关闭应用程序后如何发送通知?

我希望每当从我的Firebase数据库添加一个孩子时都发生通知.在应用运行或在后台运行时，我在onChildAdded()中运行以下代码，但显然在关闭时却没有.我读了一些有关启动服务的信息，但有点不知所措.

I want a notification to happen whenever a child is added from my firebase database. I have the below code working within onChildAdded() while the app is running or in the background, but obviously not when it's closed. I read something about starting a service but was a bit overwhelmed.

NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(mRecyclerView.getContext())
                        .setSmallIcon(R.drawable.icon)
                        .setContentTitle("Title")
                        .setContentText("Content Text");

                Intent newAnimalIntent = new Intent(mRecyclerView.getContext(), MainActivity.class);

                TaskStackBuilder stackBuilder = TaskStackBuilder.create(mRecyclerView.getContext());
                stackBuilder.addNextIntent(newAnimalIntent);
                PendingIntent newAnimalPendingIntent = stackBuilder.getPendingIntent(
                        0,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );
                mBuilder.setContentIntent(newAnimalPendingIntent);
                NotificationManager NM = (NotificationManager) getActivity().getSystemService(getActivity().NOTIFICATION_SERVICE);
                NM.notify(0,mBuilder.build());