Android Notifications

Rule 1: Always create channels before posting (Android 8+)

// ✅ Channel setup — do this in Application.onCreate()
class MyApplication : Application() {
    override fun onCreate() {
        super.onCreate()
        createNotificationChannels()
    }

    private fun createNotificationChannels() {
        val channels = listOf(
            NotificationChannelCompat.Builder(
                NotificationChannels.MESSAGES,
                NotificationManagerCompat.IMPORTANCE_HIGH
            ).apply {
                setName("Messages")
                setDescription("Direct messages from other users")
                setVibrationEnabled(true)
                setLightsEnabled(true)
            }.build(),

            NotificationChannelCompat.Builder(
                NotificationChannels.UPDATES,
                NotificationManagerCompat.IMPORTANCE_DEFAULT
            ).apply {
                setName("Updates")
                setDescription("App updates and announcements")
            }.build(),

            NotificationChannelCompat.Builder(
                NotificationChannels.BACKGROUND_SYNC,
                NotificationManagerCompat.IMPORTANCE_MIN
            ).apply {
                setName("Background sync")
                setDescription("Silent background sync status")
            }.build()
        )

        NotificationManagerCompat.from(this).createNotificationChannelsCompat(channels)
    }
}

object NotificationChannels {
    const val MESSAGES = "messages"
    const val UPDATES = "updates"
    const val BACKGROUND_SYNC = "background_sync"
}

Rule 2: NotificationCompat — correct builder pattern

// ✅ Full notification with actions and deep link
class NotificationHelper @Inject constructor(
    @ApplicationContext private val context: Context
) {
    private val notificationManager = NotificationManagerCompat.from(context)

    fun showMessageNotification(
        id: Int,
        senderName: String,
        messagePreview: String,
        conversationId: String
    ) {
        // Deep link PendingIntent
        val intent = Intent(context, MainActivity::class.java).apply {
            flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
            putExtra("conversation_id", conversationId)
        }
        val pendingIntent = PendingIntent.getActivity(
            context, id, intent,
            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
        )

        // Reply action
        val replyInput = RemoteInput.Builder("reply_text")
            .setLabel("Reply...")
            .build()
        val replyIntent = Intent(context, NotificationReceiver::class.java).apply {
            action = "REPLY"
            putExtra("notification_id", id)
            putExtra("conversation_id", conversationId)
        }
        val replyPendingIntent = PendingIntent.getBroadcast(
            context, id, replyIntent,
            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_MUTABLE  // MUTABLE required for RemoteInput
        )
        val replyAction = NotificationCompat.Action.Builder(
            R.drawable.ic_reply, "Reply", replyPendingIntent
        ).addRemoteInput(replyInput).build()

        val notification = NotificationCompat.Builder(context, NotificationChannels.MESSAGES)
            .setSmallIcon(R.drawable.ic_notification)
            .setContentTitle(senderName)
            .setContentText(messagePreview)
            .setStyle(NotificationCompat.BigTextStyle().bigText(messagePreview))
            .setContentIntent(pendingIntent)
            .addAction(replyAction)
            .setAutoCancel(true)
            .setPriority(NotificationCompat.PRIORITY_HIGH)
            .setCategory(NotificationCompat.CATEGORY_MESSAGE)
            .setVisibility(NotificationCompat.VISIBILITY_PRIVATE)  // hide on lock screen
            .build()

        if (ActivityCompat.checkSelfPermission(context, Manifest.permission.POST_NOTIFICATIONS)
            == PackageManager.PERMISSION_GRANTED) {
            notificationManager.notify(id, notification)
        }
    }

    fun cancel(id: Int) = notificationManager.cancel(id)
    fun cancelAll() = notificationManager.cancelAll()
}

Rule 3: Foreground service notification

// ✅ Required notification for foreground services
class DownloadService : Service() {
    private val notificationId = 100

    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        startForeground(notificationId, buildProgressNotification(0))
        return START_STICKY
    }

    fun updateProgress(progress: Int) {
        NotificationManagerCompat.from(this).notify(notificationId, buildProgressNotification(progress))
    }

    private fun buildProgressNotification(progress: Int) =
        NotificationCompat.Builder(this, NotificationChannels.BACKGROUND_SYNC)
            .setSmallIcon(R.drawable.ic_download)
            .setContentTitle("Downloading...")
            .setProgress(100, progress, progress == 0)
            .setOngoing(true)  // can't be dismissed by user
            .setSilent(true)
            .build()
}

Common Mistakes

❌ Not creating channel before posting — notification silently dropped on Android 8+ ❌ FLAG_MUTABLE on PendingIntent without RemoteInput — security vulnerability ❌ FLAG_IMMUTABLE on PendingIntent with RemoteInput — reply won't work ❌ Not checking POST_NOTIFICATIONS permission on Android 13+ ❌ Hardcoded notification IDs — use consistent ID per notification type, not random ❌ No setAutoCancel(true) — notification stays after user taps it