Alarmmanager-每分钟的顶部都有火灾警报

Question

I am trying to fire an Alarm on top of every minute. But as for some reason it doesn't want to work. It is for an Widget Clock which should update every minute. For battery reasons i've made an reciever for screen off and on. So my Alarm only fires when the screen is on. The minutes should be syncron with my System clock.

        public static void startClockTickAlarm() {
            AlarmManager alarmManager = (AlarmManager)_context.getSystemService(Context.ALARM_SERVICE);             
            Calendar calendar = Calendar.getInstance();
            calendar.setTimeInMillis(System.currentTimeMillis());
            calendar.add(Calendar.SECOND, 1);       
            calendar.add(Calendar.HOUR, 1);
            alarmManager.setRepeating(AlarmManager.RTC, utcMillisNextMin(), 60000, createClockTickIntent(_context));
        }

        public static final long utcMillisNextMin() {
            Time t = new Time();
            t.setToNow();
            t.second = 0;
            t.minute++;
            System.out.println("Next Alarm: " + t.hour + ":" + t.minute + ":" + t.second);
            return t.normalize(true);
        }

My System.out gives me exactly what it should. For example i enable my widget at 11:30:15 the returning result is 11:31:00 - This means my alarm should firstly fire at 11:31. According to my System Clock, the alarm is 5-15 seconds too late. The seconds which the alarm fires too late are not always the same (between 5 and 15 seconds).

Thanks in advance.

Answer 1

Like @mighter said, API >= 19 if you must use exact timing, use the setExact() API.

if(android.os.Build.VERSION.SDK_INT < 19) {
    alarmManager.setRepeating(AlarmManager.RTC, utcMillisNextMin(), 60000, createClockTickIntent(_context));
} else {
    alarmManager.setExact(...);
}

Answer 2

There's following statement in docs :

Note: as of API 19, all repeating alarms are inexact. If your application needs precise delivery times then it must use one-time exact alarms, rescheduling each time as described above. Legacy applications whose targetSdkVersion is earlier than API 19 will continue to have all of their alarms, including repeating alarms, treated as exact.

If you're running on Android 4.4.4 then this might be the reason.