I'm wondering is there a way to get all of the messages in Notification Center.
I already know how to post a message, but I don't know whether I can get the messages posted by other Apps into my App for usage.
Yes, you can:
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), nil, { (_, observer, name, _, _) in
print("Notification \(name) received")
}, nil, nil, .deliverImmediately)
But be careful, there are a lot of notifications. :-)
Please consider also this SO question.