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

及之后的版本号採用非精准闹钟机制。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()

部分解决问题，但依旧不能在休眠状态下精准唤醒。

关于

alarm api

的支持与使用请參考下图：

(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版本号为例，须要改动内核下的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

  ，而且须要使用带有

  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：须要将

  wakeup

  类型的

  alarm

  特殊处理：即精准闹铃。在

  setImpl

  中加入例如以下代码：

  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);
  }
  }

  对应的

  IntentService

  例如以下所看到的：

  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);
  }
  }