Singleton with parameter in Kotlin

Question

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

Answer 1

I am not entirely sure why would you need such code, but here is my best shot at it:

class TasksLocalDataSource private constructor(context: Context) : TasksDataSource {
    private val mDbHelper = TasksDbHelper(context)

    companion object {
        private var instance : TasksLocalDataSource? = null

        fun  getInstance(context: Context): TasksLocalDataSource {
            if (instance == null)  // NOT thread safe!
                instance = TasksLocalDataSource(context)

            return instance!!
        }
    }
}

This is similar to what you wrote, and has the same API.

A few notes:

• Do not use lateinit here. It has a different purpose, and a nullable variable is ideal here.

• What does checkNotNull(context) do? context is never null here, this is guarantied by Kotlin. All checks and asserts are already implemented by the compiler.

UPDATE:

If all you need is a lazily initialised instance of class TasksLocalDataSource, then just use a bunch of lazy properties (inside an object or on the package level):

val context = ....

val dataSource by lazy {
    TasksLocalDataSource(context)
}