Kotlin 中带参数的单例 [英] Singleton with parameter in Kotlin

问题描述

我正在尝试将 Android 应用程序从 Java 转换为 Kotlin.应用程序中有一些单身人士.我为没有构造函数参数的单例使用了一个伴随对象.还有一个单例需要一个构造函数参数.

I am trying to convert an Android app from Java to Kotlin. There are a few singletons in the app. I used a companion object for the singletons without constructor parameters. There is another singleton that takes a constructor parameter.

Java 代码:

public class TasksLocalDataSource implements TasksDataSource {

    private static TasksLocalDataSource INSTANCE;

    private TasksDbHelper mDbHelper;

    // Prevent direct instantiation.
    private TasksLocalDataSource(@NonNull Context context) {
        checkNotNull(context);
        mDbHelper = new TasksDbHelper(context);
    }

    public static TasksLocalDataSource getInstance(@NonNull Context context) {
        if (INSTANCE == null) {
            INSTANCE = new TasksLocalDataSource(context);
        }
        return INSTANCE;
    }
}

我在 kotlin 中的解决方案:

My solution in kotlin:

class TasksLocalDataSource private constructor(context: Context) : TasksDataSource {

    private val mDbHelper: TasksDbHelper

    init {
        checkNotNull(context)
        mDbHelper = TasksDbHelper(context)
    }

    companion object {
        lateinit var INSTANCE: TasksLocalDataSource
        private val initialized = AtomicBoolean()

        fun getInstance(context: Context) : TasksLocalDataSource {
            if(initialized.getAndSet(true)) {
                INSTANCE = TasksLocalDataSource(context)
            }
            return INSTANCE
        }
    }
}

我错过了什么吗?线程安全?懒惰?

Am I missing anything? Thread safety? Laziness ?

有几个类似的问题，但我不喜欢答案:)

There were a few similar questions but I don't like the answers :)

推荐答案

这里有一个来自 Google 架构组件的巧妙替代方案 示例代码，它使用了also函数:

Here's a neat alternative from Google's architecture components sample code, which uses the also function:

class UsersDatabase : RoomDatabase() {

    companion object {

        @Volatile private var INSTANCE: UsersDatabase? = null

        fun getInstance(context: Context): UsersDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
            }

        private fun buildDatabase(context: Context) =
            Room.databaseBuilder(context.applicationContext,
                    UsersDatabase::class.java, "Sample.db")
                    .build()
    }
}

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