Kotlin-在Android中转换Singleton DatabaseController的最佳方法 [英] Kotlin - Best way to convert Singleton DatabaseController in Android

问题描述

我正在从"Kotlin in Action"中学习Kotlin，并且正在慢慢将Android应用程序代码转换为Kotlin.但是我在转换以下类时发现了一些问题.

I'm learning Kotlin from "Kotlin in Action" and I'm slowly converting an Android app code to it. But I have found some problem in converting the following class.

public class DatabaseController {
  private static DatabaseController sDatabaseController;

  private SQLiteDatabase mDatabase;

  private DatabaseController(Context context) {
    mDatabase = new SQLiteOpenHelperImpl(context.getApplicationContext())
            .getWritableDatabase();
  }

  public static DatabaseController getDatabaseController(Context context) {
    if (sDatabaseController == null) {
        sDatabaseController = new DatabaseController(context);
    }
    return sDatabaseController;
  }

  public void addElement(Element element) {
    if (element != null) {
        ContentValues values = getContentValues(element);

        mDatabase.beginTransaction();
        try {
            // insert element
            mDatabase.setTransactionSuccessful();
        } finally {
            mDatabase.endTransaction();
        }
    }
}

我想出了两种不同的Kotlin实现，但是它们都没有完全说服我.哪一个可以被认为是更好的解决方案?还是存在更好的第三个?

I've come up with two different Kotlin implementation, but neither of them fully convince me. Which one can be considered a better solution? Or does exist a third one that is better?

使用对象的第一个实现

object DatabaseControllerObject {
  private var mDatabase : SQLiteDatabase? = null

  fun initDatabase(context: Context) {
    mDatabase = mDatabase?: SQLiteOpenHelperImpl(context.applicationContext).writableDatabase
  }

  fun addElement(context: Context, element: Element) {
    initDatabase(context)
    // insert alarm
    mDatabase?.let {
    // CODE
    }
}

第二个实现，所有内容都放在一个文件中，在每个需要数据库的活动的onCreate()中，我在其中调用initDatabase(..)

Second implementation with everything in a single file, where I call initDatabase(..) in the onCreate() of each activity that needs the database

private var mDatabase: SQLiteDatabase? = null

fun initDatabase(context: Context) {
    mDatabase = mDatabase ?: SQLiteOpenHelperImpl(context.applicationContext).writableDatabase
}

fun addElement(element: Element) {
    val values = getContentValues(element)

    mDatabase?.let {
        it.beginTransaction()
        try {
          // insert
          it.setTransactionSuccessful()
        } finally {
          it.endTransaction()
        }
    }
}

推荐答案

我认为您想要的是同伴对象:

I think what you want is a companion object:

class DatabaseController
{
    private constructor(context: Context)
    {
        // ...
    }

    companion object
    {
        private var instance: DatabaseController? = null
        fun getInstance(context: Context): DatabaseController
        {
            if(instance == null)
            {
                instance = DatabaseController(context)
            }

            return instance!!
        }
    }
}

然后您可以这样称呼它:

Then you can just call it like this:

val databaseController = DatabaseController.getInstance(context)

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