AlarmManager.setRepeating不准确的问题，被setWindow()或者setExact()替代

背景：

当我们想让Android应用程序定时为做一件工作时，我们往往会在一个BroadcastReceiver中使用AlarmManager.setRepeating（）方法来实现。在API 19（即Kitkat）之后，这一方法将不再准确地保证每次工作都在你设置的时间开始。

 

解释：

Note: Beginning in API 19, the trigger time passed to this method is treated as inexact: the alarm will not be delivered before this time, but may be deferred and delivered some time later. The OS will use this policy in order to "batch" alarms together across the entire system, minimizing the number of times the device needs to "wake up" and minimizing battery use. In general, alarms scheduled in the near future will not be deferred as long as alarms scheduled far in the future.

With the new batching policy, delivery ordering guarantees are not as strong as they were previously. If the application sets multiple alarms, it is possible that these alarms' actual delivery ordering may not match the order of their requesteddelivery times. If your application has strong ordering requirements there are other APIs that you can use to get the necessary behavior; see setWindow(int, long, long, PendingIntent) and setExact(int, long, PendingIntent).

原来，操作系统为了节能省电，将会调整alarm唤醒的时间。故通过AlarmManager.setRepeating()方法将不再保证你定义的工作能按时开始。

 

解决办法：

1. 按照官方网站说的，调用API 19之后出现的setWindow()或者setExact()方法。

2、先设置一次性的闹钟，收到闹钟广播的提醒之后，再设置下一次的；（以下是部分关键代码，仅供参考）

  public static void setAlarmTime(Context context, long timeInMillis, Intent intent) {
        AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
        PendingIntent sender = PendingIntent.getBroadcast(context, intent.getIntExtra("id", 0),
                intent, PendingIntent.FLAG_UPDATE_CURRENT);
        int interval = (int) intent.getLongExtra("intervalMillis", 0);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            am.setExact(AlarmManager.RTC_WAKEUP, timeInMillis, sender);
        }
    }

public class AlarmReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        String msg = intent.getStringExtra("msg");
        int id = intent.getIntExtra("id", -1);

        long intervalMillis = intent.getLongExtra("intervalMillis", 0);
        if (intervalMillis != 0) {
            AlarmManagerUtil.setAlarmTime(context, System.currentTimeMillis() + intervalMillis,
                    intent);
        }
    }
}

/**
 * @param flag            周期性时间间隔的标志,flag = 0 表示一次性的闹钟, flag = 1 表示每天提醒的闹钟(1天的时间间隔),flag = 2
 *                        表示按周每周提醒的闹钟（一周的周期性时间间隔）
 * @param hour            时
 * @param minute          分
 * @param id              闹钟的id
 * @param week            week=0表示一次性闹钟或者按天的周期性闹钟，非0 的情况下是几就代表以周为周期性的周几的闹钟
 * @param tips            闹钟提示信息
 * @param soundOrVibrator 2表示声音和震动都执行，1表示只有铃声提醒，0表示只有震动提醒
 */   
 public static void setAlarm(Context context, int flag, int hour, int minute, int id, int
            week, String tips, int soundOrVibrator) {
        AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
        Calendar calendar = Calendar.getInstance();
        long intervalMillis = 0;
        calendar.set(calendar.get(Calendar.YEAR), calendar.get(Calendar.MONTH), calendar.get
                (Calendar.DAY_OF_MONTH), hour, minute, 3);
        if (flag == 0) {
            intervalMillis = 0;
        } else if (flag == 1) {
            intervalMillis = 24 * 3600 * 1000;
        } else if (flag == 2) {
            intervalMillis = 24 * 3600 * 1000 * 7;
        }
        Intent intent = new Intent(ALARM_ACTION);
        intent.putExtra("intervalMillis", intervalMillis);
        intent.putExtra("msg", tips);
        intent.putExtra("hour", hour);
        intent.putExtra("minute", minute);
        intent.putExtra("id", id);
        intent.putExtra("soundOrVibrator", soundOrVibrator);
        PendingIntent sender = PendingIntent.getBroadcast(context, id, intent, PendingIntent
                .FLAG_IMMUTABLE);

        LogUtils.d("utils", "clock里的week：" + week);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            am.setExact(AlarmManager.RTC_WAKEUP, calMethod(week, calendar.getTimeInMillis()), sender);
        } else {
            if (flag == 0) {
                am.set(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), sender);
            } else {
                am.setRepeating(AlarmManager.RTC_WAKEUP, calMethod(week, calendar.getTimeInMillis
                        ()), intervalMillis, sender);
            }
        }
    }

public static void cancelAlarm(Context context, String action, int id) {
    Intent intent = new Intent(action);
    PendingIntent pi = PendingIntent.getBroadcast(context, id, intent, PendingIntent
            .FLAG_UPDATE_CURRENT);
    AlarmManager am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
    am.cancel(pi);
}