Notification handling in background in Firebase - Mini Project: Build & Apply

Notification handling in background

📖 Scenario: You are building a mobile app that needs to receive notifications even when the app is not open. This means the app must handle notifications in the background to alert users about important updates.

🎯 Goal: Create a Firebase Cloud Messaging (FCM) service worker script that handles notifications when the app is in the background. You will set up the initial service worker, configure notification options, implement the background message handler, and finalize the notification display.

📋 What You'll Learn

Create a service worker file named firebase-messaging-sw.js.

Initialize Firebase messaging in the service worker.

Add a configuration variable for notification title.

Implement the background message handler to show notifications.

Complete the notification display with icon and click action.

💡 Why This Matters

🌍 Real World

Mobile apps often need to notify users about updates even when the app is closed. Handling notifications in the background ensures users stay informed.

💼 Career

Understanding background notification handling is essential for mobile app developers and cloud engineers working with Firebase Cloud Messaging.

1

Create the Firebase messaging service worker

Create a file named firebase-messaging-sw.js and write the line importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-app-compat.js'); to import Firebase app scripts.

Firebase

# Your code here

Hint

This line imports Firebase app scripts needed for messaging in the service worker.

2

Add Firebase messaging import and initialize app

In firebase-messaging-sw.js, add the line importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-messaging-compat.js'); to import messaging scripts. Then initialize Firebase app with firebase.initializeApp({messagingSenderId: '1234567890'});.

Firebase

importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-app-compat.js');
# Your code here

Hint

Import messaging scripts and initialize Firebase with your sender ID.

3

Implement background message handler with notification title

Create a constant notificationTitle with value 'Background Notification'. Then get messaging instance with const messaging = firebase.messaging();. Use messaging.onBackgroundMessage with a callback that receives payload and calls self.registration.showNotification(notificationTitle, {body: payload.data.body});.

Firebase

importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-app-compat.js');
importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-messaging-compat.js');
firebase.initializeApp({messagingSenderId: '1234567890'});
# Your code here

Hint

Set notification title, get messaging instance, and handle background messages to show notifications.

4

Complete notification with icon and click action

In the showNotification call, add an icon property with value '/firebase-logo.png' and a data property containing click_action with value 'https://www.example.com' inside the options object.

Firebase

importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-app-compat.js');
importScripts('https://www.gstatic.com/firebasejs/9.22.1/firebase-messaging-compat.js');
firebase.initializeApp({messagingSenderId: '1234567890'});
const notificationTitle = 'Background Notification';
const messaging = firebase.messaging();
messaging.onBackgroundMessage(function(payload) {
  // Add icon and click_action properties below
  self.registration.showNotification(notificationTitle, {body: payload.data.body});
});

Hint

Add icon and click action URL to the notification options for better user experience.

Practice

1. What is the main purpose of using onBackgroundMessage in Firebase Cloud Messaging?

easy

A. To authenticate users before sending notifications

B. To send messages from the client to the server

C. To handle notifications when the app is not open or in the background

D. To delete old notifications from the device

Solution

1. Step 1: Understand the role of onBackgroundMessage

   This function is designed to catch messages when the app is not active or open.

2. Step 2: Compare with other options

   Sending messages, authentication, or deleting notifications are unrelated to background message handling.

3. Final Answer:

   To handle notifications when the app is not open or in the background -> Option C

4. Quick Check:

   Background message handling = To handle notifications when the app is not open or in the background [OK]

Hint: Remember: background means app closed or hidden [OK]

Common Mistakes:

• Confusing sending messages with receiving them
• Thinking it manages user authentication
• Assuming it deletes notifications automatically

2. Which of the following is the correct way to import and use onBackgroundMessage in a Firebase service worker?

easy

A. import { onBackgroundMessage } from 'firebase/messaging/sw'; onBackgroundMessage(handler);

B. import { onBackgroundMessage } from 'firebase/messaging/sw'; onBackgroundMessage(messaging, handler);

C. import { onBackgroundMessage } from 'firebase/messaging'; messaging.onBackgroundMessage(handler);

D. import { onBackgroundMessage } from 'firebase/messaging'; onBackgroundMessage(messaging, handler);

Solution

1. Step 1: Identify correct import path

   The correct import for service worker usage is from 'firebase/messaging/sw'.

2. Step 2: Use correct function signature

   onBackgroundMessage requires only the handler function as argument (no messaging instance needed).

3. Final Answer:

   import { onBackgroundMessage } from 'firebase/messaging/sw'; onBackgroundMessage(handler); -> Option A

4. Quick Check:

   Correct import and call signature = import { onBackgroundMessage } from 'firebase/messaging/sw'; onBackgroundMessage(handler); [OK]

Hint: Remember: import from 'firebase/messaging/sw' and pass handler only [OK]

Common Mistakes:

• Importing from wrong path like 'firebase/messaging'
• Passing messaging instance to onBackgroundMessage
• Using messaging.onBackgroundMessage(handler)

3. Given this service worker code snippet, what will happen when a background message arrives?

import { onBackgroundMessage } from 'firebase/messaging/sw';
import { messaging } from './firebase-config';

onBackgroundMessage((payload) => {
  console.log('Received background message ', payload);
  self.registration.showNotification(payload.notification.title, {
    body: payload.notification.body
  });
});

medium

A. Only the message payload is logged, no notification is shown

B. The notification shows but the payload is not logged

C. An error occurs because showNotification is not a valid method

D. The message payload is logged and a notification with title and body is shown

Solution

1. Step 1: Analyze the callback function

   The callback logs the payload and calls showNotification with title and body from the payload.

2. Step 2: Confirm method validity

   self.registration.showNotification is the correct way to show notifications in service workers.

3. Final Answer:

   The message payload is logged and a notification with title and body is shown -> Option D

4. Quick Check:

   Payload logged + notification shown = The message payload is logged and a notification with title and body is shown [OK]

Hint: Check if both console.log and showNotification are called [OK]

Common Mistakes:

• Thinking showNotification is invalid in service workers
• Assuming logging disables notification
• Ignoring the payload content usage

4. You wrote this service worker code but notifications do not appear when the app is in background:

import { onBackgroundMessage } from 'firebase/messaging';
import { messaging } from './firebase-config';

onBackgroundMessage((payload) => {
  self.registration.showNotification(payload.notification.title, {
    body: payload.notification.body
  });
});

What is the main issue?

medium

A. Using self.registration.showNotification instead of Notification.show

B. Wrong import path for onBackgroundMessage ('firebase/messaging' instead of 'firebase/messaging/sw')

C. Not importing messaging from Firebase

D. Payload does not contain notification object

Solution

1. Step 1: Check onBackgroundMessage usage

   The import path is wrong for service worker; must be from 'firebase/messaging/sw'.

2. Step 2: Verify notification method and imports

   self.registration.showNotification is correct and messaging is imported but not needed.

3. Final Answer:

   Wrong import path for onBackgroundMessage ('firebase/messaging' instead of 'firebase/messaging/sw') -> Option B

4. Quick Check:

   Service worker import path = Wrong import path for onBackgroundMessage ('firebase/messaging' instead of 'firebase/messaging/sw') [OK]

Hint: Remember: import from 'firebase/messaging/sw' [OK]

Common Mistakes:

• Using wrong import path for onBackgroundMessage
• Confusing notification API names
• Assuming payload structure is always wrong

5. You want to ensure your Firebase app shows a notification with a custom icon and vibrates when a background message arrives. Which code snippet correctly adds these features inside onBackgroundMessage?

hard

A. onBackgroundMessage((payload) => { self.registration.showNotification(payload.notification.title, { body: payload.notification.body, icon: '/icon.png', vibrate: [100, 50, 100] }); });

B. onBackgroundMessage((payload) => { self.showNotification(payload.notification.title, { body: payload.notification.body, icon: '/icon.png', vibrate: [100, 50, 100] }); });

C. onBackgroundMessage((payload) => { Notification.show(payload.notification.title, { body: payload.notification.body, icon: '/icon.png', vibrate: [100, 50, 100] }); });

D. onBackgroundMessage((payload) => { self.registration.showNotification(payload.title, { body: payload.body, icon: '/icon.png', vibrate: [100, 50, 100] }); });

Solution

1. Step 1: Use correct notification method and parameters

   self.registration.showNotification is correct. The notification title and body come from payload.notification.

2. Step 2: Add icon and vibrate options properly

   Options icon and vibrate are correctly included in the options object.

3. Final Answer:

   onBackgroundMessage((payload) => { self.registration.showNotification(payload.notification.title, { body: payload.notification.body, icon: '/icon.png', vibrate: [100, 50, 100] }); }); -> Option A

4. Quick Check:

   Correct method, payload fields, and options = onBackgroundMessage((payload) => { self.registration.showNotification(payload.notification.title, { body: payload.notification.body, icon: '/icon.png', vibrate: [100, 50, 100] }); }); [OK]

Hint: Use self.registration.showNotification with full payload.notification fields [OK]

Common Mistakes:

• Using wrong notification method like self.showNotification
• Accessing payload.title instead of payload.notification.title
• Omitting icon or vibrate options