RoomActivity.kt

class RoomActivity: AppCompatActivity() {
  
  private lateinit var mAdapter: RoomAdapter
  private val userManager: UserManager by inject() // koin user injection
  
  override fun onCreate(savedInstance: Bundle?) {
    super.onCreate(savedInstance)
    
    mAdapter = RoomAdapter(userManager.currentUser.id)
    
    // recycler view in layout
    room_list_chat.apply {
          adapter = mAdapter
          layoutManager = LinearLayoutManager(this@RoomActivity).also { it.stackFromEnd = true }
    }
    
    // subscribe to room; getting all messages
    val messageEvent = { message: Message -> mAdapter.addMessage(message) }

    val errorEvent = { error: elements.Error -> makeToast(error.reason) }

    val roomListeners = RoomListeners(
            onMultipartMessage = messageEvent,
            onErrorOccurred = errorEvent
    )

    val messageCallback: (Subscription) -> Unit = {
        runOnUiThread {
            mAdapter.notifyDataSetChanged()
        }
    }

    if (room.id.isNotEmpty()) {
        currentUser?.subscribeToRoomMultipart(
                roomId = room.id,
                messageLimit = 30,
                listeners = roomListeners,
                callback = messageCallback
        )
    }
  }
}

Initially, I did not include the runOnUiThread function but added it because it seemed to display the messages without the user touching the screen, but it doesn't display all the messages.