在Android中重用 - 为什么我不能设置重复警报并使用相同的PendingIntent实例取消它？

Question

将activity_main.xml中的onClick绑定到setAlarm为一个按钮并将unsetAlarm绑定到同一活动中的另一个按钮时，单击链接到unsetAlarm方法的按钮时，以下代码将不允许您取消设置警报。

...package name and includes ommitted...

public class MainActivity extends Activity {

    private AlarmManager alarmManager;
    private PendingIntent notifyIntent;
    private static final String TAG = "MainActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
    }


    public void setAlarm(View v) {
        Intent myIntent = new Intent(MainActivity.this,
                NotificationService.class);

        notifyIntent = PendingIntent.getService(MainActivity.this, 0,
                myIntent, 0);
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.MINUTE, 1);
        Log.v(TAG, "time for alarm trigger:" + calendar.getTime().toString());
        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP,
                calendar.getTimeInMillis(), 1 * 60 * 1000, notifyIntent);
    }

    public void unsetAlarm(View v) {
        alarmManager.cancel(notifyIntent);
        Log.v(TAG, "cancelling notification");
    }

}

解决方法是（正如我在Notifications and AlarmManager - cancelling the alarm I set中提供的那样）在unsetAlarm方法中重新创建pendingIntent：

  public void unsetAlarm(View v) {
     Intent myIntent = new Intent(MainActivity.this,
            NotificationService.class);
    notifyIntent = PendingIntent.getService(MainActivity.this, 0,
            myIntent, 0);  // recreate it here before calling cancel

    alarmManager.cancel(notifyIntent);
    Log.v(TAG, "cancelling notification");
} 

我的问题是：为什么我不能重复使用PendingIntent，存储在第一个代码片段中的“notifyIntent”字段中？为什么我必须重新创建它才能取消它？我已经将MainActivity设置为具有属性 android：launchMode =“singleInstance”所以当我点击NotificationService中创建的通知时我应该相信它使用相同的实例（我已经省略了它但是它只是显示通知）。