问题
With the change of GCM to FCM through Google, we are looking to bring our push notifications for iOS into FCM as well. Without providing an update, which would register each user to the Firebase push service, is it possible to subscribe all current users tokens to the firebase service, therefore generating FCM keys for them?
回答1:
Yes, you can use the batchImport method of the InstanceID API to import a list of APNs tokens which will return a mapping of InstanceID tokens that you can then use to send messages to iOS app instances using FCM.
来源：https://stackoverflow.com/questions/37464706/firebase-fcm-get-tokens-for-old-ios-apns-tokens