SingletonWithContext.kt

/**
 * Provides a single instance of a class requiring a context.  Can be inherited by a companion
 * object to easily make it a singleton
 */
abstract class SingletonWithContext<T> {
    @Volatile
    private var instance: T? = null

    fun getInstance(context: Context): T =
            instance ?: synchronized(this) {
                instance ?: buildInstance(context).also { instance = it }
            }

    protected abstract fun buildInstance(context: Context): T
}

/**
 * A singleton that uses Shared Preferences on Android
 */
class PrefsHelper private constructor(val sharedPreferences: SharedPreferences) {

    companion object : SingletonWithContext<PrefsHelper>() {
        override fun buildInstance(context: Context): PrefsHelper {
            val sharedPreferences = context.applicationContext.getSharedPreferences("MyPreferences", Context.MODE_PRIVATE)
            return PrefsHelper(sharedPreferences)
        }
    }
}

/**
 * Example usage of our singleton
 */
class ExampleActivity : FragmentActivity() {
    val prefsHelper by lazy { PrefsHelper.getInstance(this) }

}