I have a question about the push notification. When I install the app in android and don’t open it. During the app installation, is the push notification service auto started? or Ionic 2 requires that the user open the app first, then the push notification service should be started.
No, the app has to actually be opened for any code of the app being run. Only then will it register for push and get a push ID. In general the “installation” of an app is a thing you don’t know about at all, it is only the “first start” you can do anything - like registering for Push.
So if someone downloads your app and never opens it, you have no way to contact them.
(The “notification service” is a OS thing unrelated to the app. It always runs.)