Note: This article was completed by AI (Claude Opus 4.7) from my initial notes and thoughts.
Implementing a clock alarm app on Android these days involves quite a few pain points. Battery optimizations, background restrictions, and permission requirements have all grown stricter over the years. This post walks through how I built one and the tricks needed to make the alarm fire reliably and show up on screen.
Regular AlarmManager.set() is inexact — Android may delay it to batch wake-ups. For a clock alarm that must fire at the right moment, use setExact() (or setExactAndAllowWhileIdle() to also fire in Doze mode).
fun scheduleAlarm(context: Context, alarm: Alarm) {
val alarmManager = context.getSystemService(Context.ALARM_SERVICE) as AlarmManager
val intent = Intent(context, AlarmReceiver::class.java).apply {
action = alarm.id.toString() // use id as action for easy cancellation later
}
val pendingIntent = PendingIntent.getBroadcast(
context,
0,
intent,
PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
)
alarmManager.setExactAndAllowWhileIdle(
AlarmManager.RTC_WAKEUP,
alarm.triggerAtMillis,
pendingIntent
)
}
On Android 12 (API 31) and above, the app must hold the SCHEDULE_EXACT_ALARM permission (or USE_EXACT_ALARM for always-allowed cases like clock apps). Declare it in AndroidManifest.xml:
<uses-permission android:name="android.permission.SCHEDULE_EXACT_ALARM" />
SCHEDULE_EXACT_ALARM can be revoked by the user in system settings, so check before scheduling:
if (alarmManager.canScheduleExactAlarms()) {
scheduleAlarm(context, alarm)
} else {
val intent = Intent(Settings.ACTION_REQUEST_SCHEDULE_EXACT_ALARM)
startActivity(intent)
}
When the alarm fires, we handle it in a BroadcastReceiver and launch the alarm screen from there.
class AlarmReceiver : BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent) {
val launchIntent = Intent(context, AlarmActivity::class.java).apply {
flags = Intent.FLAG_ACTIVITY_NEW_TASK
}
context.startActivity(launchIntent)
}
}
Register the receiver in the manifest. From Android 12 onward android:exported must be declared explicitly:
<receiver
android:name=".AlarmReceiver"
android:exported="true" />
Starting from Android 10, apps cannot start activities freely from the background. Simply calling startActivity() from AlarmReceiver will be silently ignored on a locked device. The correct approach is to post a high-priority notification with a full-screen intent, and let the system decide whether to show it as a heads-up notification or launch the activity directly (e.g. when the screen is off).
First, declare the permission in the manifest:
<uses-permission android:name="android.permission.USE_FULL_SCREEN_INTENT" />
Then in AlarmReceiver, post the notification instead of calling startActivity() directly:
class AlarmReceiver : BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent) {
val fullScreenIntent = Intent(context, AlarmActivity::class.java).apply {
flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP
}
val fullScreenPendingIntent = PendingIntent.getActivity(
context,
0,
fullScreenIntent,
PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
)
val notification = NotificationCompat.Builder(context, ALARM_CHANNEL_ID)
.setSmallIcon(R.drawable.ic_alarm)
.setContentTitle("Alarm")
.setContentText("Time to wake up!")
.setPriority(NotificationCompat.PRIORITY_MAX)
.setCategory(NotificationCompat.CATEGORY_ALARM)
.setFullScreenIntent(fullScreenPendingIntent, true)
.setAutoCancel(true)
.build()
NotificationManagerCompat.from(context).notify(ALARM_NOTIFICATION_ID, notification)
}
}
On the AlarmActivity side, add these window flags so it appears over the lock screen even when the screen was off:
class AlarmActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {
setShowWhenLocked(true)
setTurnScreenOn(true)
} else {
@Suppress("DEPRECATION")
window.addFlags(
WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED or
WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON
)
}
setContentView(R.layout.activity_alarm)
}
}
SYSTEM_ALERT_WINDOW is sometimes suggested for this use case but it is intended for drawing overlays (like chat bubbles), not launching activities. Google Play also restricts it to specific app categories, so setFullScreenIntent is the right tool here.
PendingIntent cancellation is usually tricky because you need to keep the original request code around. There is a cleaner trick: set the alarm's own id as the Intent action when scheduling. Since Android matches PendingIntents by action (among other things), you can reconstruct the exact same PendingIntent at cancel time — no request code bookkeeping needed.
fun cancelAlarm(context: Context, alarm: Alarm) {
val alarmManager = context.getSystemService(Context.ALARM_SERVICE) as AlarmManager
val intent = Intent(context, AlarmReceiver::class.java).apply {
action = alarm.id.toString()
}
val pendingIntent = PendingIntent.getBroadcast(
context,
0,
intent,
PendingIntent.FLAG_NO_CREATE or PendingIntent.FLAG_IMMUTABLE
) ?: return // already cancelled or never scheduled
alarmManager.cancel(pendingIntent)
pendingIntent.cancel()
}
Using FLAG_NO_CREATE ensures we get null back (and bail early) if the alarm was never set, rather than creating a new PendingIntent by accident.
setExactAndAllowWhileIdle() so the alarm fires even in Doze mode.canScheduleExactAlarms() and request SCHEDULE_EXACT_ALARM if needed.BroadcastReceiver and post a high-priority notification with setFullScreenIntent().FLAG_SHOW_WHEN_LOCKED and FLAG_TURN_SCREEN_ON on the activity to appear over the lock screen.Intent action to make cancellation straightforward without extra bookkeeping.