안드로이드 실시간 채팅: Firebase Realtime Database 🔥💬

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.constraintlayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent">

    <androidx.recyclerview.widget.recyclerview android:id="@+id/recyclerViewChatRooms" android:layout_width="0dp" android:layout_height="0dp" app:layout_constraintbottom_totopof="@+id/buttonCreateChat" app:layout_constraintend_toendof="parent" app:layout_constraintstart_tostartof="parent" app:layout_constrainttop_totopof="parent"></androidx.recyclerview.widget.recyclerview>

    <button android:id="@+id/buttonCreateChat" android:layout_width="0dp" android:layout_height="wrap_content" android:text="새 채팅방 만들기" app:layout_constraintbottom_tobottomof="parent" app:layout_constraintend_toendof="parent" app:layout_constraintstart_tostartof="parent"></button>

</androidx.constraintlayout.widget.constraintlayout>

import android.os.Bundle
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.database.*
import kotlinx.android.synthetic.main.activity_chat_list.*

class ChatListActivity : AppCompatActivity() {

    private lateinit var database: DatabaseReference
    private lateinit var chatRoomAdapter: ChatRoomAdapter
    private val chatRooms = mutableListOf<chatroom>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_chat_list)

        database = FirebaseDatabase.getInstance().reference

        setupRecyclerView()
        loadChatRooms()

        buttonCreateChat.setOnClickListener {
            createNewChatRoom()
        }
    }

    private fun setupRecyclerView() {
        chatRoomAdapter = ChatRoomAdapter(chatRooms) { chatRoom ->
            // TODO: 채팅방으로 이동
        }
        recyclerViewChatRooms.apply {
            layoutManager = LinearLayoutManager(this@ChatListActivity)
            adapter = chatRoomAdapter
        }
    }

    private fun loadChatRooms() {
        val userId = FirebaseAuth.getInstance().currentUser?.uid ?: return
        database.child("users").child(userId).child("chatRooms")
            .addValueEventListener(object : ValueEventListener {
                override fun onDataChange(snapshot: DataSnapshot) {
                    chatRooms.clear()
                    for (childSnapshot in snapshot.children) {
                        val chatRoom = childSnapshot.getValue(ChatRoom::class.java)
                        chatRoom?.let { chatRooms.add(it) }
                    }
                    chatRoomAdapter.notifyDataSetChanged()
                }

                override fun onCancelled(error: DatabaseError) {
                    Toast.makeText(this@ChatListActivity, "채팅방 목록을 불러오는데 실패했습니다.", Toast.LENGTH_SHORT).show()
                }
            })
    }

    private fun createNewChatRoom() {
        val userId = FirebaseAuth.getInstance().currentUser?.uid ?: return
        val newChatRoomRef = database.child("chatRooms").push()
        val chatRoomId = newChatRoomRef.key ?: return

        val chatRoom = ChatRoom(chatRoomId, "새로운 채팅방", System.currentTimeMillis())
        
        newChatRoomRef.setValue(chatRoom)
        database.child("users").child(userId).child("chatRooms").child(chatRoomId).setValue(true)

        Toast.makeText(this, "새로운 채팅방이 생성되었습니다.", Toast.LENGTH_SHORT).show()
    }
}

data class ChatRoom(
    val id: String = "",
    val name: String = "",
    val lastMessageTime: Long = 0
)
</chatroom>

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import java.text.SimpleDateFormat
import java.util.*

class ChatRoomAdapter(
    private val chatRooms: List<chatroom>,
    private val onItemClick: (ChatRoom) -> Unit
) : RecyclerView.Adapter<chatroomadapter.viewholder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(android.R.layout.simple_list_item_2, parent, false)
        return ViewHolder(view)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val chatRoom = chatRooms[position]
        holder.bind(chatRoom)
    }

    override fun getItemCount() = chatRooms.size

    inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        private val text1: TextView = itemView.findViewById(android.R.id.text1)
        private val text2: TextView = itemView.findViewById(android.R.id.text2)

        fun bind(chatRoom: ChatRoom) {
            text1.text = chatRoom.name
            text2.text = SimpleDateFormat("yyyy-MM-dd HH:mm", Locale.getDefault())
                .format(Date(chatRoom.lastMessageTime))
            itemView.setOnClickListener { onItemClick(chatRoom) }
        }
    }
}
</chatroomadapter.viewholder></chatroom>

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.constraintlayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent">

    <androidx.recyclerview.widget.recyclerview android:id="@+id/recyclerViewMessages" android:layout_width="0dp" android:layout_height="0dp" app:layout_constraintbottom_totopof="@+id/editTextMessage" app:layout_constraintend_toendof="parent" app:layout_constraintstart_tostartof="parent" app:layout_constrainttop_totopof="parent"></androidx.recyclerview.widget.recyclerview>

    <edittext android:id="@+id/editTextMessage" android:layout_width="0dp" android:layout_height="wrap_content" android:hint="메시지를 입력하세요" app:layout_constraintbottom_tobottomof="parent" app:layout_constraintend_tostartof="@+id/buttonSend" app:layout_constraintstart_tostartof="parent"></edittext>

    <button android:id="@+id/buttonSend" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="전송" app:layout_constraintbottom_tobottomof="parent" app:layout_constraintend_toendof="parent"></button>

</androidx.constraintlayout.widget.constraintlayout>

import android.os.Bundle
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.database.*
import kotlinx.android.synthetic.main.activity_chat.*

class ChatActivity : AppCompatActivity() {

    private lateinit var database: DatabaseReference
    private lateinit var messageAdapter: MessageAdapter
    private val messages = mutableListOf<message>()
    private lateinit var chatRoomId: String

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_chat)

        chatRoomId = intent.getStringExtra("CHAT_ROOM_ID") ?: return finish()
        database = FirebaseDatabase.getInstance().reference.child("chatRooms").child(chatRoomId).child("messages")

        setupRecyclerView()
        loadMessages()

        buttonSend.setOnClickListener {
            sendMessage()
        }
    }

    private fun setupRecyclerView() {
        messageAdapter = MessageAdapter(messages)
        recyclerViewMessages.apply {
            layoutManager = LinearLayoutManager(this@ChatActivity).apply {
                stackFromEnd = true
            }
            adapter = messageAdapter
        }
    }

    private fun loadMessages() {
        database.addChildEventListener(object : ChildEventListener {
            override fun onChildAdded(snapshot: DataSnapshot, previousChildName: String?) {
                val message = snapshot.getValue(Message::class.java)
                message?.let {
                    messages.add(it)
                    messageAdapter.notifyItemInserted(messages.size - 1)
                    recyclerViewMessages.scrollToPosition(messages.size - 1)
                }
            }

            override fun onChildChanged(snapshot: DataSnapshot, previousChildName: String?) {}
            override fun onChildRemoved(snapshot: DataSnapshot) {}
            override fun onChildMoved(snapshot: DataSnapshot, previousChildName: String?) {}
            override fun onCancelled(error: DatabaseError) {}
        })
    }

    private fun sendMessage() {
        val messageText = editTextMessage.text.toString().trim()
        if (messageText.isNotEmpty()) {
            val userId = FirebaseAuth.getInstance().currentUser?.uid ?: return
            val message = Message(userId, messageText, System.currentTimeMillis())
            database.push().setValue(message)
            editTextMessage.text.clear()
        }
    }
}

data class Message(
    val senderId: String = "",
    val text: String = "",
    val timestamp: Long = 0
)
</message>

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.google.firebase.auth.FirebaseAuth
import java.text.SimpleDateFormat
import java.util.*

class MessageAdapter(private val messages: List<message>) : RecyclerView.Adapter<messageadapter.messageviewholder>() {

    private val currentUserId = FirebaseAuth.getInstance().currentUser?.uid

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MessageViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(android.R.layout.simple_list_item_2, parent, false)
        return MessageViewHolder(view)
    }

    override fun onBindViewHolder(holder: MessageViewHolder, position: Int) {
        val message = messages[position]
        holder.bind(message)
    }

    override fun getItemCount() = messages.size

    inner class MessageViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        private val text1: TextView = itemView.findViewById(android.R.id.text1)
        private val text2: TextView = itemView.findViewById(android.R.id.text2)

        fun bind(message: Message) {
            text1.text = message.text
            text2.text = SimpleDateFormat("HH:mm", Locale.getDefault()).format(Date(message.timestamp))

            if (message.senderId == currentUserId) {
                itemView.setBackgroundResource(android.R.color.holo_green_light)
            } else {
                itemView.setBackgroundResource(android.R.color.white)
            }
        }
    }
}
</messageadapter.messageviewholder></message>

FirebaseDatabase.getInstance().setPersistenceEnabled(true)