转换为 kotlin

Question

我正在关注this tutorial，但现在我想将 WordRepository 类转换为 kotlin。我对 kotlin 非常陌生，所以我尝试使用 this online kotlin converter 并从第一个链接复制粘贴 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;
       }
   }
}

以下是我从在线转换器获得的 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，为了使上面的代码工作，我改变了

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 功能。这是 WordRoomDatabase 的样子

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，特别是这部分：

mWordDao = db.wordDao();

显示错误

只有安全 (?.) 或非空断言 (!!.) 调用才允许在 AppDatabase 类型的可空接收器？

Answer 1

这里的问题是您在调用 WordRoomDatabase.getInstance() 时返回一个 可为空 实例（问题标记类型），因为它是签名。

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
    }

---

为了进一步了解可空类型和安全调用，这里有一些有用的链接：

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

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