Android4.4及之后休眠状态下Alarm不准时的问题

为了减轻功耗，延长电池使用时间，Android 4.4及之后的版本采用非精准闹钟机制，以及休眠状态下的wakeup类型的alarm不会实时唤醒设备，而会等到机器被物理唤醒时才触发alarm。Android 6.0提供了新的api：setExactAndAllowWhileIdle()部分解决这个问题，但依然不能在休眠状态下精准唤醒。

关于alarm api 的支持与使用请参考下图：

(图片来源：https://plus.google.com/+AndroidDevelopers/posts/GdNrQciPwqo)

此外，应用层面不要使用不持有wakelock的BroadcastReceiver，而要使用WakefulBroadcastReceiver。

为了修复这个问题，以5.0.2版本为例，需要修改内核下的alarm-dev.c以及framework下的AlarmManagerService。

• 内核 alarm-dev.c：其原因是使用普通的static struct wakeup_source alarm_wake_lock;，而非带有WAKE_LOCK_SUSPEND类别信息的struct wake_lock，并且需要使用带有android_前缀的wakeup lock相关函数。即：

  android_wake_lock_init
  android_wake_lock_destroy
  android_wake_lock
  android_wake_lock_timeout
  android_wake_unlock

  而非普通的wake lock操作函数：

  wake_lock_init
  wake_lock_destroy
  wake_lock
  wake_lock_timeout
  wake_unlock

• framework AlarmManagerService.java：需要将wakeup类型的alarm特殊处理：即精准闹铃。在setImpl中添加如下代码：

  public boolean isWakeup(int type)
  {
      return (type & TYPE_NONWAKEUP_MASK) == 0;
  }
  
  void setImpl(int type, long triggerAtTime, long windowLength, long interval,
          PendingIntent operation, boolean isStandalone, WorkSource workSource,
          AlarmManager.AlarmClockInfo alarmClock) {
      if (operation == null) {
          Slog.w(TAG, "set/setRepeating ignored because there is no intent");
          return;
      }
  
      if (isWakeup(type)) {
          windowLength = AlarmManager.WINDOW_EXACT;
          isStandalone = true;
      }

  并在alarm被触发时多取几个满足条件的batch做处理：

  boolean triggerAlarmsLocked(ArrayList<Alarm> triggerList, final long nowELAPSED,
          final long nowRTC) {
      boolean hasWakeup = false;
      // batches are temporally sorted, so we need only pull from the
      // start of the list until we either empty it or hit a batch
      // that is not yet deliverable
  
      ArrayList<Alarm> repeatList = new ArrayList<Alarm>();
      ListIterator<Batch> it = mAlarmBatches.listIterator();
      while (it.hasNext()) {
          Batch batch = it.next();
          if (batch.start > nowELAPSED) {
              // Everything else is scheduled for the future
              break;
          }
  
          // We will (re)schedule some alarms now; don't let that interfere
          // with delivery of this current batch
          it.remove();
  
          final int N = batch.size();
          for (int i = 0; i < N; i++) {
              Alarm alarm = batch.get(i);
              alarm.count = 1;
              triggerList.add(alarm);
  
              // Recurring alarms may have passed several alarm intervals while the
              // phone was asleep or off, so pass a trigger count when sending them.
              if (alarm.repeatInterval > 0) {
                  // this adjustment will be zero if we're late by
                  // less than one full repeat interval
                  alarm.count += (nowELAPSED - alarm.whenElapsed) / alarm.repeatInterval;
  
                  // Also schedule its next recurrence
                  repeatList.add(alarm);
              }
  
              if (alarm.wakeup) {
                  hasWakeup = true;
                  mNextWakeup = 0;
              }
  
              // We removed an alarm clock. Let the caller recompute the next alarm clock.
              if (alarm.alarmClock != null) {
                  mNextAlarmClockMayChange = true;
              }
          }
      }
  
      if (repeatList.size() > 0) {
          for (Alarm alarm : repeatList) {
              final long delta = alarm.count * alarm.repeatInterval;
              final long nextElapsed = alarm.whenElapsed + delta;
              setImplLocked(alarm.type, alarm.when + delta, nextElapsed, alarm.windowLength,
                      maxTriggerTime(nowELAPSED, nextElapsed, alarm.repeatInterval),
                      alarm.repeatInterval, alarm.operation, alarm.windowLength == AlarmManager.WINDOW_EXACT, true,
                      alarm.workSource, alarm.alarmClock, alarm.userId);
          }
      }
  
      // This is a new alarm delivery set; bump the sequence number to indicate that
      // all apps' alarm delivery classes should be recalculated.
      mCurrentSeq++;
      calculateDeliveryPriorities(triggerList);
      Collections.sort(triggerList, mAlarmDispatchComparator);
  
      return hasWakeup;
  }

• 应用层面使用WakefulBroadcastReceiver:

  import android.support.v4.content.WakefulBroadcastReceiver;
  
  public class AutoUpdateAlarmReceiver extends WakefulBroadcastReceiver {
  
      @Override
      public void onReceive(Context context, Intent intent) {
          // Start the service, keeping the device awake while the service is
          // launching. This is the Intent to deliver to the service.
          Intent service = new Intent(context, AutoUpdateIntentService.class);
          service.setAction(intent.getAction());
          startWakefulService(context, service);
      }
  }

  相应的IntentService如下所示：

  public class AutoUpdateIntentService extends IntentService {
      public AutoUpdateIntentService() {
          super("AutoUpdateIntentService");
      }
  
      @Override
      protected void onHandleIntent(Intent intent) {
          String action = intent.getAction();
  
          // do your work here.
          // ...
  
          // Release the wake lock provided by the WakefulBroadcastReceiver.
          AutoUpdateAlarmReceiver.completeWakefulIntent(intent);
      }
  }