如何在Room框架下注册onUpgrade回调及自定义DatabaseErrorHandler

  在 Android 中，Room 为 SQLite 提供了高效稳定的抽象层，简化开发流程。RoomDatabase.java 是初始化数据库的重要构建组件，通过它我们可以添加RoomDatabase#Callback监听，RoomDatabase#Callback提供了以下回调接口：

/**
 * Callback for {@link RoomDatabase}.
 */
public abstract static class Callback {

    /**
     * Called when the database is created for the first time. This is called after all the
     * tables are created.
     *
     * @param db The database.
     */
    public void onCreate(@NonNull SupportSQLiteDatabase db) {
    }

    /**
     * Called when the database has been opened.
     *
     * @param db The database.
     */
    public void onOpen(@NonNull SupportSQLiteDatabase db) {
    }

    /**
     * Called after the database was destructively migrated
     *
     * @param db The database.
     */
    public void onDestructiveMigration(@NonNull SupportSQLiteDatabase db){
    }
}

但是对于RoomDatabase#Callback提供的回调接口有时很难满足我们的开发需求，因为它没有将数据库打开过程的完整生命周期事件暴露出来。我们更需要的是SupportSQLiteOpenHelper#Callback开放的生命周期事件：

• onConfigure 在数据库创建或打开之前回调，在这可以添加一些数据库能力，如打开 WAL 日志模式
• onCreate 数据库首次创建时回调
• onUpgrade 数据库版本低于请求版本时回调，进行升级处理
• onDowngrade 类似于 onUpgrade，数据库版本高于请求版本时回调，进行降级处理
• onOpen 数据库连接打开后回调
• onCorruption 在数据库损坏时调用，默认删除db及相关日志文件

  Room框架中SupportSQLiteOpenHelper#Callback的实现类是RoomOpenHelper，幸运的是这个 callback 被注入到SupportSQLiteOpenHelper.Configuration中，并且 Room 允许我们使用SupportSQLiteOpenHelper.Configuration来提供自定义的SupportSQLiteOpenHelper。因此我们可以对FrameworkSQLiteOpenHelper和RoomOpenHelper进行一层代理并开放出想要的生命周期回调。
值得一提的是Room框架下,FrameworkSQLiteOpenHelper使用SupportSQLiteOpenHelper#Callback的onCorruption方法来处理数据库损坏的情况，而其处理策略是删除db及其关联的日志文件。

OpenHelper(Context context, String name, final FrameworkSQLiteDatabase[] dbRef,
        final Callback callback) {
    super(context, name, null, callback.version,
            new DatabaseErrorHandler() {
                @Override
                public void onCorruption(SQLiteDatabase dbObj) {
                    callback.onCorruption(getWrappedDb(dbRef, dbObj));
                }
            });
    mCallback = callback;
    mDbRef = dbRef;
}

很多时候这并不是我们想要的处理方式，通过代理RoomOpenHelper，替换掉onCorruption的默认实现方式，这样就实现了自定义 DatabaseErrorHandler。

下面给出具体的代码示例展示如何进行代理：

// 自定义 SupportSQLiteOpenHelper.Factory 
public class DecoratedOpenHelperFactory implements SupportSQLiteOpenHelper.Factory {
    @NonNull
    private final SupportSQLiteOpenHelper.Factory delegate;
    @Nullable
    private final SupportSQLiteOpenHelper.Callback customListener;

    public DecoratedOpenHelperFactory(@NonNull SupportSQLiteOpenHelper.Factory factory, @Nullable SupportSQLiteOpenHelper.Callback customListener) {
        this.delegate = factory;
        this.customListener = customListener;
    }

    @Override
    public SupportSQLiteOpenHelper create(@NonNull SupportSQLiteOpenHelper.Configuration configuration) {
        final SupportSQLiteOpenHelper.Configuration sqliteConfig = SupportSQLiteOpenHelper.Configuration.builder(configuration.context)
                .name(configuration.name)
                .callback(new DecoratedCallback(configuration.callback, customListener))
                .build();
        return delegate.create(sqliteConfig);
    }
}

// 自定义 SupportSQLiteOpenHelper.Callback
public class DecoratedCallback extends SupportSQLiteOpenHelper.Callback {
    @NonNull
    private final SupportSQLiteOpenHelper.Callback delegate;
    @Nullable
    private final SupportSQLiteOpenHelper.Callback customListener;

    public DecoratedCallback(@NonNull SupportSQLiteOpenHelper.Callback supportSqLiteOpenHelperCallback, @Nullable SupportSQLiteOpenHelper.Callback customListener) {
        super(supportSqLiteOpenHelperCallback.version);
        this.delegate = supportSqLiteOpenHelperCallback;
        this.customListener = customListener;
    }

    @Override
    public void onCreate(@Nullable SupportSQLiteDatabase db) {
        delegate.onCreate(db);
        Optional.ofNullable(customListener).ifPresent(customListener -> customListener.onCreate(db));
    }

    @Override
    public void onUpgrade(SupportSQLiteDatabase db, int oldVersion, int newVersion) {
        delegate.onUpgrade(db, oldVersion, newVersion);
        Optional.ofNullable(customListener).ifPresent(customListener -> customListener.onUpgrade(db, oldVersion, newVersion));
    }

    @Override
    public void onDowngrade(SupportSQLiteDatabase db, int oldVersion, int newVersion) {
        delegate.onDowngrade(db, oldVersion, newVersion);
        Optional.ofNullable(customListener).ifPresent(customListener -> customListener.onDowngrade(db, oldVersion, newVersion));
    }

    @Override
    public void onOpen(SupportSQLiteDatabase db) {
        delegate.onOpen(db);
        Optional.ofNullable(customListener).ifPresent(customListener -> customListener.onOpen(db));
    }

    @Override
    public void onCorruption(SupportSQLiteDatabase db) {
        Optional.ofNullable(customListener).ifPresent(customListener -> customListener.onCorruption(db));
    }
}

最后，Room的构造使用如下：

Room.databaseBuilder(context, databaseName)
    .openHelperFactory(new DecoratedOpenHelperFactory(
        new FrameworkSQLiteOpenHelperFactory(), new SupportSQLiteOpenHelper.Callback() {
            @Override
            public void onCreate(SupportSQLiteDatabase db) {

            }

            @Override
            public void onUpgrade(SupportSQLiteDatabase db, int oldVersion, int newVersion) {

            }

            // etc
        }))
    .build();