用alarmmanager 多次发送PendingIntent

遇到如下问题

service中得一随机数 用alarmmanager 发送PendingIntent的时候，receiver收到的随机数不变。

pendingintent传值经常获取到的值是第一次的值或者null，这个跟第二个参数和最后一个参数选择有关系.
将最后一个参数设置成

PendingIntent.FLAG_UPDATE_CURRENT 

即可解决问题。

总结一下pendingIntent的常用FLAG标签：

FLAG_ONE_SHOT:this PendingIntent can only be used once. If set, after send() is called on it, it will be automatically canceled for you and any future attempt to send through it will fail.

FLAG_NO_CREATE:if the described PendingIntent does not already exist, then simply return null instead of creating it.

FLAG_CANCEL_CURRENT:if the described PendingIntent already exists, the current one is canceled before generating a new one. You can use this to retrieve a new PendingIntent when you are only changing the extra data in the Intent; by >canceling the previous pending intent, this ensures that only entities given the new data will be able to launch it. If this assurance is not an issue, consider FLAG_UPDATE_CURRENT.

FLAG_UPDATE_CURRENT: if the described PendingIntent already exists, then keep it but its replace its extra data with what is in this new Intent. This can be used if you are creating intents where only the extras change, and don't care >that any entities that received your previous PendingIntent will be able to launch it with your new extras even if they are not explicitly given to it.

上 面4个flag中最经常使用的是FLAG_UPDATE_CURRENT，因为描述的Intent有更新的时候需要用到这个flag去更新你的描述，否则 组件在下次事件发生或时间到达的时候extras永远是第一次Intent的extras。使用FLAG_CANCEL_CURRENT也能做到更新 extras，只不过是先把前面的extras清除，另外FLAG_CANCEL_CURRENT和FLAG_UPDATE_CURRENT的区别在于能 否新new一个Intent，FLAG_UPDATE_CURRENT能够新new一个Intent，而FLAG_CANCEL_CURRENT则不能， 只能使用第一次的Intent。

另外两flag就比较少用，利用FLAG_ONE_SHOT获取的PendingIntent只能使用一次，再使用PendingIntent也将失败，利用FLAG_NO_CREAT获取的PendingIntent若描述的Intent不存在则返回NULL值.

代码
注意：此代码仅仅实现延时操作，若需要循环发送，需要在receiver将service启动。

 @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // TODO Auto-generated method stub
        AlarmManager alarmManager = (AlarmManager)getSystemService(ALARM_SERVICE);
        int random;
        // get a random number
        random = (int)(Math.random() * 10 + 1);

        long triggerTime = SystemClock.elapsedRealtime() + 5 * 1000;

        // get pid and tid
        int pid = Process.myPid();
        long tid = Process.myTid();

        Intent i = new Intent(SEND_ACTION);
        i.putExtra(RANDOM_NUM, random);
        i.putExtra(SERVICE_PID, pid);
        i.putExtra(SERVICE_TID, tid);

        // get a pending intent which can send braodcast
        PendingIntent pi = PendingIntent
                .getBroadcast(this, 0, i, PendingIntent.FLAG_CANCEL_CURRENT);
        // 5seconds later send the pendingintent
        alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, triggerTime, pi);

        return super.onStartCommand(intent, flags, startId);
    }