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();

巴特西

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

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