Android Jetpack之WorkManager源码分析
Android WorkManager简介
WorkManager 负责用来管理后台任务，它适用于需要保证系统即使应用程序退出也会运行的任务，
WorkManager根据设备API级别和应用程序状态等因素选择适当的方式来运行任务。如果WorkManager在应用程序运行时执行的任务，WorkManager可以在应用程序进程的新线程中运行您的任务。如果您的应用程序未运行，WorkManager会选择一种合适的方式来安排后台任务 。具体取决于设备API级别和包含的依赖项，WorkManager可能会使用 JobScheduler，Firebase JobDispatcher或AlarmManager调度任务。
为什么在有了service以后，google还有出WorkManager框架呢？
1.service的滥用导致手机后台任务不断执行，耗电量大。
2.从开发者来说，Android8.0以后，Android对于后台service管理的更加严格，应用在后台启动的服务必须是前台服务，否则会导致应用crash。当然你也可以选择降低targetSdkVersion。
3.针对targetSdkVersion Google也针对的出了一些限制。ps:2019 年起，每次发布新版本后，所有应用都必须在一年内将 Target API 更新到最新版本。
官方指导地址：官方指地址
WorkManager的使用
官方DEMO 官方DEMO
2.1 Gradle依赖配置
def work = "2.1.0"
implementation"androidx.work:work-runtime:$work"
implementation"androidx.work:work-testing:$work"
// implementation"androidx.work:work-firebase:$work"
implementation"androidx.work:work-runtime-ktx:$work"
2.2 定义Worker类
自定义Worker类，继承自Worker，然后复写doWork() 方法，返回当前任务的结果 Result。doWork方法是执行在子线程的。
class JetpackWork(context: Context,workerParameters: WorkerParameters) : Worker(context,workerParameters){
override fun doWork(): Result {
Log.e("workermanager","work start:")
Thread.sleep(2_000)
Log.e("workermanager","do work thread msg :"+Thread.currentThread().name)
return Result.success()
}
}
2.3 执行任务
（1）使用 OneTimeWorkRequest.Builder 创建对象Worker，将任务加入WorkManager队列。并且OneTimeWorkRequest.Builder创建的是一个单次执行的任务。
（2）将任务排入WorkManager队列，等待执行。
Worker不一定是立即执行的。WorkManager会选择适当的时间运行Worker，平衡诸如系统负载，设备是否插入等考虑因素。但是如果我们没有指定任何约束条件，WorkManager会立即运行我们的任务。
var request = OneTimeWorkRequest.Builder(JetpackWork::class.java)
.build()
WorkManager.getInstance(this).enqueue(request)
2.4 重复执行任务
（1）使用PeriodicWorkRequest.Builder类创建循环任务，创建一个PeriodicWorkRequest对象
（2）然后将任务添加到WorkManager的任务队列，等待执行。
（3）最小时间间隔是15分钟。
public static final long MIN_PERIODIC_INTERVAL_MILLIS = 15 * 60 * 1000L; // 15 minutes.
var pRequest = PeriodicWorkRequest.Builder(JetpackWork::class.java,1,TimeUnit.SECONDS).build()
WorkManager.getInstance(this).enqueue(pRequest)
2.4 任务的状态
通过获取LiveData查看任务的状态WorkInfo.State，只有当Activityh处于活跃状态下才可以监听成功。
WorkManager.getInstance(this).getWorkInfoByIdLiveData(request.id).observe(this, Observer {
Log.e("workermanager","state :"+it?.state?.name)
})
2.5 任务约束Constraints
WorkManager 允许我们指定任务执行的环境，比如网络已连接、电量充足时等，在满足条件的情况下任务才会执行。
（1）使用Constraints.Builder()创建并配置Constraints对象，可以指定上诉任务运行时间时的约束。
（2）创建Worker时调用setConstraints指定约束条件。
var constraint = Constraints.Builder()
.setRequiredNetworkType(NetworkType.CONNECTED)
.setRequiresBatteryNotLow(true)
.setRequiresCharging(true).build()
var request = OneTimeWorkRequest.Builder(JetpackWork::class.java)
.setConstraints(constraint)
.build()
WorkManger提供了以下的约束作为Work执行的条件：
（1）setRequiredNetworkType：网络连接设置
（2）setRequiresBatteryNotLow：是否为低电量时运行 默认false
（3）setRequiresCharging：是否要插入设备（接入电源），默认false
（4）setRequiresDeviceIdle：设备是否为空闲，默认false
（5）setRequiresStorageNotLow：设备可用存储是否不低于临界阈值
2.6 取消任务
（1）从WorkRequest()获取Worker的ID
（2 调用WorkManager.getInstance().cancelWorkById(workRequest.id)根据ID取消任务。
WorkManager 对于已经正在运行或完成的任务是无法取消任务的。
WorkManager.getInstance(this).cancelWorkById(request.id)
2.7 添加TAG
通过为WorkRequest对象分配标记字符串来对任务进行分组
var twoRequest = OneTimeWorkRequest.Builder(JetpackTwoWork::class.java)
.setConstraints(constraint)
.addTag("jetpack")
.build()
WorkManager.getStatusesByTag() 返回该标记的所有任务的列表信息。
WorkManager.getInstance(this).getWorkInfosByTag("jetpack")
WorkManager.cancelAllWorkByTag() 取消具有特定标记的所有任务
WorkManager.getInstance(this).cancelAllWorkByTag("jetpack")
通过获取LiveData查看具有特定标记的所有任务的状态WorkInfo.State
WorkManager.getInstance(this).getWorkInfosByTagLiveData("jetpack").observe(this, Observer {
})
进阶使用
3.1 数据交互
WorkManager可以将参数传递给任务，并让任务返回结果。传递和返回值都是键值对形式。
（1）使用 Data.Builder创建 Data 对象，保存参数的键值对。
（2）在创建WorkQuest之前调用WorkRequest.Builder.setInputData()传递Data的实例
var requestData = Data.Builder().putString("jetpack", "workermanager").build()
var request = OneTimeWorkRequest.Builder(JetpackWork::class.java)
.setConstraints(constraint)
.setInputData(requestData)
.build()
（3 在JetpackWork.doWork方法中通过getInputData获取传递值。
（4）在构建Data对象，跟随着任务的结果返回。
class JetpackWork(context: Context,workerParameters: WorkerParameters) : Worker(context,workerParameters){
override fun doWork(): Result {
Log.e("workermanager","work start:"+inputData.getString("jetpack"))
Thread.sleep(2_000)
Log.e("workermanager","do work thread msg :"+Thread.currentThread().name)
var outData = Data.Builder().putString("back","hi,jetpack").build()
return Result.success(outData)
}
}
(5) 通过LiveData监听Worker返回的数据。
WorkManager.getInstance(this).getWorkInfoByIdLiveData(request.id).observe(this, Observer {
Log.e("workermanager", "out data :" + it?.outputData?.getString("back"))
})
3.2 链式任务
- WorkManager允许拥有多个任务的工作序列按照顺序执行任务。
（）使用该WorkManager.beginWith() 方法创建一个序列 ，并传递一个OneTimeWorkRequest对象;，该方法返回一个WorkContinuation对象。
（2）使用 WorkContinuation.then()添加剩余的任务。
（3）最后调用WorkContinuation.enqueue()将整个序列排入队列 。
如果中间有任何任务返回 Result.failure()，则整个序列结束。并且上一个任务的结果数据可以作为下一个任务的输入数据，实现任务之间的数据传递。
WorkManager.getInstance(this).beginWith(request).then(twoRequest).then(threeRequest).enqueue()
- 可以使用WorkContinuation.combine()方法连接多个链来创建更复杂的序列。
在这里插入图片描述
要建立这个序列，先创建两个单独的链，然后将它们连接在一起成为第三个链：
WorkContinuation chainAC = WorkManager.getInstance()
.beginWith(worker A)
.then(worker C);
WorkContinuation chainBD = WorkManager.getInstance()
.beginWith(worker B)
.then(worker D);
WorkContinuation chainAll = WorkContinuation
.combine(chainAC, chainBD)
.then(worker E);
chainAll.enqueue();
在这种情况下，WorkManager在worker C之前运行workA，它也在workD之前运行workB， WorkB和workD都完成后，WorkManager 运行workE。
注意：虽然WorkManager依次运行每个子链，但不能保证链1中的任务与 链2中的任务重叠，例如，workB可能在workC之前或之后运行，或者它们可能同时运行。唯一可以保证的是每个子链中的任务将按顺序运行。
3.3 唯一的工作序列
我们可以创建一个唯一的工作序列，在任务队列里，同一个任务只存在一个，避免任务的重复执行。通过调用 beginUniqueWork() 来创建唯一的工作序列。
参数含义：1、工作序列的名称 2、当有相同名称序列时采取的策略方式 3、需要执行的Worker
WorkManager.getInstance(this).beginUniqueWork("jetpack",ExistingWorkPolicy.APPEND,request).enqueue()
ExistingWorkPolicy提供以下策略：
（1）ExistingWorkPolicy.REPLACE：取消现有序列并将其替换为新序列
（2）ExistingWorkPolicy.KEEP：保留现有序列并忽略您的新请求
（3）ExistingWorkPolicy.APPEND：将新序列附加到现有序列，在现有序列的最后一个任务完成后运行新序列的第一个任务。
源码分析
下面我们带着三个问题来看代码，梳理一下WorkManager的源码
1.没有任务约束Constraints的任务是如何执行的。
2.添加任务约束Constraints的任务是如何被触发的。
4.1 WorkManager类
- 通过上面我们知道任务的执行，是通过WorkManager.getInstance(this).enqueue(request)执行的。
WorkManager是个抽象类，通过WorkManager.getInstance方法返回的是，它的子类WorkManagerImpl的单例对象。
（1）单例模式初始化WorkManagerImpl对象
（2）调用getInstance方法返回sDelegatedInstance对象。这里sDelegatedInstance对象已经不是nulll了。下面我们就来分析一下sDelegatedInstance的初始化过程。
public static @NonNull WorkManagerImpl getInstance(@NonNull Context context) {
synchronized (sLock) {
WorkManagerImpl instance = getInstance();
if (instance == null) {
Context appContext = context.getApplicationContext();
if (appContext instanceof Configuration.Provider) {
initialize(
appContext,
((Configuration.Provider) appContext).getWorkManagerConfiguration());
instance = getInstance(appContext);
} else {
throw new IllegalStateException("WorkManager is not initialized properly. You "
+ "have explicitly disabled WorkManagerInitializer in your manifest, "
+ "have not manually called WorkManager#initialize at this point, and "
+ "your Application does not implement Configuration.Provider.");
}
}
return instance;
}
}
public static @Nullable WorkManagerImpl getInstance() {
synchronized (sLock) {
if (sDelegatedInstance != null) {
return sDelegatedInstance;
}
return sDefaultInstance;
}
}
- 通过反编译我们的APP，我们在AndroidManifest.xml文件中找到了一个provider的配置项。WorkManagerInitializer类又继承自ContentProvider，关于ContentProvider的启动过程这里不过多介绍，在应用的启动时候，会通过ActivityThread初始化ContentProvider（WorkManagerInitializer），即执行了onCreate方法。
<provider
android:name="androidx.work.impl.WorkManagerInitializer"
android:exported="false"
android:multiprocess="true"
android:authorities="com.jandroid.multivideo.workmanager-init"
android:directBootAware="false" />
在WorkManagerInitializer的onCreate方法中调用了WorkManager.initialize的方法进行初始化。
public boolean onCreate() {
// Initialize WorkManager with the default configuration.
WorkManager.initialize(getContext(), new Configuration.Builder().build());
return true;
}
在WorkManager.initialize内部通过调用 WorkManagerImpl.initialize(context, configuration)完成WorkManagerImpl的初始化任务。
- 下面重点看一下WorkManagerImpl.initializ内部做了那些初始化操作。
（1）sDelegatedInstance和sDefaultInstance都不为空，说明已经初始化过，抛出异常
（2）调用WorkManagerImpl的构造方法完成初始化任务。
（3）configuration.getTaskExecutor())内部返回默认的线程池。
（4）WorkManagerTaskExecutor内部通过调用SerialExecutor实现线程的调度。
public static void initialize(@NonNull Context context, @NonNull Configuration configuration) {
synchronized (sLock) {
if (sDelegatedInstance != null && sDefaultInstance != null) {
throw new IllegalStateException("WorkManager is already initialized. Did you "
+ "try to initialize it manually without disabling "
+ "WorkManagerInitializer? See "
+ "WorkManager#initialize(Context, Configuration) or the class level"
+ "Javadoc for more information.");
}
if (sDelegatedInstance == null) {
context = context.getApplicationContext();
if (sDefaultInstance == null) {
sDefaultInstance = new WorkManagerImpl(
context,
configuration,
new WorkManagerTaskExecutor(configuration.getTaskExecutor()));
}
sDelegatedInstance = sDefaultInstance;
}
}
}
private @NonNull Executor createDefaultExecutor() {
return Executors.newFixedThreadPool(
// This value is the same as the core pool size for AsyncTask#THREAD_POOL_EXECUTOR.
Math.max(2, Math.min(Runtime.getRuntime().availableProcessors() - 1, 4)));
}
- initialize方法内部通过调用WorkManagerImpl的构造方法完成初始化任务。
（1）WorkDatabase创建数据库擦操作，内部使用的是Room框架。
（2）createSchedulers创建调度者集合。这里面主要有两种：GreedyScheduler和SystemJobScheduler（如果系统版本号大于23的话）。
（3）创建Processor类。接下来会分析该类的作用和代码。
public WorkManagerImpl(
@NonNull Context context,
@NonNull Configuration configuration,
@NonNull TaskExecutor workTaskExecutor,
boolean useTestDatabase) {
Context applicationContext = context.getApplicationContext();
WorkDatabase database = WorkDatabase.create(
applicationContext, configuration.getTaskExecutor(), useTestDatabase);
Logger.setLogger(new Logger.LogcatLogger(configuration.getMinimumLoggingLevel()));
List<Scheduler> schedulers = createSchedulers(applicationContext, workTaskExecutor);
Processor processor = new Processor(
context,
configuration,
workTaskExecutor,
database,
schedulers);
internalInit(context, configuration, workTaskExecutor, database, schedulers, processor);
}
- createSchedulers主要是为了创建调度者集合。
（1）createBestAvailableBackgroundScheduler创建一个最有效的后台调度者。
（2）创建GreedyScheduler调度者。
public @NonNull List<Scheduler> createSchedulers(Context context, TaskExecutor taskExecutor) {
return Arrays.asList(
Schedulers.createBestAvailableBackgroundScheduler(context, this),
// Specify the task executor directly here as this happens before internalInit.
// GreedyScheduler creates ConstraintTrackers and controllers eagerly.
new GreedyScheduler(context, taskExecutor, this));
}
- createBestAvailableBackgroundScheduler方法
（1）如果Android版本号>=23，返回SystemJobScheduler，内部主要是使用JobScheduler完成调度
（2）如果手机支持GCM，则返回GcmScheduler调度者，国内基本告别了。
（3）其他情况下返回SystemAlarmScheduler，内部使用AlarmManager实现原理。
static Scheduler createBestAvailableBackgroundScheduler(
@NonNull Context context,
@NonNull WorkManagerImpl workManager) {
Scheduler scheduler;
if (Build.VERSION.SDK_INT >= WorkManagerImpl.MIN_JOB_SCHEDULER_API_LEVEL) {
scheduler = new SystemJobScheduler(context, workManager);
setComponentEnabled(context, SystemJobService.class, true);
Logger.get().debug(TAG, "Created SystemJobScheduler and enabled SystemJobService");
} else {
scheduler = tryCreateGcmBasedScheduler(context);
if (scheduler == null) {
scheduler = new SystemAlarmScheduler(context);
setComponentEnabled(context, SystemAlarmService.class, true);
Logger.get().debug(TAG, "Created SystemAlarmScheduler");
}
}
return scheduler;
}
4.2 任务的入执行队列enqueue方法
- 通过以上分析我们知道WorkManager.getInstance返回的是WorkManagerImpl实例，所以我们进入到enqueue方法中看看。调用WorkContinuationImpl实例的enqueue方法。
public Operation enqueue(
@NonNull List<? extends WorkRequest> workRequests) {
// This error is not being propagated as part of the Operation, as we want the
// app to crash during development. Having no workRequests is always a developer error.
if (workRequests.isEmpty()) {
throw new IllegalArgumentException(
"enqueue needs at least one WorkRequest.");
}
return new WorkContinuationImpl(this, workRequests).enqueue();
}
- 直接进入WorkContinuationImpl.enqueue方法看看。
（1）创建EnqueueRunnable继承自Runnable
（2）getWorkTaskExecutor获取WorkManagerTaskExecutor对象。
（3）通过之前在Configuration创建的线程池中执行EnqueueRunnable任务。
public @NonNull Operation enqueue() {
// Only enqueue if not already enqueued.
if (!mEnqueued) {
// The runnable walks the hierarchy of the continuations
// and marks them enqueued using the markEnqueued() method, parent first.
EnqueueRunnable runnable = new EnqueueRunnable(this);
mWorkManagerImpl.getWorkTaskExecutor().executeOnBackgroundThread(runnable);
mOperation = runnable.getOperation();
} else {
Logger.get().warning(TAG,
String.format("Already enqueued work ids (%s)", TextUtils.join(", ", mIds)));
}
return mOperation;
}
- EnqueueRunnable类
在scheduleWorkInBackground方法中调度任务执行，内部调用Schedulers类的schedule方法，分配任务。
public void run() {
try {
if (mWorkContinuation.hasCycles()) {
throw new IllegalStateException(
String.format("WorkContinuation has cycles (%s)", mWorkContinuation));
}
boolean needsScheduling = addToDatabase();
if (needsScheduling) {
// Enable RescheduleReceiver, only when there are Worker's that need scheduling.
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));
}
}
public void scheduleWorkInBackground() {
WorkManagerImpl workManager = mWorkContinuation.getWorkManagerImpl();
Schedulers.schedule(
workManager.getConfiguration(),
workManager.getWorkDatabase(),
workManager.getSchedulers());
}
- Schedulers类。
在调用Scheduler的schedule的掉配任务。在分析WorkManager初始化的时候，我们知道主要有GreedyScheduler等调度类。下面重点分析一下该类。
public static void schedule(
@NonNull Configuration configuration,
@NonNull WorkDatabase workDatabase,
List<Scheduler> schedulers) {
if (schedulers == null || schedulers.size() == 0) {
return;
}
WorkSpecDao workSpecDao = workDatabase.workSpecDao();
List<WorkSpec> eligibleWorkSpecs;
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);
}
}
}
- GreedyScheduler类
（1）判断时候有约束条件。没有则调用startWork执行任务。有则将任务入集合
public void schedule(@NonNull WorkSpec... workSpecs) {
registerExecutionListenerIfNeeded();
// Keep track of the list of new WorkSpecs whose constraints need to be tracked.
// Add them to the known list of constrained WorkSpecs and call replace() on
// WorkConstraintsTracker. That way we only need to synchronize on the part where we
// are updating mConstrainedWorkSpecs.
List<WorkSpec> constrainedWorkSpecs = new ArrayList<>();
List<String> constrainedWorkSpecIds = new ArrayList<>();
for (WorkSpec workSpec: workSpecs) {
if (workSpec.state == WorkInfo.State.ENQUEUED
&& !workSpec.isPeriodic()
&& workSpec.initialDelay == 0L
&& !workSpec.isBackedOff()) {
if (workSpec.hasConstraints()) {
// Exclude content URI triggers - we don't know how to handle them here so the
// background scheduler should take care of them.
if (Build.VERSION.SDK_INT < 24
|| !workSpec.constraints.hasContentUriTriggers()) {
constrainedWorkSpecs.add(workSpec);
constrainedWorkSpecIds.add(workSpec.id);
}
} else {
Logger.get().debug(TAG, String.format("Starting work for %s", workSpec.id));
mWorkManagerImpl.startWork(workSpec.id);
}
}
}
// onExecuted() which is called on the main thread also modifies the list of mConstrained
// WorkSpecs. Therefore we need to lock here.
synchronized (mLock) {
if (!constrainedWorkSpecs.isEmpty()) {
Logger.get().debug(TAG, String.format("Starting tracking for [%s]",
TextUtils.join(",", constrainedWorkSpecIds)));
mConstrainedWorkSpecs.addAll(constrainedWorkSpecs);
mWorkConstraintsTracker.replace(mConstrainedWorkSpecs);
}
}
}
- WorkManagerImpl.startWork方法
还是调用WorkManagerTaskExecutor的executeOnBackgroundThread方法执行StartWorkRunnable。
public void startWork(String workSpecId, WorkerParameters.RuntimeExtras runtimeExtras) {
mWorkTaskExecutor
.executeOnBackgroundThread(
new StartWorkRunnable(this, workSpecId, runtimeExtras));
}
- StartWorkRunnable类
getProcessor方法的是我们在创建WorkManager时创建的Processor对象。这里会调用Processor的startWork方法。
public void run() {
mWorkManagerImpl.getProcessor().startWork(mWorkSpecId, mRuntimeExtras);
}
- Processor类
内部通过构建Work的包装类WorkerWrapper，然后再次调用WorkManagerTaskExecutor类执行WorkerWrapper任务。
public boolean startWork(String id, WorkerParameters.RuntimeExtras runtimeExtras) {
WorkerWrapper workWrapper;
synchronized (mLock) {
// Work may get triggered multiple times if they have passing constraints
// and new work with those constraints are added.
if (mEnqueuedWorkMap.containsKey(id)) {
Logger.get().debug(
TAG,
String.format("Work %s is already enqueued for processing", id));
return false;
}
workWrapper =
new WorkerWrapper.Builder(
mAppContext,
mConfiguration,
mWorkTaskExecutor,
mWorkDatabase,
id)
.withSchedulers(mSchedulers)
.withRuntimeExtras(runtimeExtras)
.build();
ListenableFuture<Boolean> future = workWrapper.getFuture();
future.addListener(
new FutureListener(this, id, future),
mWorkTaskExecutor.getMainThreadExecutor());
mEnqueuedWorkMap.put(id, workWrapper);
}
mWorkTaskExecutor.getBackgroundExecutor().execute(workWrapper);
Logger.get().debug(TAG, String.format("%s: processing %s", getClass().getSimpleName(), id));
return true;
}
- WorkerWrapper类
（1）反射机制获取到ListenableWorker对象。其中Worker类继承自ListenableWorker类。
（2）调用ListenableWorker.startWork，实际上是调用Worker类的startWork方法。
（3）在Worker类的startWork方法中又会调用doWork方法，也就是我们复写的doWork方法。
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;
}
// Do a quick check to make sure we don't need to bail out in case this work is already
// running, finished, or is blocked.
if (mWorkSpec.state != ENQUEUED) {
resolveIncorrectStatus();
mWorkDatabase.setTransactionSuccessful();
Logger.get().debug(TAG,
String.format("%s is not in ENQUEUED state. Nothing more to do.",
mWorkSpec.workerClassName));
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();
// Allow first run of a PeriodicWorkRequest
// to go through. This is because when periodStartTime=0;
// calculateNextRunTime() always > now.
// For more information refer to b/124274584
boolean isFirstRun = mWorkSpec.periodStartTime == 0;
if (!isFirstRun && now < mWorkSpec.calculateNextRunTime()) {
Logger.get().debug(TAG,
String.format(
"Delaying execution for %s because it is being executed "
+ "before schedule.",
mWorkSpec.workerClassName));
// For AlarmManager implementation we need to reschedule this kind of Work.
// This is not a problem for JobScheduler because we will only reschedule
// work if JobScheduler is unaware of a jobId.
resolve(true);
return;
}
}
// Needed for nested transactions, such as when we're in a dependent work request when
// using a SynchronousExecutor.
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 {
Logger.get().debug(TAG, String.format("Starting work for %s",
mWorkSpec.workerClassName));
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 {
Logger.get().debug(TAG, String.format("%s returned a %s result.",
mWorkSpec.workerClassName, result));
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();
}
}
小结
（1）Worker：指定我们需要执行的任务。 WorkManager API包含一个抽象的Worker类WorkManagerImpl，我们需要继承这个类并且在这里执行工作。
（2）WorkRequest：代表一个单独的任务。一个WorkRequest 对象指定哪个 Woker 类应该执行该任务，而且，我们还可以向 WorkRequest 对象添加详细信息，指定任务运行的环境等。每个 WorkRequest 都有一个自动生成的唯一ID，我们可以使用该ID来执行诸如取消排队的任务或获取任务状态等内容。 WorkRequest 是一个抽象类，在代码中，我们需要使用它的直接子类，OneTimeWorkRequest 或 PeriodicWorkRequest.。
（3）WorkRequest.Builder：用于创建WorkRequest对象的辅助类，同样，我们要使用它的一个子OneTimeWorkRequest.Builder 和PeriodicWorkRequest.Builder 。
（4）Constraints：指定任务在何时运行（例如，“仅在连接到网络时”）。我们可以通过Constraints.Builder 来创建Constraints对象，并在创建WorkRequest之前，将 Constraints 对象传递给 WorkRequest.Builder。
（5）WorkManager：将WorkRequest入队和管理WorkRequest。我们要将WorkRequest对象传递给 WorkManager ，WorkManager 以这样的方式调度任务，以便分散系统资源的负载，同时遵守我们指定的约束条件。
（6）WorkStatus：包含有关特定任务的信息。WorkManager 为每个 WorkRequest 对象提供一个（）LiveData，LiveData持有一个WorkStatus对象，通过观察LiveData，我们可以确定任务的当前状态，并在任务完成后获取返回的任何值。
下面以贴一张执行的类图信息。
image
任务约束Constraints的任务是如何被触发
- 结下来分析一下如果带条件约束的任务是如何被触发的。以网络变化为例分析该场景。
通过反编译我们的APP，我们在AndroidManifest.xml文件中找到了一个receiver的配置项。
通过action，我们知道主要是为了监听网络的变化的。
<receiver
android:name="androidx.work.impl.background.systemalarm.ConstraintProxy$NetworkStateProxy"
android:enabled="false"
android:exported="false"
android:directBootAware="false">
<intent-filter>
<action
android:name="android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>
</receiver>
- NetworkStateProxy类
public static class NetworkStateProxy extends ConstraintProxy {
}
- ConstraintProxy类
在ConstraintProxy类的onReceive方法中，startService一个SystemAlarmService，其中ACTION
为ACTION_CONSTRAINTS_CHANGED。
@Override
public void onReceive(Context context, Intent intent) {
Logger.get().debug(TAG, String.format("onReceive : %s", intent));
Intent constraintChangedIntent = CommandHandler.createConstraintsChangedIntent(context);
context.startService(constraintChangedIntent);
}
static Intent createConstraintsChangedIntent(@NonNull Context context) {
Intent intent = new Intent(context, SystemAlarmService.class);
intent.setAction(ACTION_CONSTRAINTS_CHANGED);
return intent;
}
- SystemAlarmService类
内部调用mDispatcher.add方法
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
super.onStartCommand(intent, flags, startId);
if (mIsShutdown) {
Logger.get().info(TAG,
"Re-initializing SystemAlarmDispatcher after a request to shut-down.");
// Destroy the old dispatcher to complete it's lifecycle.
mDispatcher.onDestroy();
// Create a new dispatcher to setup a new lifecycle.
initializeDispatcher();
// Set mIsShutdown to false, to correctly accept new commands.
mIsShutdown = false;
}
if (intent != null) {
mDispatcher.add(intent, startId);
}
// If the service were to crash, we want all unacknowledged Intents to get redelivered.
return Service.START_REDELIVER_INTENT;
}
- SystemAlarmDispatcher类
内部调用processCommand方法
public boolean add(@NonNull final Intent intent, final int startId) {
Logger.get().debug(TAG, String.format("Adding command %s (%s)", intent, startId));
assertMainThread();
String action = intent.getAction();
if (TextUtils.isEmpty(action)) {
Logger.get().warning(TAG, "Unknown command. Ignoring");
return false;
}
// If we have a constraints changed intent in the queue don't add a second one. We are
// treating this intent as special because every time a worker with constraints is complete
// it kicks off an update for constraint proxies.
if (CommandHandler.ACTION_CONSTRAINTS_CHANGED.equals(action)
&& hasIntentWithAction(CommandHandler.ACTION_CONSTRAINTS_CHANGED)) {
return false;
}
intent.putExtra(KEY_START_ID, startId);
synchronized (mIntents) {
boolean hasCommands = !mIntents.isEmpty();
mIntents.add(intent);
if (!hasCommands) {
// Only call processCommand if this is the first command.
// The call to dequeueAndCheckForCompletion will process the remaining commands
// in the order that they were added.
processCommand();
}
}
return true;
}
- processCommand方法
内部调用CommandHandler.onHandleIntent方法
private void processCommand() {
assertMainThread();
PowerManager.WakeLock processCommandLock =
WakeLocks.newWakeLock(mContext, PROCESS_COMMAND_TAG);
try {
processCommandLock.acquire();
// Process commands on the background thread.
mWorkManager.getWorkTaskExecutor().executeOnBackgroundThread(new Runnable() {
@Override
public void run() {
synchronized (mIntents) {
mCurrentIntent = mIntents.get(0);
}
if (mCurrentIntent != null) {
final String action = mCurrentIntent.getAction();
final int startId = mCurrentIntent.getIntExtra(KEY_START_ID,
DEFAULT_START_ID);
Logger.get().debug(TAG,
String.format("Processing command %s, %s", mCurrentIntent,
startId));
final PowerManager.WakeLock wakeLock = WakeLocks.newWakeLock(
mContext,
String.format("%s (%s)", action, startId));
try {
Logger.get().debug(TAG, String.format(
"Acquiring operation wake lock (%s) %s",
action,
wakeLock));
wakeLock.acquire();
mCommandHandler.onHandleIntent(mCurrentIntent, startId,
SystemAlarmDispatcher.this);
} catch (Throwable throwable) {
Logger.get().error(
TAG,
"Unexpected error in onHandleIntent",
throwable);
} finally {
Logger.get().debug(
TAG,
String.format(
"Releasing operation wake lock (%s) %s",
action,
wakeLock));
wakeLock.release();
// Check if we have processed all commands
postOnMainThread(
new DequeueAndCheckForCompletion(SystemAlarmDispatcher.this));
}
}
}
});
} finally {
processCommandLock.release();
}
}
- onHandleIntent方法
onHandleIntent传入的Action 是ACTION_CONSTRAINTS_CHANGED。然后执行handleConstraintsChanged方法，在该方法内部经过一系列转化，🈶️会回到onHandleIntent方法中，而且ACTION为ACTION_DELAY_MET。
void onHandleIntent(
@NonNull Intent intent,
int startId,
@NonNull SystemAlarmDispatcher dispatcher) {
String action = intent.getAction();
if (ACTION_CONSTRAINTS_CHANGED.equals(action)) {
handleConstraintsChanged(intent, startId, dispatcher);
} else if (ACTION_RESCHEDULE.equals(action)) {
handleReschedule(intent, startId, dispatcher);
} else {
Bundle extras = intent.getExtras();
if (!hasKeys(extras, KEY_WORKSPEC_ID)) {
Logger.get().error(TAG,
String.format("Invalid request for %s, requires %s.",
action,
KEY_WORKSPEC_ID));
} else {
if (ACTION_SCHEDULE_WORK.equals(action)) {
handleScheduleWorkIntent(intent, startId, dispatcher);
} else if (ACTION_DELAY_MET.equals(action)) {
handleDelayMet(intent, startId, dispatcher);
} else if (ACTION_STOP_WORK.equals(action)) {
handleStopWork(intent, startId, dispatcher);
} else if (ACTION_EXECUTION_COMPLETED.equals(action)) {
handleExecutionCompleted(intent, startId, dispatcher);
} else {
Logger.get().warning(TAG, String.format("Ignoring intent %s", intent));
}
}
}
}
- DelayMetCommandHandler
经过成成调用，会调用到DelayMetCommandHandler类onAllConstraintsMet方法。在该方法内部会调用startWork方法。而startWork方法正式Processor的方法。又回到了上面分析的正常work的工作流程了。
public void onAllConstraintsMet(@NonNull List<String> workSpecIds) {
// WorkConstraintsTracker will call onAllConstraintsMet with list of workSpecs whose
// constraints are met. Ensure the workSpecId we are interested is part of the list
// before we call Processor#startWork().
if (!workSpecIds.contains(mWorkSpecId)) {
return;
}
synchronized (mLock) {
if (mCurrentState == STATE_INITIAL) {
mCurrentState = STATE_START_REQUESTED;
Logger.get().debug(TAG, String.format("onAllConstraintsMet for %s", mWorkSpecId));
// Constraints met, schedule execution
// Not using WorkManagerImpl#startWork() here because we need to know if the
// processor actually enqueued the work here.
boolean isEnqueued = mDispatcher.getProcessor().startWork(mWorkSpecId);
if (isEnqueued) {
// setup timers to enforce quotas on workers that have
// been enqueued
mDispatcher.getWorkTimer()
.startTimer(mWorkSpecId, WORK_PROCESSING_TIME_IN_MS, this);
} else {
// if we did not actually enqueue the work, it was enqueued before
// cleanUp and pretend this never happened.
cleanUp();
}
} else {
Logger.get().debug(TAG, String.format("Already started work for %s", mWorkSpecId));
}
}
}
小结
实现原理就是通过监听各种约束条件变化的广播，然后经过层层转化，最终的处理逻辑和无限制条件的work流程一致。
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