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转换为Kotlin

kotlin

imin · 技术社区 · 7 年前

我在跟踪 this tutorial 但现在我想把wordrepository类转换成kotlin。我对科特林非常陌生,所以我试着用 this online kotlin converter 并从第一个链接复制粘贴了java wordrepository类。下面是原始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,为了使上面的代码起作用,我更改了

WordRoomDatabase.getDatabase(application)

到

WordRoomDatabase.getInstance(application)

因为我还将wordroomdatabase更改为kotlin,它不再具有getdatabase函数。以下是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

只有安全的?)或非空断言(!!)允许在 appdatabase类型的可空接收器?

1 回复 | 直到 7 年前

crgarridos 7 年前

这里的问题是你要返回一个 可为空 调用时的实例(问号类型) WordRoomDatabase.getInstance() 因为它的签名。

你可以改变你的 getInstance 致:

    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/