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。
- 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
@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()); }复制代码 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 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); } } }复制代码 下面看下Scheduler的子类
@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 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 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(); } }复制代码 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());复制代码 下面看下核心的Worker类
@Override public final @NonNull ListenableFuturestartWork() { mFuture = SettableFuture.create(); getBackgroundExecutor().execute(new Runnable() { @Override public void run() { Result result = doWork(); mFuture.set(result); } }); return mFuture; }复制代码
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 cancelled2. 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)复制代码 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。