BuyPreferences.kt

interface BuyPreferences {
    fun incrementBuyCount()
    fun getBuyCount(): Int
}

class BuyPreferencesImpl(context: Context) : BuyPreferences {
    // ...
    private val sharedPreferences: SharedPreferences = context.getSharedPreferences(...)

    override fun incrementBuyCount() {
        val count = getBuyCount()
        sharedPreferences.edit().putInt(BUY_COUNT_KEY, count + 1).apply()
    }

    override fun getBuyCount() = sharedPreferences.getInt(BUY_COUNT_KEY, 0)
}