firebase_functions_code.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

/**
 * Triggered when a new message is created in any chat.
 * Sends an FCM notification to the receiver.
 */
exports.onTextMessageCreated = functions.firestore
    .document('chats/{chatId}/messages/{messageId}')
    .onCreate(async (snapshot, context) => {
        const messageData = snapshot.data();
        const chatId = context.params.chatId;
        const messageId = context.params.messageId;

        const senderId = messageData.senderId;
        const receiverId = messageData.receiverId;
        const text = messageData.text;

        try {
            // 1. Fetch receiver's FCM token
            const receiverDoc = await admin.firestore().collection('users').doc(receiverId).get();
            if (!receiverDoc.exists) return null;

            const receiverData = receiverDoc.data();
            const fcmToken = receiverData.fcmToken;
            if (!fcmToken) return null;

            // 2. Fetch sender's name
            const senderDoc = await admin.firestore().collection('users').doc(senderId).get();
            const senderName = senderDoc.exists ? senderDoc.data().displayName : "Someone";

            // 3. Prepare FCM payload
            const payload = {
                notification: {
                    title: senderName,
                    body: text.length > 50 ? text.substring(0, 47) + "..." : text,
                },
                data: {
                    senderId: senderId,
                    receiverId: receiverId,
                    chatId: chatId,
                    messageId: messageId,
                    type: "text",
                    click_action: "FLUTTER_NOTIFICATION_CLICK",
                },
                token: fcmToken,
            };

            // 4. Send notification
            await admin.messaging().send(payload);
            console.log(`Notification sent to ${receiverId} from ${senderId}`);
            
            return null;
        } catch (error) {
            console.error('Error sending notification:', error);
            return null;
        }
    });