我正在使用 firebase 创建一个 Ionic 2 应用程序，我需要一种方法来在应用程序关闭时(即在前台、后台和被终止)收听数据库更改(特别是在 child_add 上)
I am creating an Ionic 2 app with firebase and I need a way to listen to database changes (specifically on child_added) when the app is closed (I.e. in foreground,background and killed)
基本上，我想使用 WebRTC 在应用程序中拨打电话，例如 whatsapp，我正在关注这篇文章 - https://websitebeaver.com/insanely-simple-webrtc-video-chat-using-firebase-with-codepen-demo
Basically, I want to use WebRTC to make calls within the app like whatsapp and I am following this post - https://websitebeaver.com/insanely-simple-webrtc-video-chat-using-firebase-with-codepen-demo
但是，唯一让我感到困惑的是，当应用程序关闭时它会如何工作.谁能帮我理解一下?
However, the only thing that puzzles me is how it will work when the app is closed. Can anyone please help me understand?
谢谢！
使用 Firebase 客户端 SDK 来主动监听数据库更改是不可能的，就像在应用代码运行时一样.
It's not possible to actively listen to database changes using the Firebase client SDK in exactly the same way that you can when your app's code is running.
如果您希望您的应用接收有关数据库更改的信息，您可以改用 Firebase Cloud Messaging 向您的应用发送通知，其中包含有关更改的信息的小负载.当您的应用收到通知时，它可以决定要做什么.网络支持有一些限制，所以请务必阅读那个.
If you want your app to receive information about changes to your database, you can instead use Firebase Cloud Messaging to send your app a notification with a small payload that contains information about the change. When your app receives the notification, it can then make a decision about what to do. There are some limitations with web support, so be sure to read about that.
还可以查看 Cloud Functions for Firebase，以便更轻松地编写一些服务器端可以触发以响应数据库更改并在这些更改发生时发送通知的代码.
Also look into Cloud Functions for Firebase to make it easier to write some server side code that can trigger in response to a database change and send a notification when those changes happen.
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