Question

我正在关注this tutorial，但现在我想将WordRepository类转换为kotlin。我对kotlin极为陌生，因此我尝试使用this online kotlin converter并从第一个链接复制粘贴的Java WordRepository类。下面是原始的Java代码btw：

public class WordRepository {

   private WordDao mWordDao;
   private LiveData<List<Word>> mAllWords;

   WordRepository(Application application) {
       WordRoomDatabase db = WordRoomDatabase.getDatabase(application);
       mWordDao = db.wordDao();
       mAllWords = mWordDao.getAllWords();
   }

   LiveData<List<Word>> getAllWords() {
       return mAllWords;
   }


   public void insert (Word word) {
       new insertAsyncTask(mWordDao).execute(word);
   }

   private static class insertAsyncTask extends AsyncTask<Word, Void, Void> {

       private WordDao mAsyncTaskDao;

       insertAsyncTask(WordDao dao) {
           mAsyncTaskDao = dao;
       }

       @Override
       protected Void doInBackground(final Word... params) {
           mAsyncTaskDao.insert(params[0]);
           return null;
       }
   }
}

下面是我从在线转换器中获得的kotlin代码：

class WordRepository internal constructor(application:Application) {
  private val mWordDao:WordDao
  internal val allWords:LiveData<List<Word>>
  init{
    val db = WordRoomDatabase.getDatabase(application)
    mWordDao = db.wordDao()
    allWords = mWordDao.getAllWords()
  }
  fun insert(word:Word) {
    insertAsyncTask(mWordDao).execute(word)
  }
  private class insertAsyncTask internal constructor(dao:WordDao):AsyncTask<Word, Void, Void>() {
    private val mAsyncTaskDao:WordDao
    init{
      mAsyncTaskDao = dao
    }
    protected fun doInBackground(vararg params:Word):Void {
      mAsyncTaskDao.insert(params[0])
      return null
    }
  }
}

请注意，为了使上面的代码有效，我进行了更改

WordRoomDatabase.getDatabase(application)

到

WordRoomDatabase.getInstance(application)

因为我也将WordRoomDatabase更改为kotlin，并且它不再具有getDatabase函数。这是WordRoomDatabase的外观

abstract class WordRoomDatabase: RoomDatabase(){

    companion object {
        private var INSTANCE: WordRoomDatabase? = null

        fun getInstance(context: Context): WordRoomDatabase? {
            if (INSTANCE == null){
                synchronized(WordRoomDatabase::class){
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                            WordRoomDatabase::class.java, "sps.db")
                            .build()
                }
            }
            return INSTANCE
        }

        fun destroyInstance(){
            INSTANCE == null
        }
    }
}

但是，代码不起作用，特别是这一部分：

mWordDao = db.wordDao();

显示错误

仅允许在服务器上安全（？。）或非空声明（!!。）的调用 AppDatabase类型的可为空的接收器？

Answer 1

这里的问题是，由于WordRoomDatabase.getInstance()的签名，您将返回一个 nullable 实例（带问号的类型）。

您可以将getInstance更改为：

    fun getInstance(context: Context): WordRoomDatabase {
        if (INSTANCE == null){
            synchronized(WordRoomDatabase::class){
                INSTANCE = when you call [https://kotlinlang.org/docs/reference/null-safety.html#safe-calls](https://kotlinlang.org/docs/reference/null-safety.html#safe-calls)Room.databaseBuilder(context.getApplicationContext(),
                        WordRoomDatabase::class.java, "sps.db")
                        .build()
            }
        }
        return INSTANCE as WordRoomDatabase
    }

要进一步了解可为null的类型和安全调用，请参见以下有用链接：

https://kotlinlang.org/docs/reference/null-safety.html#safe-calls

https://www.callicoder.com/kotlin-nullable-types-null-safety/

转换为Kotlin

1 个答案: