package com.cdc.viewModel;
import android.content.Context;import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.room.migration.Migration;
import androidx.sqlite.db.SupportSQLiteDatabase;
@Database(entities = {Word.class},version=5,exportSchema = false)
public abstract class WordDataBase extends RoomDatabase {private static WordDataBase INSTANCE;//单例模式public static synchronized WordDataBase getInstance(Context context){if(INSTANCE==null){INSTANCE= Room.databaseBuilder(context.getApplicationContext(),WordDataBase.class,"Word_database")//.allowMainThreadQueries() 强制要求在主线程运行(真实环境中不使用)//.fallbackToDestructiveMigration()//破坏试的迁移.原有的数据被清空，重新建立新表.addMigrations(MIGRATION_4_5).build();}return INSTANCE;}public abstract WordDao getWordDao();//版本号  3到4static final Migration MIGRATION_3_4=new Migration(3,4) {@Overridepublic void migrate(@NonNull SupportSQLiteDatabase database) {//没有布尔值，用INTEGER代替//使用"ALTER TABLE Word  ADD COLUMN bar_data2 INTEGER  NOT NULL DEFAULT 1"出错。//使用下面分开的形式,可以正确执行database.execSQL("ALTER TABLE Word "+" ADD COLUMN bar_data2 INTEGER "+" NOT NULL DEFAULT 1");}};//版本号  4到5  没有办法直接删除某一列。可以通过以下四步完成删除某一列的效果static final Migration MIGRATION_4_5=new Migration(4,5) {@Overridepublic void migrate(@NonNull SupportSQLiteDatabase database) {//1.创建一个新表WordTemp，只设定想要的字段database.execSQL("CREATE TABLE WordTemp "+"( id  INTEGER "+" PRIMARY KEY NOT NULL ,"+" english_word TEXT ,"+" chinese_meaning TEXT "+" ) ");//2.将原来表中的数据复制过来，database.execSQL(" INSERT INTO WordTemp (id,english_word,chinese_meaning) " +"SELECT id,english_word,chinese_meaning  FROM Word ");//3. 将原表删除database.execSQL(" DROP TABLE Word ");//4.将新建的表改名database.execSQL(" ALTER  TABLE WordTemp  RENAME to Word");}};
}