在 Kotlin 中获取抽象 class 中的对象

Question

我正在尝试在 Kotlin 中复制以下 Java class。

@Database(entities = {Word.class}, version = 1)
public abstract class WordRoomDatabase extends RoomDatabase {

   public abstract WordDao wordDao();

   private static WordRoomDatabase INSTANCE;


   static WordRoomDatabase getDatabase(final Context context) {
       if (INSTANCE == null) {
           synchronized (WordRoomDatabase.class) {
               if (INSTANCE == null) {
                   INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                           WordRoomDatabase.class, "word_database")
                           .build();                
               }
           }
       }
       return INSTANCE;
   }
}

我正在关注代码实验室的 tutorial，以使用 Room 实现一个简单的 SQLite 数据库。以下是我对 class.

的完整 Kotlin 实现

@Database(entities = [Word::class], version = 1)
abstract class WordRoomDatabase : RoomDatabase() {

    abstract fun wordDao(): WordDao

    private class PopulateDbAsync(db: WordRoomDatabase) : AsyncTask<Unit, Unit, Unit>() {

        private val dao = db.wordDao()

        override fun doInBackground(vararg params: Unit) {
            dao.deleteAll()
            var word = Word("Hello")
            dao.insert(word)
            word = Word("World")
            dao.insert(word)
        }
    }

    companion object {
        private var INSTANCE: WordRoomDatabase? = null

        private val roomDatabaseCallback = object : RoomDatabase.Callback() {

            override fun onOpen(db: SupportSQLiteDatabase) {
                super.onOpen(db)
                PopulateDbAsync(INSTANCE as WordRoomDatabase).execute()
            }
        }

        fun getDatabase(context: Context): WordRoomDatabase {
            if (INSTANCE == null) {
                synchronized(WordRoomDatabase::class) {
                    if (INSTANCE == null) {
                        INSTANCE = Room.databaseBuilder(context.applicationContext,
                                WordRoomDatabase::class.java, "word_database")
                                .addCallback(roomDatabaseCallback)
                                .build()
                    }
                }
            }

            return INSTANCE as WordRoomDatabase
        }
    }
}

当我尝试访问伴随对象中的数据时，出现以下错误。 cannot find implementation for com.fifthgen.androidroomtest.data.WordRoomDatabase.WordRoomDatabase_Impl does not exist

这是发生错误的代码行。

private val db = WordRoomDatabase.getDatabase(application)

Answer 1

按照 Luksprog 的建议向 build.gradle 添加了房间编译器和 Kotlin 插件，一切正常。

apply plugin: 'kotlin-kapt'

// .....

kapt 'android.arch.persistence.room:compiler:1.1.0'

在 Kotlin 中获取抽象 class 中的对象

Get an object inside an abstract class in Kotlin

android

kotlin

android-room