AlarmManager.setRepeating将不再准确

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. 按照如下方法写scheduleAlarm()方法：

public class PollReceiver extends BroadcastReceiver {
   private static final int PERIOD = 60000; // 1 minutes

   @Override
   public void onReceive(Context ctxt, Intent i) {
       if (i.getAction().equals("great")) {
           scheduleAlarms(ctxt);
             //Do your work
       }
   }

   static void scheduleAlarms(Context ctxt) {
       AlarmManager mgr = (AlarmManager) ctxt.getSystemService(Context.ALARM_SERVICE);
       Intent i = new Intent(ctxt, PollReceiver.class);
       i.setAction("great");
       PendingIntent pi = PendingIntent.getBroadcast(ctxt, 0, i, 0);.
       mgr.cancel(pi);
       mgr.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime()+PERIOD, pi);

   }

}