【起航计划 017】2015 起航计划 Android APIDemo的魔鬼步伐 16 App->Alarm->Alarm Controller Alarm事件 PendingIntent Schedule AlarmManager

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：

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：

            // When the alarm goes off, we want to broadcast an Intent to our
            // BroadcastReceiver.  Here we make an Intent with an explicit class
            // name to have our own receiver (which has been published in
            // AndroidManifest.xml) instantiated and called, and then create an
            // IntentSender to have the intent executed as a broadcast.
            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：

            // When the alarm goes off, we want to broadcast an Intent to our
            // BroadcastReceiver.  Here we make an Intent with an explicit class
            // name to have our own receiver (which has been published in
            // AndroidManifest.xml) instantiated and called, and then create an
            // IntentSender to have the intent executed as a broadcast.
            // Note that unlike above, this IntentSender is configured to
            // allow itself to be sent multiple times.
            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。

            // Create the same intent, and thus a matching IntentSender, for
            // the one that was scheduled.
            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);

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。