Android ApiDemos示例解析(19)：App->Alarm->Alarm Controller
Alarm Controller演示如何在Android应用中使用Alarm事件，其功能和java.util.Timer ,TimerTask类似。但Alarm可以即使当前应用退出后也可以做到Schedule一个任务在指定的时刻执行。
AlarmManager 用于管理Alarm事件，支持单次执行或重复执行。 和大都数Android服务一样，AlarmManager也是通过getSystemService来获取服务对象：
[java]
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
与TimerTask对应的任务描述类型为PendingIntent，PendingIntent描述了将要执行的Intent，PendingIntent没有提供构造函数，需要通过static 函数getActivity(Context, int, Intent, int), getBroadcast(Context, int, Intent, int), getService(Context, int, Intent, int) 来或得想要执行的Activity，Broadcast，Service描述。
本例中是取得对Broadcast OneShotAlarm 和RepeatingAlarm的描述，分别对应于单次执行时执行的Broadcast事件和多次执行时Broadcast事件，它们在AndroidManifest.xml定义为Broadcast Receiver：
<receiver android:name=”.app.OneShotAlarm” android:process=”:remote” />
<receiver android:name=”.app.RepeatingAlarm” android:process=”:remote” />
Schedule单次Alarm事件代码如下：
[java]
Intent intent = new Intent(AlarmController.this, OneShotAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0);
// We want the alarm to go off 30 seconds from now.
Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.add(Calendar.SECOND, 30);
// Schedule the alarm!
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.set(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), sender);
Intent intent = new Intent(AlarmController.this, OneShotAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0);
// We want the alarm to go off 30 seconds from now.
Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.add(Calendar.SECOND, 30);
// Schedule the alarm!
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.set(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), sender);
其中sender为对发给Broadcast Receiver OneShotAlarm的Intent的描述，当到达指定的时间（例子中为30秒），AlarmManager将给OneShotAlarm发出一个Broadcast Intent，OneShotAlarm接到后，将使用Toast在屏幕上显示一个消息。 如果你多次点击“One Shot Alarm”并不会Schedule多个Alarm事件，这是因为Schedule同一个Sender对象，后一次将取消上此Scheduled的事件。
Schedule一个重复事件代码如下：
[java]
Intent intent = new Intent(AlarmController.this, RepeatingAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0);
// We want the alarm to go off 30 seconds from now.
long firstTime = SystemClock.elapsedRealtime();
firstTime += 15*1000;
// Schedule the alarm!
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP,
firstTime, 15*1000, sender);
Intent intent = new Intent(AlarmController.this, RepeatingAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0);
// We want the alarm to go off 30 seconds from now.
long firstTime = SystemClock.elapsedRealtime();
firstTime += 15*1000;
// Schedule the alarm!
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP,
firstTime, 15*1000, sender);
上述代码每15秒给RepeatingAlarm 发出一个Broadcast事件，RepeatingAlarm接受到后，也在屏幕上显示一个消息。
对于与Schedule的事件，单次或多次的，都可以调用AlarmManager 的cancel方法取消Schedule的Alarm事件，下面代码取消多次Alarm事件。
[java]
Intent intent = new Intent(AlarmController.this, RepeatingAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0);
// And cancel the alarm.
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.cancel(sender);
Intent intent = new Intent(AlarmController.this, RepeatingAlarm.class);
PendingIntent sender = PendingIntent.getBroadcast(AlarmController.this,
0, intent, 0); www.zzzyk.com
// And cancel the alarm.
AlarmManager am = (AlarmManager)getSystemService(ALARM_SERVICE);
am.cancel(sender);
注：如果你没有Cancel这个多次Alarm事件，每隔15秒屏幕上都会显示一个消息，即使你退出这个例子或是启动其它应用，直到Reboot之后才中止。
作者：mapdigit
补充：移动开发 , Android ,
