Database

package ua.leomovskii.demomessenger;

import com.google.firebase.firestore.*;
import com.google.android.gms.tasks.Task;
import java.util.*;

public class Database {

    private static final FirebaseFirestore db = FirebaseFirestore.getInstance();
    private static final CollectionReference messages = db.collection("messages");

    private static ListenerRegistration messagesListener;

    public interface OnMessagesUpdatedListener {
        void onMessagesUpdated(List<MessageModel> messages);
        void onError(String error);
    }

    public static Task<MessageModel> TryGetMessage(int id) {
        return messages.whereEqualTo("id", id).limit(1).get()
                .continueWith(task -> {
                    if (!task.getResult().isEmpty()) {
                        return task.getResult().getDocuments().get(0).toObject(MessageModel.class);
                    }
                    return null;
                });
    }

    public static Task<List<MessageModel>> GetAllMessages() {
        return messages.orderBy("timestamp", Query.Direction.ASCENDING).get()
                .continueWith(task -> {
                    List<MessageModel> list = new ArrayList<>();
                    for (DocumentSnapshot doc : task.getResult()) {
                        MessageModel m = doc.toObject(MessageModel.class);
                        if (m != null) {
                            m.documentId = doc.getId();
                            list.add(m);
                        }
                    }
                    return list;
                });
    }

    public static void listenForAddedMessages(OnMessagesUpdatedListener callback) {
        if (messagesListener != null) {
            messagesListener.remove();
        }

        messagesListener = messages.orderBy("timestamp", Query.Direction.ASCENDING)
                .addSnapshotListener((snap, e) -> {
                    if (e != null) {
                        callback.onError("Fetch messages error: " + e.getMessage());
                        return;
                    }

                    if (snap != null) {
                        List<MessageModel> newMessages = new ArrayList<>();

                        for (DocumentChange dc : snap.getDocumentChanges()) {
                            if (dc.getType() == DocumentChange.Type.ADDED) {
                                MessageModel m = dc.getDocument().toObject(MessageModel.class);
                                if (m != null) {
                                    m.documentId = dc.getDocument().getId();
                                    newMessages.add(m);
                                }
                            }
                        }

                        if (!newMessages.isEmpty()) {
                            callback.onMessagesUpdated(newMessages);
                        }
                    }
                });
    }

    public static Task<Void> SendMessage(String text, String userId) {
        MessageModel m = new MessageModel();
        m.text = text;
        m.userId = userId;
        m.timestamp = System.currentTimeMillis();

        return messages.add(m).continueWith(task -> null);
    }

    public static void stopListening() {
        if (messagesListener != null) {
            messagesListener.remove();
            messagesListener = null;
        }
    }
}