The following type definitions are available globally.
The completion handler invoked once the data connection with FIRMessaging is established. The data connection is used to send a continous stream of data and all the FIRMessaging data notifications arrive through this connection. Once the connection is established we invoke the callback with
nilerror. Correspondingly if we get an error while trying to establish a connection we invoke the handler with an appropriate error object and do an exponential backoff to try and connect again unless successful.
typealias FIRMessagingConnectCompletion = (Error?) -> Void
typedef void (^FIRMessagingConnectCompletion)(NSError *_Nullable)
The error object if any describing why the data connection to FIRMessaging failed.