上一篇讲解了如何使用WorkManager；本篇我们就按照上一篇的使用步骤来分析源码
第一步生成一个待执行的request请求
val request = OneTimeWorkRequestBuilder() .setConstraints(constraints) .addTag("tagCountWorker") .setInputData(Data.Builder().putString("parameter1", "value of parameter1").build()) .setBackoffCriteria(BackoffPolicy.EXPONENTIAL, 1, TimeUnit.HOURS) .build() 我们看类WorkRequest的源码部分：
public abstract class WorkRequest { private @NonNull UUID mId; private @NonNull WorkSpec mWorkSpec; private @NonNull Set mTags; public abstract static class Builder, W extends WorkRequest> { boolean mBackoffCriteriaSet = false; UUID mId; WorkSpec mWorkSpec; Set mTags = new HashSet<>(); Class<? extends ListenableWorker> mWorkerClass; Builder(@NonNull Class<? extends ListenableWorker> workerClass) { mId = UUID.randomUUID(); mWorkerClass = workerClass; mWorkSpec = new WorkSpec(mId.toString(), workerClass.getName()); addTag(workerClass.getName()); } public final @NonNull W build() { W returnValue = buildInternal(); // Create a new id and WorkSpec so this WorkRequest.Builder can be used multiple times. mId = UUID.randomUUID(); mWorkSpec = new WorkSpec(mWorkSpec); mWorkSpec.id = mId.toString(); return returnValue; } abstract @NonNull W buildInternal(); abstract @NonNull B getThis(); }} @Entity( indices = { @Index(value = {"schedule_requested_at"}), @Index(value = {"period_start_time"}) })public final class WorkSpec { private static final String TAG = Logger.tagWithPrefix("WorkSpec"); public static final long SCHEDULE_NOT_REQUESTED_YET = -1; @ColumnInfo(name = "id") @PrimaryKey @NonNull public String id; @ColumnInfo(name = "state") @NonNull public WorkInfo.State state = ENQUEUED; @ColumnInfo(name = "worker_class_name") @NonNull public String workerClassName; @ColumnInfo(name = "input_merger_class_name") public String inputMergerClassName; @ColumnInfo(name = "input") @NonNull public Data input = Data.EMPTY; @ColumnInfo(name = "output") @NonNull public Data output = Data.EMPTY; @ColumnInfo(name = "initial_delay") public long initialDelay; @ColumnInfo(name = "interval_duration") public long intervalDuration; @ColumnInfo(name = "flex_duration") public long flexDuration; @Embedded @NonNull public Constraints constraints = Constraints.NONE; @ColumnInfo(name = "run_attempt_count") @IntRange(from = 0) public int runAttemptCount; @ColumnInfo(name = "backoff_policy") @NonNull public BackoffPolicy backoffPolicy = BackoffPolicy.EXPONENTIAL; @ColumnInfo(name = "backoff_delay_duration") public long backoffDelayDuration = WorkRequest.DEFAULT_BACKOFF_DELAY_MILLIS; @ColumnInfo(name = "period_start_time") public long periodStartTime; @ColumnInfo(name = "minimum_retention_duration") public long minimumRetentionDuration; @ColumnInfo(name = "schedule_requested_at") public long scheduleRequestedAt = SCHEDULE_NOT_REQUESTED_YET; /** * This is {@code true} when the WorkSpec needs to be hosted by a foreground service. */ @ColumnInfo(name = "run_in_foreground") public boolean runInForeground; public WorkSpec(@NonNull String id, @NonNull String workerClassName) { this.id = id; this.workerClassName = workerClassName; } public WorkSpec(@NonNull WorkSpec other) { id = other.id; workerClassName = other.workerClassName; state = other.state; inputMergerClassName = other.inputMergerClassName; input = new Data(other.input); output = new Data(other.output); initialDelay = other.initialDelay; intervalDuration = other.intervalDuration; flexDuration = other.flexDuration; constraints = new Constraints(other.constraints); runAttemptCount = other.runAttemptCount; backoffPolicy = other.backoffPolicy; backoffDelayDuration = other.backoffDelayDuration; periodStartTime = other.periodStartTime; minimumRetentionDuration = other.minimumRetentionDuration; scheduleRequestedAt = other.scheduleRequestedAt; runInForeground = other.runInForeground; }}我们看到上面使用了Room的Entity注解来注解WorkSpec，说明这是一个持有化存储的类，会被存储在数据库中； 它里面存储了一个WorkRequest的几乎所有信息，包括唯一标识id，workerClassName，input，output，constraints等，这些信息在我们生成WorkRequest中都有涉及
WorkManager是一个抽象类，它的实现类是WorkManagerImpl，采用单例模式返回WorkManagerImpl对象； 下面注释1的地方调用getInstance其实返回值已经不为空了，下面我们来分析：
public static @NonNull WorkManager getInstance(@NonNull Context context) { return WorkManagerImpl.getInstance(context);}@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)public static @NonNull WorkManagerImpl getInstance(@NonNull Context context) { synchronized (sLock) { //-----1----- 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; }}getInstance源码如下，我们看到有两个WorkManagerImpl对象，这里sDelegatedInstance已经不为空，它是在哪里赋值的呢？
private static WorkManagerImpl sDelegatedInstance = null;private static WorkManagerImpl sDefaultInstance = null;@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)public static @Nullable WorkManagerImpl getInstance() { synchronized (sLock) { if (sDelegatedInstance != null) { return sDelegatedInstance; } return sDefaultInstance; }}在源码中有这么一个类WorkManagerInitializer，它继承ContentProvider，源码如下：
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)public class WorkManagerInitializer extends ContentProvider { @Override public boolean onCreate() { // Initialize WorkManager with the default configuration. WorkManager.initialize(getContext(), new Configuration.Builder().build()); return true; }} 我们都知道ContentProvider的创建时机是在程序的入口ActivityThread.main中，通过调用thread.attach最终回调到activityThread.handleBindApplication方法，在这个方法中ActivityThread会创建Application对象并加载ContentProvider，但是有一点要注意，就是会先加载ContentProvider然后在调用Application的onCreate方法
这样在程序启动前就会调用WorkManagerInitializer的onCreate方法，从而调用WorkManager的initialize方法，我们来看一下这个方法：
public boolean onCreate() { // Initialize WorkManager with the default configuration. //-----1----- WorkManager.initialize(getContext(), new Configuration.Builder().build()); return true;}public static void initialize(@NonNull Context context, @NonNull Configuration configuration) { WorkManagerImpl.initialize(context, configuration);}@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)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) { //-----2----- sDefaultInstance = new WorkManagerImpl( context, configuration, new WorkManagerTaskExecutor(configuration.getTaskExecutor())); } sDelegatedInstance = sDefaultInstance; } }}在注释1的地方，传入了一个参数Configuration, 下面我们先来看一下Configuration的源码：
public final class Configuration { public static final int MIN_SCHEDULER_LIMIT = 20; final @NonNull Executor mExecutor; final @NonNull Executor mTaskExecutor; final @NonNull WorkerFactory mWorkerFactory; final @NonNull InputMergerFactory mInputMergerFactory; final int mLoggingLevel; final int mMinJobSchedulerId; final int mMaxJobSchedulerId; final int mMaxSchedulerLimit; private final boolean mIsUsingDefaultTaskExecutor; Configuration(@NonNull Configuration.Builder builder) { if (builder.mExecutor == null) { mExecutor = createDefaultExecutor(); } else { mExecutor = builder.mExecutor; } if (builder.mTaskExecutor == null) { mIsUsingDefaultTaskExecutor = true; mTaskExecutor = createDefaultExecutor(); } else { mIsUsingDefaultTaskExecutor = false; mTaskExecutor = builder.mTaskExecutor; } if (builder.mWorkerFactory == null) { mWorkerFactory = WorkerFactory.getDefaultWorkerFactory(); } else { mWorkerFactory = builder.mWorkerFactory; } if (builder.mInputMergerFactory == null) { mInputMergerFactory = InputMergerFactory.getDefaultInputMergerFactory(); } else { mInputMergerFactory = builder.mInputMergerFactory; } mLoggingLevel = builder.mLoggingLevel; mMinJobSchedulerId = builder.mMinJobSchedulerId; mMaxJobSchedulerId = builder.mMaxJobSchedulerId; mMaxSchedulerLimit = builder.mMaxSchedulerLimit; }}我们截取了Configuration部分重要的源码； 它是采用的建造者模式，从上面的源码看出，它是一个配置类，里面保存了很多配置信息，比如Executor，WorkerFactory，InputMergerFactory等，这里的Executor默认是一个采用Executors.newFixedThreadPool的线程池
然后我们接着看上面注释2的地方使用构造方法初始化了sDefaultInstance,并将其赋值给了sDelegatedInstance，所以程序在一开始就初始化了这两个对象，当我们调用的时候就不为空了
我们看内部实现：
sDefaultInstance = new WorkManagerImpl( context, configuration, //-----1----- new WorkManagerTaskExecutor(configuration.getTaskExecutor()));public WorkManagerImpl( @NonNull Context context, @NonNull Configuration configuration, @NonNull TaskExecutor workTaskExecutor) { this(context, configuration, workTaskExecutor, //-----2----- context.getResources().getBoolean(R.bool.workmanager_test_configuration));}public WorkManagerImpl( @NonNull Context context, @NonNull Configuration configuration, @NonNull TaskExecutor workTaskExecutor, boolean useTestDatabase) { this(context, configuration, workTaskExecutor, //-----3----- WorkDatabase.create( context.getApplicationContext(), workTaskExecutor.getBackgroundExecutor(), useTestDatabase) );}public WorkManagerImpl( @NonNull Context context, @NonNull Configuration configuration, @NonNull TaskExecutor workTaskExecutor, @NonNull WorkDatabase database) { Context applicationContext = context.getApplicationContext(); Logger.setLogger(new Logger.LogcatLogger(configuration.getMinimumLoggingLevel())); //-----4----- List schedulers = createSchedulers(applicationContext, workTaskExecutor); /-----5----- Processor processor = new Processor( context, configuration, workTaskExecutor, database, schedulers); //-----6----- internalInit(context, configuration, workTaskExecutor, database, schedulers, processor);} @Database(entities = { Dependency.class, WorkSpec.class, WorkTag.class, SystemIdInfo.class, WorkName.class, WorkProgress.class, Preference.class}, version = 10)@TypeConverters(value = {Data.class, WorkTypeConverters.class})public abstract class WorkDatabase extends RoomDatabase {...}public static WorkDatabase create( @NonNull final Context context, @NonNull Executor queryExecutor, boolean useTestDatabase) { RoomDatabase.Builder builder; if (useTestDatabase) { builder = Room.inMemoryDatabaseBuilder(context, WorkDatabase.class) .allowMainThreadQueries(); } else { String name = WorkDatabasePathHelper.getWorkDatabaseName(); builder = Room.databaseBuilder(context, WorkDatabase.class, name); builder.openHelperFactory(new SupportSQLiteOpenHelper.Factory() { @NonNull @Override public SupportSQLiteOpenHelper create( @NonNull SupportSQLiteOpenHelper.Configuration configuration) { SupportSQLiteOpenHelper.Configuration.Builder configBuilder = SupportSQLiteOpenHelper.Configuration.builder(context); configBuilder.name(configuration.name) .callback(configuration.callback) .noBackupDirectory(true); FrameworkSQLiteOpenHelperFactory factory = new FrameworkSQLiteOpenHelperFactory(); return factory.create(configBuilder.build()); } }); }} public List createSchedulers( @NonNull Context context, @NonNull 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));} enqueue实际调用的是WorkContinuationImpl的enqueue方法，源码如下：
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. //-----1----- 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;}public class EnqueueRunnable implements Runnable { private static final String TAG = Logger.tagWithPrefix("EnqueueRunnable"); private final WorkContinuationImpl mWorkContinuation; private final OperationImpl mOperation; public EnqueueRunnable(@NonNull WorkContinuationImpl workContinuation) { mWorkContinuation = workContinuation; mOperation = new OperationImpl(); } @Override 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); //-----1----- scheduleWorkInBackground(); } mOperation.setState(Operation.SUCCESS); } catch (Throwable exception) { mOperation.setState(new Operation.State.FAILURE(exception)); } } public Operation getOperation() { return mOperation; }}我们关注注释1处的scheduleWorkInBackground方法：
public void scheduleWorkInBackground() { WorkManagerImpl workManager = mWorkContinuation.getWorkManagerImpl(); Schedulers.schedule( workManager.getConfiguration(), workManager.getWorkDatabase(), workManager.getSchedulers());}public static void schedule( @NonNull Configuration configuration, @NonNull WorkDatabase workDatabase, List schedulers) { if (schedulers == null || schedulers.size() == 0) { return; } WorkSpecDao workSpecDao = workDatabase.workSpecDao(); List eligibleWorkSpecs; workDatabase.beginTransaction(); try { eligibleWorkSpecs = workSpecDao.getEligibleWorkForScheduling( configuration.getMaxSchedulerLimit()); if (eligibleWorkSpecs != null && eligibleWorkSpecs.size() > 0) { long now = System.currentTimeMillis(); // Mark all the WorkSpecs as scheduled. // Calls to Scheduler#schedule() could potentially result in more schedules // on a separate thread. Therefore, this needs to be done first. for (WorkSpec workSpec : eligibleWorkSpecs) { workSpecDao.markWorkSpecScheduled(workSpec.id, now); } } workDatabase.setTransactionSuccessful(); } finally { workDatabase.endTransaction(); } if (eligibleWorkSpecs != null && eligibleWorkSpecs.size() > 0) { WorkSpec[] eligibleWorkSpecsArray = eligibleWorkSpecs.toArray(new WorkSpec[0]); // Delegate to the underlying scheduler. for (Scheduler scheduler : schedulers) { //-----1----- scheduler.schedule(eligibleWorkSpecsArray); } }} 在注释1处调用了scheduler.schedule方法，我们前面将GreedyScheduler加入到队列中，我们这里主要分析该类：
public void schedule(@NonNull WorkSpec... workSpecs) { if (mIsMainProcess == null) { // The default process name is the package name. mIsMainProcess = TextUtils.equals(mContext.getPackageName(), getProcessName()); } if (!mIsMainProcess) { Logger.get().info(TAG, "Ignoring schedule request in non-main process"); return; } registerExecutionListenerIfNeeded(); List constrainedWorkSpecs = new ArrayList<>(); List constrainedWorkSpecIds = new ArrayList<>(); for (WorkSpec workSpec : workSpecs) { //-----1----- if (workSpec.state == WorkInfo.State.ENQUEUED && !workSpec.isPeriodic() && workSpec.initialDelay == 0L && !workSpec.isBackedOff()) { if (workSpec.hasConstraints()) { if (SDK_INT >= 23 && workSpec.constraints.requiresDeviceIdle()) { // Ignore requests that have an idle mode constraint. Logger.get().debug(TAG, String.format("Ignoring WorkSpec %s, Requires device idle.", workSpec)); } else if (SDK_INT >= 24 && workSpec.constraints.hasContentUriTriggers()) { // Ignore requests that have content uri triggers. Logger.get().debug(TAG, String.format("Ignoring WorkSpec %s, Requires ContentUri triggers.", workSpec)); } else { constrainedWorkSpecs.add(workSpec); constrainedWorkSpecIds.add(workSpec.id); } } else { Logger.get().debug(TAG, String.format("Starting work for %s", workSpec.id)); //-----2----- mWorkManagerImpl.startWork(workSpec.id); } } } synchronized (mLock) { if (!constrainedWorkSpecs.isEmpty()) { Logger.get().debug(TAG, String.format("Starting tracking for [%s]", TextUtils.join(",", constrainedWorkSpecIds))); mConstrainedWorkSpecs.addAll(constrainedWorkSpecs); mWorkConstraintsTracker.replace(mConstrainedWorkSpecs); } }} 首先在注释1处判断是否有约束条件，有的话将其任务和任务id加入到集合中，没有的话直接执行注释2处的startWork方法。我们先关注没有约束条件时候的startWork方法：
public void startWork( @NonNull String workSpecId, @Nullable WorkerParameters.RuntimeExtras runtimeExtras) { mWorkTaskExecutor .executeOnBackgroundThread( new StartWorkRunnable(this, workSpecId, runtimeExtras));}接着通过线程池调用了StartWorkRunnable，它是一个Runnable：
public class StartWorkRunnable implements Runnable { private WorkManagerImpl mWorkManagerImpl; private String mWorkSpecId; private WorkerParameters.RuntimeExtras mRuntimeExtras; public StartWorkRunnable( WorkManagerImpl workManagerImpl, String workSpecId, WorkerParameters.RuntimeExtras runtimeExtras) { mWorkManagerImpl = workManagerImpl; mWorkSpecId = workSpecId; mRuntimeExtras = runtimeExtras; } @Override public void run() { //-----1----- mWorkManagerImpl.getProcessor().startWork(mWorkSpecId, mRuntimeExtras); }}接着上面注释1处的getProcessor().startWork方法，Processor的startWorker源码如下：
public boolean startWork( @NonNull String id, @Nullable 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, this, mWorkDatabase, id) .withSchedulers(mSchedulers) .withRuntimeExtras(runtimeExtras) .build(); ListenableFuture future = workWrapper.getFuture(); future.addListener( new FutureListener(this, id, future), mWorkTaskExecutor.getMainThreadExecutor()); mEnqueuedWorkMap.put(id, workWrapper); } //-----1----- mWorkTaskExecutor.getBackgroundExecutor().execute(workWrapper); Logger.get().debug(TAG, String.format("%s: processing %s", getClass().getSimpleName(), id)); return true;} 它是一个work的包装类WorkWrapper，然后执行注释1处的方法, 我们看WorkWrapper的源码:
public class WorkerWrapper implements Runnable {public void run() { mTags = mWorkTagDao.getTagsForWorkSpecId(mWorkSpecId); mWorkDescription = createWorkDescription(mTags); runWorker();}private void runWorker() {...if (mWorker == null) {//-----1----- mWorker = mConfiguration.getWorkerFactory().createWorkerWithDefaultFallback( mAppContext, mWorkSpec.workerClassName, params); } mWorkTaskExecutor.getMainThreadExecutor() .execute(new Runnable() { @Override public void run() { try { Logger.get().debug(TAG, String.format("Starting work for %s", mWorkSpec.workerClassName)); //-----2----- mInnerFuture = mWorker.startWork(); future.setFuture(mInnerFuture); } catch (Throwable e) { future.setException(e); } } });}}public abstract class Worker extends ListenableWorker {public abstract @NonNull Result doWork(); @Override public final @NonNull ListenableFuture startWork() { mFuture = SettableFuture.create(); getBackgroundExecutor().execute(new Runnable() { @Override public void run() { try { Result result = doWork(); mFuture.set(result); } catch (Throwable throwable) { mFuture.setException(throwable); } } }); return mFuture; }} 我反编译了我的apk，拿到了androidManifest.xml：
在最开始注释1的地方有一个provider，正是我们分析初始化workManager的时候的那个Provider。
而且我们还看到有很多的Receiver，我们以注释2处的NetworkStateProxy为例进行分析，它有一个action是CONNECTIVITY_CHANGE，所以当网络状态发生变化的时候会触发这个Receiver。我们看源码部分：
abstract class ConstraintProxy extends BroadcastReceiver { private static final String TAG = Logger.tagWithPrefix("ConstraintProxy"); @Override public void onReceive(Context context, Intent intent) { Logger.get().debug(TAG, String.format("onReceive : %s", intent)); //-----1----- Intent constraintChangedIntent = CommandHandler.createConstraintsChangedIntent(context); context.startService(constraintChangedIntent); } /** * Proxy for Battery Not Low constraint */ public static class BatteryNotLowProxy extends ConstraintProxy { } /** * Proxy for Battery Charging constraint */ public static class BatteryChargingProxy extends ConstraintProxy { } /** * Proxy for Storage Not Low constraint */ public static class StorageNotLowProxy extends ConstraintProxy { } /** * Proxy for Network State constraints */ public static class NetworkStateProxy extends ConstraintProxy { }}static final String ACTION_CONSTRAINTS_CHANGED = "ACTION_CONSTRAINTS_CHANGED";static Intent createConstraintsChangedIntent(@NonNull Context context) { Intent intent = new Intent(context, SystemAlarmService.class); intent.setAction(ACTION_CONSTRAINTS_CHANGED); return intent;}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;}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 (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(); //-----1----- 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. //-----2----- processCommand(); } } return true;}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) { //-----1----- 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(); //-----2----- 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(); }}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, dispatcher); } else if (ACTION_EXECUTION_COMPLETED.equals(action)) { handleExecutionCompleted(intent, startId); } else { Logger.get().warning(TAG, String.format("Ignoring intent %s", intent)); } } }}private void handleConstraintsChanged( @NonNull Intent intent, int startId, @NonNull SystemAlarmDispatcher dispatcher) { Logger.get().debug(TAG, String.format("Handling constraints changed %s", intent)); // Constraints changed command handler is synchronous. No cleanup // is necessary. ConstraintsCommandHandler changedCommandHandler = new ConstraintsCommandHandler(mContext, startId, dispatcher); //-----1----- changedCommandHandler.handleConstraintsChanged();}void handleConstraintsChanged() { List candidates = mDispatcher.getWorkManager().getWorkDatabase() .workSpecDao() .getScheduledWork(); // Update constraint proxy to potentially disable proxies for previously // completed WorkSpecs. ConstraintProxy.updateAll(mContext, candidates); // This needs to be done to populate matching WorkSpec ids in every constraint controller. mWorkConstraintsTracker.replace(candidates); List eligibleWorkSpecs = new ArrayList<>(candidates.size()); // Filter candidates should have already been scheduled. long now = System.currentTimeMillis(); for (WorkSpec workSpec : candidates) { String workSpecId = workSpec.id; long triggerAt = workSpec.calculateNextRunTime(); if (now >= triggerAt && (!workSpec.hasConstraints() || mWorkConstraintsTracker.areAllConstraintsMet(workSpecId))) { eligibleWorkSpecs.add(workSpec); } } for (WorkSpec workSpec : eligibleWorkSpecs) { String workSpecId = workSpec.id; //-----1----- Intent intent = CommandHandler.createDelayMetIntent(mContext, workSpecId); Logger.get().debug(TAG, String.format( "Creating a delay_met command for workSpec with id (%s)", workSpecId)); mDispatcher.postOnMainThread( //-----2----- new SystemAlarmDispatcher.AddRunnable(mDispatcher, intent, mStartId)); } mWorkConstraintsTracker.reset();} static class AddRunnable implements Runnable { private final SystemAlarmDispatcher mDispatcher; private final Intent mIntent; private final int mStartId; AddRunnable(@NonNull SystemAlarmDispatcher dispatcher, @NonNull Intent intent, int startId) { mDispatcher = dispatcher; mIntent = intent; mStartId = startId; } @Override public void run() { mDispatcher.add(mIntent, mStartId); }}我们看到又调用了mDispatcher.add方法，最终还是会 -> processCommand -> mCommandHandler.onHandleIntent , 但是这时候在onHandleIntent中的action就不是ACTION_CONSTRAINTS_CHANGED，而是ACTION_DELAY_MET了，所以要执行handleDelayMet方法：
private void handleDelayMet( @NonNull Intent intent, int startId, @NonNull SystemAlarmDispatcher dispatcher) { Bundle extras = intent.getExtras(); synchronized (mLock) { String workSpecId = extras.getString(KEY_WORKSPEC_ID); Logger.get().debug(TAG, String.format("Handing delay met for %s", workSpecId)); // Check to see if we are already handling an ACTION_DELAY_MET for the WorkSpec. // If we are, then there is nothing for us to do. if (!mPendingDelayMet.containsKey(workSpecId)) { DelayMetCommandHandler delayMetCommandHandler = new DelayMetCommandHandler(mContext, startId, workSpecId, dispatcher); mPendingDelayMet.put(workSpecId, delayMetCommandHandler); //-----1----- delayMetCommandHandler.handleProcessWork(); } else { Logger.get().debug(TAG, String.format("WorkSpec %s is already being handled for ACTION_DELAY_MET", workSpecId)); } }}void handleProcessWork() { mWakeLock = WakeLocks.newWakeLock( mContext, String.format("%s (%s)", mWorkSpecId, mStartId)); Logger.get().debug(TAG, String.format("Acquiring wakelock %s for WorkSpec %s", mWakeLock, mWorkSpecId)); mWakeLock.acquire(); WorkSpec workSpec = mDispatcher.getWorkManager() .getWorkDatabase() .workSpecDao() .getWorkSpec(mWorkSpecId); // This should typically never happen. Cancelling work should remove alarms, but if an // alarm has already fired, then fire a stop work request to remove the pending delay met // command handler. if (workSpec == null) { stopWork(); return; } // Keep track of whether the WorkSpec had constraints. This is useful for updating the // state of constraint proxies when onExecuted(). mHasConstraints = workSpec.hasConstraints(); if (!mHasConstraints) { Logger.get().debug(TAG, String.format("No constraints for %s", mWorkSpecId)); onAllConstraintsMet(Collections.singletonList(mWorkSpecId)); } else { // Allow tracker to report constraint changes //-----1----- mWorkConstraintsTracker.replace(Collections.singletonList(workSpec)); }} ** @param workSpecs A list of {@link WorkSpec}s to monitor constraints for*/@SuppressWarnings("unchecked")public void replace(@NonNull Iterable workSpecs) { synchronized (mLock) { for (ConstraintController<?> controller : mConstraintControllers) { controller.setCallback(null); } for (ConstraintController<?> controller : mConstraintControllers) { //-----1----- controller.replace(workSpecs); } for (ConstraintController<?> controller : mConstraintControllers) { controller.setCallback(this); } }}public void replace(@NonNull Iterable workSpecs) { mMatchingWorkSpecIds.clear(); for (WorkSpec workSpec : workSpecs) { if (hasConstraint(workSpec)) { mMatchingWorkSpecIds.add(workSpec.id); } } if (mMatchingWorkSpecIds.isEmpty()) { mTracker.removeListener(this); } else { mTracker.addListener(this); } //-----2----- updateCallback(mCallback, mCurrentValue);} private void updateCallback( @Nullable OnConstraintUpdatedCallback callback, @Nullable T currentValue) { // We pass copies of references (callback, currentValue) to updateCallback because public // APIs on ConstraintController may be called from any thread, and onConstraintChanged() is // called from the main thread. if (mMatchingWorkSpecIds.isEmpty() || callback == null) { return; } if (currentValue == null || isConstrained(currentValue)) { callback.onConstraintNotMet(mMatchingWorkSpecIds); } else { //-----1----- callback.onConstraintMet(mMatchingWorkSpecIds); }}public void onAllConstraintsMet(@NonNull List workSpecIds) { for (String workSpecId : workSpecIds) { Logger.get().debug( TAG, String.format("Constraints met: Scheduling work ID %s", workSpecId)); mWorkManagerImpl.startWork(workSpecId); }} public void startWork( @NonNull String workSpecId, @Nullable WorkerParameters.RuntimeExtras runtimeExtras) { mWorkTaskExecutor .executeOnBackgroundThread( new StartWorkRunnable(this, workSpecId, runtimeExtras));}这里的StartWorkRunnable就和前面分析的连接上了，最终会调用我们自定义的doWork方法
有需要文章中完整代码的同学 ： 现在私信发送： “底层源码” 即可 免费获取
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WorkManager是一个很优秀的框架，使用起来很方便，只需要自定义Worker，创建请求，加入workManager队列等待执行任务即可，并且能保证任务肯定执行，至于其中的原理，需要好好研究一下源码
最后我想说：
学习没有捷径可言，我们要注意记学习，不仅要记，还要写心得体会，文字笔记、画图、总结等，方式很多，但是一定要自己认真去做，不要太相信自己的记忆，只有反复记忆，加深理解才行
同时，对于程序员而言，不单单是死记硬背，我们有更好的方式去学习，比如写demo去验证。复习知识点时，要及时跟你做过的项目结合起来，这样在面试时就知道怎么聊了，由项目讲到知识点，由一个知识点串联到另一个知识点。复习到一定阶段，可以尝试着去把这些东西串联起来，由点及面，形成知识体系
对于程序员来说，要学习的知识内容、技术有太多太多，要想不被环境淘汰就只有不断提升自己，从来都是我们去适应环境，而不是环境来适应我们
技术是无止境的，你需要对自己提交的每一行代码、使用的每一个工具负责，不断挖掘其底层原理，才能使自己的技术升华到更高的层面
Android 架构师之路还很漫长，与君共勉
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