Android WorkManager浅谈

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，也希望大家多多支持我们。