mFuture = SettableFuture.create();
    getBackgroundExecutor().execute(new Runnable() {
    @Override
    public void run() {
    Result result = doWork();
    mFuture.set(result);
    }
    });
    return mFuture;
    }

    可见,在调用doWork()后,任务执行完调用了set方法,此时会回调addListener方法。

    addListener回调中主要用来判断当前任务的状态,所以如果任务被停止,此处展示捕获的异常信息。

    比如调用一个任务的cancel方法,会展示下面的信息。

    1. 2019-02-02 15:35:41.682 30526-30542/com.outman.study.workmanagerdemo I/WM-WorkerWrapper: Work [ id=3d775394-e0d7-44e3-a670-c3527a3245ee, tags={ com.outman.study.workmanagerdemo.SimpleWorker } ] was cancelled
    2.   java.util.concurrent.CancellationException: Task was cancelled.
    3.     at androidx.work.impl.utils.futures.AbstractFuture.cancellationExceptionWithCause(AbstractFuture.java:1184)
    4.     at androidx.work.impl.utils.futures.AbstractFuture.getDoneValue(AbstractFuture.java:514)
    5.     at androidx.work.impl.utils.futures.AbstractFuture.get(AbstractFuture.java:475)
    6.     at androidx.work.impl.WorkerWrapper$2.run(WorkerWrapper.java:264)
    7.     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
    8.     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
    9.     at java.lang.Thread.run(Thread.java:764)

    以上就是我的简单分析,还有好多没有说到,后面有时间会继续。

    有不对的欢迎批评指正。希望对大家的学习有所帮助

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