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一  Room数据变化LiveData如何收到onChanged回调的？

1.1  LiveData是如何创建的

这里讨论的LiveData的创建是特指Dao定义的方法的返回类型，而不是所有的LiveData。

以NoteDao 举例：

@Dao
public interface NoteDao {@Query("select * from note")LiveData<List<EntityNote>> getAll();@Updateint update(EntityNote note);@Deleteint delete(EntityNote note);@Insertvoid insert(EntityNote note);
}

 Room会通过APT自动为NoteDao 创建实体类NoteDao_Impl.java

package com.example.sourcecode.jetpack.dao;import android.database.Cursor;
import androidx.lifecycle.LiveData;
import androidx.room.EntityDeletionOrUpdateAdapter;
import androidx.room.EntityInsertionAdapter;
import androidx.room.RoomDatabase;
import androidx.room.RoomSQLiteQuery;
import androidx.room.util.CursorUtil;
import androidx.room.util.DBUtil;
import androidx.sqlite.db.SupportSQLiteStatement;
import com.example.sourcecode.jetpack.entity.EntityNote;
import java.lang.Class;
import java.lang.Exception;
import java.lang.Override;
import java.lang.String;
import java.lang.SuppressWarnings;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Callable;@SuppressWarnings({"unchecked", "deprecation"})
public final class NoteDao_Impl implements NoteDao {private final RoomDatabase __db;private final EntityInsertionAdapter<EntityNote> __insertionAdapterOfEntityNote;private final EntityDeletionOrUpdateAdapter<EntityNote> __deletionAdapterOfEntityNote;private final EntityDeletionOrUpdateAdapter<EntityNote> __updateAdapterOfEntityNote;public NoteDao_Impl(RoomDatabase __db) {this.__db = __db;this.__insertionAdapterOfEntityNote = new EntityInsertionAdapter<EntityNote>(__db) {@Overridepublic String createQuery() {return "INSERT OR ABORT INTO `note` (`id`,`uuid`,`title`,`content`,`searchContent`) VALUES (nullif(?, 0),?,?,?,?)";}@Overridepublic void bind(SupportSQLiteStatement stmt, EntityNote value) {stmt.bindLong(1, value.id);if (value.uuid == null) {stmt.bindNull(2);} else {stmt.bindString(2, value.uuid);}if (value.title == null) {stmt.bindNull(3);} else {stmt.bindString(3, value.title);}if (value.content == null) {stmt.bindNull(4);} else {stmt.bindString(4, value.content);}if (value.searchContent == null) {stmt.bindNull(5);} else {stmt.bindString(5, value.searchContent);}}};this.__deletionAdapterOfEntityNote = new EntityDeletionOrUpdateAdapter<EntityNote>(__db) {@Overridepublic String createQuery() {return "DELETE FROM `note` WHERE `id` = ?";}@Overridepublic void bind(SupportSQLiteStatement stmt, EntityNote value) {stmt.bindLong(1, value.id);}};this.__updateAdapterOfEntityNote = new EntityDeletionOrUpdateAdapter<EntityNote>(__db) {@Overridepublic String createQuery() {return "UPDATE OR ABORT `note` SET `id` = ?,`uuid` = ?,`title` = ?,`content` = ?,`searchContent` = ? WHERE `id` = ?";}@Overridepublic void bind(SupportSQLiteStatement stmt, EntityNote value) {stmt.bindLong(1, value.id);if (value.uuid == null) {stmt.bindNull(2);} else {stmt.bindString(2, value.uuid);}if (value.title == null) {stmt.bindNull(3);} else {stmt.bindString(3, value.title);}if (value.content == null) {stmt.bindNull(4);} else {stmt.bindString(4, value.content);}if (value.searchContent == null) {stmt.bindNull(5);} else {stmt.bindString(5, value.searchContent);}stmt.bindLong(6, value.id);}};}@Overridepublic void insert(final EntityNote note) {__db.assertNotSuspendingTransaction();__db.beginTransaction();try {__insertionAdapterOfEntityNote.insert(note);__db.setTransactionSuccessful();} finally {__db.endTransaction();}}@Overridepublic int delete(final EntityNote note) {__db.assertNotSuspendingTransaction();int _total = 0;__db.beginTransaction();try {_total +=__deletionAdapterOfEntityNote.handle(note);__db.setTransactionSuccessful();return _total;} finally {__db.endTransaction();}}@Overridepublic int update(final EntityNote note) {__db.assertNotSuspendingTransaction();int _total = 0;__db.beginTransaction();try {_total +=__updateAdapterOfEntityNote.handle(note);__db.setTransactionSuccessful();return _total;} finally {__db.endTransaction();}}@Overridepublic LiveData<List<EntityNote>> getAll() {final String _sql = "select * from note";final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 0);return __db.getInvalidationTracker().createLiveData(new String[]{"note"}, false, new Callable<List<EntityNote>>() {@Overridepublic List<EntityNote> call() throws Exception {final Cursor _cursor = DBUtil.query(__db, _statement, false, null);try {final int _cursorIndexOfId = CursorUtil.getColumnIndexOrThrow(_cursor, "id");final int _cursorIndexOfUuid = CursorUtil.getColumnIndexOrThrow(_cursor, "uuid");final int _cursorIndexOfTitle = CursorUtil.getColumnIndexOrThrow(_cursor, "title");final int _cursorIndexOfContent = CursorUtil.getColumnIndexOrThrow(_cursor, "content");final int _cursorIndexOfSearchContent = CursorUtil.getColumnIndexOrThrow(_cursor, "searchContent");final List<EntityNote> _result = new ArrayList<EntityNote>(_cursor.getCount());while(_cursor.moveToNext()) {final EntityNote _item;_item = new EntityNote();_item.id = _cursor.getInt(_cursorIndexOfId);if (_cursor.isNull(_cursorIndexOfUuid)) {_item.uuid = null;} else {_item.uuid = _cursor.getString(_cursorIndexOfUuid);}if (_cursor.isNull(_cursorIndexOfTitle)) {_item.title = null;} else {_item.title = _cursor.getString(_cursorIndexOfTitle);}if (_cursor.isNull(_cursorIndexOfContent)) {_item.content = null;} else {_item.content = _cursor.getString(_cursorIndexOfContent);}if (_cursor.isNull(_cursorIndexOfSearchContent)) {_item.searchContent = null;} else {_item.searchContent = _cursor.getString(_cursorIndexOfSearchContent);}_result.add(_item);}return _result;} finally {_cursor.close();}}@Overrideprotected void finalize() {_statement.release();}});}public static List<Class<?>> getRequiredConverters() {return Collections.emptyList();}
}

通过InvalidationTracker#createLiveData方法创建需要返回的LiveData对象。

// InvalidationTracker.javapublic <T> LiveData<T> createLiveData(String[] tableNames, Callable<T> computeFunction) {return createLiveData(tableNames, false, computeFunction);
}public <T> LiveData<T> createLiveData(String[] tableNames, boolean inTransaction,Callable<T> computeFunction) {return mInvalidationLiveDataContainer.create(validateAndResolveTableNames(tableNames), inTransaction, computeFunction);
}

// InvalidationLiveDataContainer.java<T> LiveData<T> create(String[] tableNames, boolean inTransaction,Callable<T> computeFunction) {return new RoomTrackingLiveData<>(mDatabase, this, inTransaction, computeFunction,tableNames);
}

InvalidationLiveDataContainer的功能比较简单：

• 创建RoomTrackingLiveData对象；
• 维护一个装载LiveData对象的set集合。

总结：

1. room会根据开发者定义的dataBae和各个dao类自动创建各自的对应的实体类；
2. DAO_Impl的实体方法会委托InvalidationTracker类创建需要返回的LiveData对象，并将数据库操作方法以参数的形式向下传递。
3. InvalidationTracker类委托InvalidationLiveDataContainer类创建RoomTrackingLiveData对象。自此LiveData对象创建成功。

1.2 RoomTrackingLiveData有何作用

class RoomTrackingLiveData<T> extends LiveData<T> {final RoomDatabase mDatabase;final boolean mInTransaction;final Callable<T> mComputeFunction;private final InvalidationLiveDataContainer mContainer;final InvalidationTracker.Observer mObserver;final AtomicBoolean mInvalid = new AtomicBoolean(true);final AtomicBoolean mComputing = new AtomicBoolean(false);final AtomicBoolean mRegisteredObserver = new AtomicBoolean(false);final Runnable mRefreshRunnable = new Runnable() {@WorkerThread@Overridepublic void run() {// 向InvalidationTracker注册一个观察者if (mRegisteredObserver.compareAndSet(false, true)) {mDatabase.getInvalidationTracker().addWeakObserver(mObserver);}boolean computed;do {computed = false;// mComputing 初始值为 falseif (mComputing.compareAndSet(false, true)) {// as long as it is invalid, keep computing.try {T value = null;// mInvalid初始值为 true// 此while循环结束后，computed == false，mInvalid == falsewhile (mInvalid.compareAndSet(true, false)) {computed = true;try {// 执行数据库操作方法，并返回结果value = mComputeFunction.call();} catch (Exception e) {// 如果SQL语句执行有误，会非常粗暴的直接报错，// liveData不能将错误状态上报给开发者。throw new RuntimeException("Exception while computing database"+ " live data.", e);}}if (computed) {// 向当前livedata的观察者们发送数据库查询结果postValue(value);}} finally {// release compute lockmComputing.set(false);}}} while (computed && mInvalid.get());}};@SuppressWarnings("WeakerAccess")final Runnable mInvalidationRunnable = new Runnable() {@MainThread@Overridepublic void run() {// 当前livedata是否有存活的观察者boolean isActive = hasActiveObservers();// 如果 mRefreshRunnable正在运行 mInvalid == true，条件不成立。// 如果 mRefreshRunnable运行结束 mInvalid == false，条件成立，重新开启任务。if (mInvalid.compareAndSet(false, true)) {if (isActive) {getQueryExecutor().execute(mRefreshRunnable);}}}};@SuppressLint("RestrictedApi")RoomTrackingLiveData(RoomDatabase database,InvalidationLiveDataContainer container,boolean inTransaction,Callable<T> computeFunction,String[] tableNames) {mDatabase = database;mInTransaction = inTransaction;mComputeFunction = computeFunction;mContainer = container;mObserver = new InvalidationTracker.Observer(tableNames) {@Overridepublic void onInvalidated(@NonNull Set<String> tables) {ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable);}};}@Overrideprotected void onActive() {super.onActive();mContainer.onActive(this);getQueryExecutor().execute(mRefreshRunnable);}@Overrideprotected void onInactive() {super.onInactive();mContainer.onInactive(this);}Executor getQueryExecutor() {if (mInTransaction) {return mDatabase.getTransactionExecutor();} else {return mDatabase.getQueryExecutor();}}
}

1. 当开发者向RoomTrackingLiveData注册了观察者后（即调用了livedata.observe方法），会调用onActive方法，在子线程里执行mRefreshRunnable任务。
2. mRefreshRunnable在初次执行时会向InvalidationTracker注册一个观察者。然后会根据SQL语句循环查询数据库，并向开发者返回查询结果。
   a. SQL语句是通过开发者在创建DAO层方法的注解自动生成的，并以方法入参的方式最终传递给RoomTrackingLiveData对象。
   b. 这里的循环不是一直执行的。在没有外界干扰情况下（指循环条件的值在没有被其他方法修改的情况），循环体只会执行一次。
3. 构造函数里创建了mObserver 对象，当mObserver被触发时,会在主线程执行mInvalidationRunnable任务。
4. mInvalidationRunnable会在子线程里开启mRefreshRunnable任务，重新查询数据库，并返回数据。

总结：

1. RoomTrackingLiveData有三个比较重要的任务：mRefreshRunnable、mInvalidationRunnable和mObserver。
2. mRefreshRunnable主要负责向数据库查询数据，并将结果返回给开发者注册的观察者。
3. mObserver负责唤醒mInvalidationRunnable。
4. mInvalidationRunnable任务分两种情况：
   • 当mRefreshRunnable还在运行时，会要求mRefreshRunnable再执行一次数据库查询任务，并按要求将结果上报。（这个逻辑是在mRefreshRunnable里实现的。）
   • 当mRefreshRunnable停止运行时，会在子线程里重新开启mRefreshRunnable任务。

由上可知，room配合livedata使用时，之所以livedata能够自动感知数据库数据变化，是由mObserver、mInvalidationRunnable、mRefreshRunnable三方共同配合的结果。

1.3 数据库变化时，是如何通知RoomTrackingLiveData

由上文可以推断出，当数据库发生变化时，是通过mObserver来启动数据库查询任务，并将结果通过
RoomTrackingLiveData#postValue方法传递给订阅者。接下来就要研究一下mObserver的调用链。

   // RoomTrackingLiveData.javafinal Runnable mRefreshRunnable = new Runnable() {@WorkerThread@Overridepublic void run() {// 1. 向InvalidationTracker注册一个观察者if (mRegisteredObserver.compareAndSet(false, true)) {mDatabase.getInvalidationTracker().addWeakObserver(mObserver);}....}};

// InvalidationTracker.javapublic void addWeakObserver(Observer observer) {// 2addObserver(new WeakObserver(this, observer));
}public void addObserver(@NonNull Observer observer) {final String[] tableNames = resolveViews(observer.mTables);    int[] tableIds = new int[tableNames.length];     final int size = tableNames.length;      for (int i = 0; i < size; i++) {         Integer tableId = mTableIdLookup.get(tableNames[i].toLowerCase(Locale.US));         if (tableId == null) {             throw new IllegalArgumentException("There is no table with name " + tableNames[i]);         }         tableIds[i] = tableId;     }     ObserverWrapper wrapper = new ObserverWrapper(observer, tableIds, tableNames);     ObserverWrapper currentObserver;     synchronized (mObserverMap) {  // 3       currentObserver = mObserverMap.putIfAbsent(observer, wrapper);     }     if (currentObserver == null && mObservedTableTracker.onAdded(tableIds)) {         syncTriggers();     }
}

• RoomTrackingLiveData创建mObserver对象，并一步步将mObserver进行包装，并存放在InvalidationTracker的mObserverMap中。
• 接下来则需要调查源码里在哪些情况下会遍历mObserverMap，并去调用mObserverMap里item的方法。

// InvalidationTracker.javaRunnable mRefreshRunnable = new Runnable() {@Overridepublic void run() {......if (invalidatedTableIds != null && !invalidatedTableIds.isEmpty()) {synchronized (mObserverMap) {// 1. 遍历了 mObserverMapfor (Map.Entry<Observer, ObserverWrapper> entry : mObserverMap) {entry.getValue().notifyByTableInvalidStatus(invalidatedTableIds);}}}}......
};public void notifyObserversByTableNames(String... tables) {synchronized (mObserverMap) {// 2. 遍历了 mObserverMapfor (Map.Entry<Observer, ObserverWrapper> entry : mObserverMap) {if (!entry.getKey().isRemote()) {entry.getValue().notifyByTableNames(tables);}}}
}

由源码可知，共有两处遍历了mObserverMap，我们先研究一下mRefreshRunnable的调用链。

/*** Enqueues a task to refresh the list of updated tables.* <p>* This method is automatically called when {@link RoomDatabase#endTransaction()} is called but* if you have another connection to the database or directly use {@link* SupportSQLiteDatabase}, you may need to call this manually.*/
public void refreshVersionsAsync() {// TODO we should consider doing this sync instead of async.if (mPendingRefresh.compareAndSet(false, true)) {if (mAutoCloser != null) {mAutoCloser.incrementCountAndEnsureDbIsOpen();}// 启动 mRefreshRunnable 任务mDatabase.getQueryExecutor().execute(mRefreshRunnable);}
}

• 从方法说明上可以看出，当RoomDatabase#endTransaction()被调用时，会启动mRefreshRunnable任务。继续跟踪refreshVersionsAsync的调用链也能发现这点。
• 接下来让我们回头研究一下room框架自动为开发者定义的dao类自动生成的xxxDAO_Impl.java。仔细研究一下各个方法的实现会发现，只要涉及到对数据库进行增、删、改的操作，都会调用到__db.endTransaction()。这里的__db就是RoomDatabase的对象。例如：

  @Overridepublic void insert(final EntityNote note) {__db.assertNotSuspendingTransaction();__db.beginTransaction();try {__insertionAdapterOfEntityNote.insert(note);__db.setTransactionSuccessful();} finally {__db.endTransaction();}}@Overridepublic int delete(final EntityNote note) {__db.assertNotSuspendingTransaction();int _total = 0;__db.beginTransaction();try {_total +=__deletionAdapterOfEntityNote.handle(note);__db.setTransactionSuccessful();return _total;} finally {__db.endTransaction();}}@Overridepublic int update(final EntityNote note) {__db.assertNotSuspendingTransaction();int _total = 0;__db.beginTransaction();try {_total +=__updateAdapterOfEntityNote.handle(note);__db.setTransactionSuccessful();return _total;} finally {__db.endTransaction();}}

 1.3.1 __db.endTransaction()中调用internalEndTransaction()

    public void endTransaction() {if (mAutoCloser == null) {internalEndTransaction();} else {mAutoCloser.executeRefCountingFunction(db -> {internalEndTransaction();return null;});}}

1.3.2 mInvalidationTracker.refreshVersionsAsync()

    private void internalEndTransaction() {mOpenHelper.getWritableDatabase().endTransaction();if (!inTransaction()) {// enqueue refresh only if we are NOT in a transaction. Otherwise, wait for the last// endTransaction call to do it.mInvalidationTracker.refreshVersionsAsync();}}

 1.3.3 mDatabase.getQueryExecutor().execute(mRefreshRunnable)

    public void refreshVersionsAsync() {// TODO we should consider doing this sync instead of async.if (mPendingRefresh.compareAndSet(false, true)) {if (mAutoCloser != null) {// refreshVersionsAsync is called with the ref count incremented from// RoomDatabase, so the db can't be closed here, but we need to be sure that our// db isn't closed until refresh is completed. This increment call must be// matched with a corresponding call in mRefreshRunnable.mAutoCloser.incrementCountAndEnsureDbIsOpen();}mDatabase.getQueryExecutor().execute(mRefreshRunnable);}}

    Runnable mRefreshRunnable = new Runnable() {@Overridepublic void run() {final Lock closeLock = mDatabase.getCloseLock();Set<Integer> invalidatedTableIds = null;closeLock.lock();try {if (!ensureInitialization()) {return;}if (!mPendingRefresh.compareAndSet(true, false)) {// no pending refreshreturn;}if (mDatabase.inTransaction()) {// current thread is in a transaction. when it ends, it will invoke// refreshRunnable again. mPendingRefresh is left as false on purpose// so that the last transaction can flip it on again.return;}// This transaction has to be on the underlying DB rather than the RoomDatabase// in order to avoid a recursive loop after endTransaction.SupportSQLiteDatabase db = mDatabase.getOpenHelper().getWritableDatabase();db.beginTransactionNonExclusive();try {invalidatedTableIds = checkUpdatedTable();db.setTransactionSuccessful();} finally {db.endTransaction();}} catch (IllegalStateException | SQLiteException exception) {// may happen if db is closed. just log.Log.e(Room.LOG_TAG, "Cannot run invalidation tracker. Is the db closed?",exception);} finally {closeLock.unlock();if (mAutoCloser != null) {mAutoCloser.decrementCountAndScheduleClose();}}if (invalidatedTableIds != null && !invalidatedTableIds.isEmpty()) {synchronized (mObserverMap) {for (Map.Entry<Observer, ObserverWrapper> entry : mObserverMap) {entry.getValue().notifyByTableInvalidStatus(invalidatedTableIds);}}}}private Set<Integer> checkUpdatedTable() {HashSet<Integer> invalidatedTableIds = new HashSet<>();Cursor cursor = mDatabase.query(new SimpleSQLiteQuery(SELECT_UPDATED_TABLES_SQL));//noinspection TryFinallyCanBeTryWithResourcestry {while (cursor.moveToNext()) {final int tableId = cursor.getInt(0);invalidatedTableIds.add(tableId);}} finally {cursor.close();}if (!invalidatedTableIds.isEmpty()) {mCleanupStatement.executeUpdateDelete();}return invalidatedTableIds;}};

1.3.4 entry.getValue().notifyByTableInvalidStatus(invalidatedTableIds)

     void notifyByTableInvalidStatus(Set<Integer> invalidatedTablesIds) {Set<String> invalidatedTables = null;final int size = mTableIds.length;for (int index = 0; index < size; index++) {final int tableId = mTableIds[index];if (invalidatedTablesIds.contains(tableId)) {if (size == 1) {// Optimization for a single-table observerinvalidatedTables = mSingleTableSet;} else {if (invalidatedTables == null) {invalidatedTables = new HashSet<>(size);}invalidatedTables.add(mTableNames[index]);}}}if (invalidatedTables != null) {mObserver.onInvalidated(invalidatedTables);}}

1.3.5 mObserver.onInvalidated(invalidatedTables)

    RoomTrackingLiveData(RoomDatabase database,InvalidationLiveDataContainer container,boolean inTransaction,Callable<T> computeFunction,String[] tableNames) {mDatabase = database;mInTransaction = inTransaction;mComputeFunction = computeFunction;mContainer = container;mObserver = new InvalidationTracker.Observer(tableNames) {@Overridepublic void onInvalidated(@NonNull Set<String> tables) {ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable);}};}

1.3.6 ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable)

    final Runnable mInvalidationRunnable = new Runnable() {@MainThread@Overridepublic void run() {boolean isActive = hasActiveObservers();if (mInvalid.compareAndSet(false, true)) {if (isActive) {getQueryExecutor().execute(mRefreshRunnable);}}}}

 1.3.7 mRefreshRunnable.run  postValue(value)

   final Runnable mRefreshRunnable = new Runnable() {@WorkerThread@Overridepublic void run() {// 向InvalidationTracker注册一个观察者if (mRegisteredObserver.compareAndSet(false, true)) {mDatabase.getInvalidationTracker().addWeakObserver(mObserver);}boolean computed;do {computed = false;// mComputing 初始值为 falseif (mComputing.compareAndSet(false, true)) {// as long as it is invalid, keep computing.try {T value = null;// mInvalid初始值为 true// 此while循环结束后，computed == false，mInvalid == falsewhile (mInvalid.compareAndSet(true, false)) {computed = true;try {// 执行数据库操作方法，并返回结果value = mComputeFunction.call();} catch (Exception e) {// 如果SQL语句执行有误，会非常粗暴的直接报错，// liveData不能将错误状态上报给开发者。throw new RuntimeException("Exception while computing database"+ " live data.", e);}}if (computed) {// 向当前livedata的观察者们发送数据库查询结果postValue(value);}} finally {// release compute lockmComputing.set(false);}}} while (computed && mInvalid.get());}};

1.3.8 observer收到onChanged回调        

LiveData<List<EntityNote>> EntityNoteLiveData = AppDatabase.getInstance().noteDao().getAll()注册的RoomLiveData的observer会回调onChanged

// 继承AndroidViewModel,带有Application环境
public class NoteViewModel extends AndroidViewModel {private MediatorLiveData<List<EntityNote >> mMediatorLiveData;public NoteViewModel(@NonNull Application application) {super(application);mMediatorLiveData = new MediatorLiveData<>();LiveData<List<EntityNote>> EntityNoteLiveData = AppDatabase.getInstance().noteDao().getAll();mMediatorLiveData.addSource(EntityNoteLiveData, new Observer<List<EntityNote>>() {private List<EntityNote> mLastEntityNoteList;@Overridepublic void onChanged(List<EntityNote> entityNotes) {if (mLastEntityNoteList == null) {mLastEntityNoteList = entityNotes;return;}if (entityNotes == null) {setValue(new ArrayList<>());return;}int lastSize = mLastEntityNoteList.size();int size = entityNotes.size();if (lastSize != size) {setValue(entityNotes);return;}for (int i = 0; i < size; i++) {EntityNote lastNote = mLastEntityNoteList.get(i);EntityNote note = entityNotes.get(i);if (!isSameNote(lastNote, note)) {setValue(entityNotes);break;}}// 没有变化不setValue不触发onChangedmLastEntityNoteList = entityNotes;}private void setValue(List<EntityNote> entityNotes) {mMediatorLiveData.setValue(entityNotes);mLastEntityNoteList = entityNotes;}private boolean isSameNote(EntityNote first, EntityNote second) {if (first == null || second == null) {return false;}return first.uuid.equals(second.uuid) && first.title.equals(second.title)&& first.id == second.id && first.content.equals(second.content);}});}//查(所有)public MediatorLiveData<List<EntityNote>> getNoteListLiveData() {return mMediatorLiveData;}
}

 

1.4 总结：

1. 数据库的增、删、改操作会调用RoomDatabase#endTransaction()；
2. RoomDatabase#endTransaction()会调用InvalidationTracker#refreshVersionsAsync();
3. refreshVersionsAsync()会开启mRefreshRunnable任务。
4. mRefreshRunnable里会遍历mObserverMap，并挨个调用其item的指定方法。
5. RoomTrackingLiveData在构造函数里创建了mObserver对象，并将此对象放置于InvalidationTracker的mObserverMap中。且此对象的方法就是用来唤醒RoomTrackingLiveData的mRefreshRunnable任务。还记得这个任务是干嘛的吗？这个任务就是根据RoomTrackingLiveData持有的数据库查询语句向数据库查询数据，并将查询结果上报给开发者指定的Observer。

至此，RoomTrackingLiveData完美实现了数据库发生变化时，会主动将新的数据上报给开发者的功能。

二  Room是如何通知其他进程的订阅者

如果有两个进程同时关联了同一个数据库，如果一个进程对此数据库的数据进行改变，那么另一个进程的RoomTrackingLiveData依旧能感知到数据变化，这是怎么做到的呢？
还记得上面在调查InvalidationTracker的mObserverMap时，发现有两个方法遍历了这个map吗。其中mRefreshRunnable已经分析过了，接下来分析另一个方法notifyObserversByTableNames。

// InvalidationTracker.javapublic void notifyObserversByTableNames(String... tables) {synchronized (mObserverMap) {//  遍历了 mObserverMapfor (Map.Entry<Observer, ObserverWrapper> entry : mObserverMap) {if (!entry.getKey().isRemote()) {entry.getValue().notifyByTableNames(tables);}}}
}

// MultiInstanceInvalidationClient.javafinal IMultiInstanceInvalidationCallback mCallback =new IMultiInstanceInvalidationCallback.Stub() {@Overridepublic void onInvalidation(final String[] tables) {mExecutor.execute(new Runnable() {@Overridepublic void run() {//1.调用了 nvalidationTracker#notifyObserversByTableNames()mInvalidationTracker.notifyObserversByTableNames(tables);}});}};final Runnable mSetUpRunnable = new Runnable() {@Overridepublic void run() {try {final IMultiInstanceInvalidationService service = mService;if (service != null) {//2. 向 service 注册 mCallbackmClientId = service.registerCallback(mCallback, mName);mInvalidationTracker.addObserver(mObserver);}} catch (RemoteException e) {Log.w(Room.LOG_TAG, "Cannot register multi-instance invalidation callback", e);}}
};final ServiceConnection mServiceConnection = new ServiceConnection() {@Overridepublic void onServiceConnected(ComponentName name, IBinder service) {mService = IMultiInstanceInvalidationService.Stub.asInterface(service);// 3. 执行 mSetUpRunnable 任务 mExecutor.execute(mSetUpRunnable);}@Overridepublic void onServiceDisconnected(ComponentName name) {mExecutor.execute(mRemoveObserverRunnable);mService = null;}};        

1. 由上可见，在MultiInstanceInvalidationClient类里绑定了一个service，并向service注册mCallback。这个mCallback会通过InvalidationTracker#notifyObserversByTableNames()通知RoomTrackingLiveData该干活了（查询和上报数据库新值）。

看到IMultiInstanceInvalidationService.Stub可以大胆猜测这里涉及到了跨进程通信。
接下来研究MultiInstanceInvalidationService

// MultiInstanceInvalidationService.javapublic class MultiInstanceInvalidationService extends Service {int mMaxClientId = 0;final HashMap<Integer, String> mClientNames = new HashMap<>();// 1. 可以理解成这是一个装载 callBack的集合final RemoteCallbackList<IMultiInstanceInvalidationCallback> mCallbackList =new RemoteCallbackList<IMultiInstanceInvalidationCallback>() {@Overridepublic void onCallbackDied(IMultiInstanceInvalidationCallback callback,Object cookie) {mClientNames.remove((int) cookie);}};private final IMultiInstanceInvalidationService.Stub mBinder =new IMultiInstanceInvalidationService.Stub() {@Overridepublic int registerCallback(IMultiInstanceInvalidationCallback callback,String name) {if (name == null) {return 0;}synchronized (mCallbackList) {int clientId = ++mMaxClientId;// 2. 将 callback 放入 mCallbackList 集合中if (mCallbackList.register(callback, clientId)) {mClientNames.put(clientId, name);return clientId;} else {--mMaxClientId;return 0;}}}@Overridepublic void unregisterCallback(IMultiInstanceInvalidationCallback callback,int clientId) {synchronized (mCallbackList) {mCallbackList.unregister(callback);mClientNames.remove(clientId);}}@Overridepublic void broadcastInvalidation(int clientId, String[] tables) {synchronized (mCallbackList) {String name = mClientNames.get(clientId);if (name == null) {Log.w(Room.LOG_TAG, "Remote invalidation client ID not registered");return;}int count = mCallbackList.beginBroadcast();try {// 这个for循环，可以理解成取出mCallbackList集合中的所有callBack，// 并调用各自的 onInvalidation方法。for (int i = 0; i < count; i++) {int targetClientId = (int) mCallbackList.getBroadcastCookie(i);String targetName = mClientNames.get(targetClientId);if (clientId == targetClientId // This is the caller itself.|| !name.equals(targetName)) { // Not the same file.continue;}try {IMultiInstanceInvalidationCallback callback =mCallbackList.getBroadcastItem(i);callback.onInvalidation(tables);} catch (RemoteException e) {Log.w(Room.LOG_TAG, "Error invoking a remote callback", e);}}} finally {mCallbackList.finishBroadcast();}}}};@Nullable@Overridepublic IBinder onBind(@NonNull Intent intent) {return mBinder;}
}

1. 由以上源码可以推断出这个service主要做了两件事：
   • 在内存中维护一个集合，这个集合装载的是所有client注册的callBack；
   • 在合适的时机调用所有client注册的callBack。这个合适的时机，就是调用broadcastInvalidation（）的时候。

回到MultiInstanceInvalidationClient，回想一下这个client向service注册了个什么玩意。

// MultiInstanceInvalidationClient.javafinal Runnable mSetUpRunnable = new Runnable() {@Overridepublic void run() {try {final IMultiInstanceInvalidationService service = mService;if (service != null) {// 1. 向service注册mCallbackmClientId = service.registerCallback(mCallback, mName);mInvalidationTracker.addObserver(mObserver);}} catch (RemoteException e) {Log.w(Room.LOG_TAG, "Cannot register multi-instance invalidation callback", e);}}
};final IMultiInstanceInvalidationCallback mCallback =new IMultiInstanceInvalidationCallback.Stub() {@Overridepublic void onInvalidation(final String[] tables) {mExecutor.execute(new Runnable() {@Overridepublic void run() {// 2. 这个方法是干什么的来着？//    是拜托InvalidationTracker通知RoomTrackingLiveData该干活了。//    上文有介绍mInvalidationTracker.notifyObserversByTableNames(tables);}});}};

接下来追踪一下MultiInstanceInvalidationService#broadcastInvalidation()

// MultiInstanceInvalidationClient.javaMultiInstanceInvalidationClient(Context context, String name, Intent serviceIntent,InvalidationTracker invalidationTracker, Executor executor) {......mObserver = new InvalidationTracker.Observer(tableNames.toArray(new String[0])) {@Overridepublic void onInvalidated(@NonNull Set<String> tables) {if (mStopped.get()) {return;}try {final IMultiInstanceInvalidationService service = mService;if (service != null) {// 1. 调用了MultiInstanceInvalidationService#broadcastInvalidation()service.broadcastInvalidation(mClientId, tables.toArray(new String[0]));}} catch (RemoteException e) {Log.w(Room.LOG_TAG, "Cannot broadcast invalidation", e);}}@Overrideboolean isRemote() {return true;}};mAppContext.bindService(serviceIntent, mServiceConnection, Context.BIND_AUTO_CREATE);
}final Runnable mSetUpRunnable = new Runnable() {@Overridepublic void run() {try {final IMultiInstanceInvalidationService service = mService;if (service != null) {mClientId = service.registerCallback(mCallback, mName);// 2. 将mObserver传递给InvalidationTrackermInvalidationTracker.addObserver(mObserver);}} catch (RemoteException e) {Log.w(Room.LOG_TAG, "Cannot register multi-instance invalidation callback", e);}}
};

看了以上2个步骤是不是似曾相识？还记得RoomTrackingLiveData的mObserver对象吗？和这里的套路是一模一样。接下来很明显，InvalidationTracker里面会有一个map来装载这个mObserver。然后会有两个方法去遍历这个map。其中一个Runnable方法会在调用数据库的增删改方法时触发，另一个方法notifyObserversByTableNames会在...会在...???
我不是在研究notifyObserversByTableNames的调用链吗？怎么又绕回来了？

这里理解起来有点绕，先明确一下前提：

1. 针对不同的进程操作同一个数据库的场景，其实每一个进程都会拥有自己独立的RoomDatabase实例。相应的MultiInstanceInvalidationClient、InvalidationTracker、RoomTrackingLiveData都是相互独立的。
2. 只有MultiInstanceInvalidationService是共同的实例。而这个共同的实例，是保证不同进程能相互感知到数据库操作的关键。
3. InvalidationTracker的mRefreshRunnable是在单进程中调用的。
4. InvalidationTracker的notifyObserversByTableNames是用于跨进程调用的。

下面重新捋一下思路。首先假设现在有两个进程会操作同一个数据库。那么这两个进程都会各自拥有一套自己的独立对象。即都会做一下事情：

1. 创建RoomTrackingLiveData对象，并将mObserver委托给InvalidationTracker管理。
2. RoomTrackingLiveData里的mRefreshRunnable会在被唤醒时重新查询数据库，并上报结果。
3. 创建MultiInstanceInvalidationClient对象，并与唯一的MultiInstanceInvalidationService进行绑定，并将callBack委托给service管理。
4. callBack里会调用InvalidationTracker#notifyObserversByTableNames()。
5. MultiInstanceInvalidationClient对象将mObserver委托给InvalidationTracker管理。
6. MultiInstanceInvalidationClient的mObserver会通知所有与MultiInstanceInvalidationService进行绑定的MultiInstanceInvalidationClient，告知它们数据库有变化。

针对进程1，我们重点关注3、4、5、6。针对进程2，我们重点关注1、2。现在开始发车：

1. 当前用户在进程1操作了数据库的修改操作，那么就会触发进程1的RoomDatabase#endTransaction(),
   进而触发了InvalidationTracker#mRefreshRunnable 任务，遍历InvalidationTracker#mObserverMap（在上一节有相关介绍）。此mObserverMap里存在一个MultiInstanceInvalidationClient添加进来的mObserver（上面第5点有提到）。
2. 进程1的MultiInstanceInvalidationClient的mObserver会调用MultiInstanceInvalidationService#broadcastInvalidation()。
3. MultiInstanceInvalidationService会遍历和执行所有MultiInstanceInvalidationClient注册的callback。这其中的一个callback就是进程2的MultiInstanceInvalidationClient注册的（上面第5点有提到）。
4. 进程2的callback会调用进程2的InvalidationTracker#notifyObserversByTableNames()。再回忆一下这个notifyObserversByTableNames()是干嘛的？没错，就是我们研究的第二个遍历InvalidationTracker的mObserverMap的方法。
5. 既然进程2已经遍历了mObserverMap，那么势必会让进程2的RoomTrackingLiveData干活（查询数据库，上报新数据）。

至此，room框架完成了一次完美的跨进程通讯。

要想当前的RoomDataBase具有跨进程通讯的能力，需要在构建databaseBuilder的时候调用enableMultiInstanceInvalidation()。例如：

@Database(entities = {EntityNote.class}, version = 1, exportSchema = false)
public abstract class AppDatabase extends RoomDatabase {private static final String DB_NAME = "note.db";private static volatile AppDatabase instance;//创建单例public static synchronized AppDatabase getInstance() {if (instance == null) {instance = create();}return instance;}/*** 创建数据库*/private static AppDatabase create() {return Room.databaseBuilder(MyApplication.getInstance(), AppDatabase.class, DB_NAME).allowMainThreadQueries().fallbackToDestructiveMigration().enableMultiInstanceInvalidation() // 跨进程通讯的能力.build();}public abstract NoteDao noteDao();
}

从源码来看，RoomDataBase正是通过此方法来间接创建MultiInstanceInvalidationClient对象，并与MultiInstanceInvalidationService建立绑定关系。