Currently, I'm wondering if it's possible to create a chat-like mechanism (server story) with an application that I'm developing personally and deliver push notifications to specified users and topics when data is added to the Firebase Database. I tried it.
The original story is Send push notifications from iOS client in Firebase.
I did it in Python because Node.js can't (deploy implementation).
--Python 3.5.2 (I forgot to put 3.6)
AppDelegate.swift
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    let settings = UIUserNotificationSettings(types: [.alert, .badge, .sound],
                                                  categories: nil)
    application.registerUserNotificationSettings(settings)
    application.registerForRemoteNotifications()
}
func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    //APNS registration
    #if DEBUG
    FIRInstanceID.instanceID().setAPNSToken(deviceToken, type: .sandbox)
    #else
    FIRInstanceID.instanceID().setAPNSToken(deviceToken, type: .prod)
    #endif
}
func application(_ application: UIApplication, didRegister notificationSettings: UIUserNotificationSettings) {
    //Sign up for a topic
    FIRMessaging.messaging().subscribe(toTopic: "/topics/news")
}
Do a hoge for later push notifications!
Hoge.swift
class Hoge: NSObject {
    class func create() {
        let ref = FIRDatabase.database().reference()
        let data: [String : String] = ["title": "title", "body": "Text"]
        ref("message").childByAutoId().setValue(data, withCompletionBlock: { (error, reference) in
            if error != nil {
                print("Success")
            } else {
                print("Failure")
            }
        }
    }
}
THE miscellaneous, but it's like this
requirements.txt
docutils==0.13.1
gcloud==0.17.0
googleapis-common-protos==1.5.0
httplib2==0.9.2
jws==0.1.3
lockfile==0.12.2
oauth2client==3.0.0
protobuf==3.2.0rc1
pyasn1==0.1.9
pyasn1-modules==0.0.8
pycryptodome==3.4.3
Pyrebase==3.0.27
python-daemon==2.1.2
python-jwt==2.0.1
requests==2.11.1
requests-toolbelt==0.7.0
rsa==3.4.2
six==1.10.0
app.py
import daemon
import json
import pyrebase
import requests
config = {
    # GoogleService-info.plist API_KEY
    "apiKey": "API_KEY",
    "authDomain": "PROJECT_NAME",
    "databaseURL": "https://PROJECT_NAME.firebaseio.com/",
    #May not be needed
    "storageBucket": "gs://PROJECT_NAME.appspot.com"
}
fcm_header = {
    'Content-type': 'application/json; charset=UTF-8',
    #See the image below this code!
    'Authorization': 'key=API_KEY'
}
firebase = pyrebase.initialize_app(config)
def stream_handler(message):
    if message['data'] == None:
        return
    if message['path'] == '/':
        return
    if 'title' in message['data']:
        title = message['data']['title']
    else:
        title = 'Delivery'
    if 'body' in message['data']:
        body = message['data']['body']
    else:
        body = ''
    payload = {'to': '/topics/news', 
               'priority': 'high', 
               'notification': {
                   'title': title,
                   'body': body,
                   'badge': 1,
                   'sound': 'default'
               }
              }
    r = requests.post('https://fcm.googleapis.com/fcm/send', headers = fcm_header, data = json.dumps(payload))
    print(r.json())
if __name__ == '__main__':
    db = firebase.database()
    dc = daemon.DaemonContext()
    with dc:
        stream = db.child("message").stream(stream_handler)

(Please protect your privacy !!)
The Python package uses Pyrebase and python-daemon.
$ pip install pyrebase python-daemon
I'm going to look at the / message directory in the Firebase Database.
if message['data'] == None:
    return
if message['path'] == '/':
    return
This is an exception handling because an error occurred (?) When there was no data in / message. Even though a push notification is sent when the monitoring code is started, the notification is coming to the iOS side, but the alert is not displayed! I put it in because there was something like that.
All you have to do is run the script on the server side.
By the way, using python-daemon will daemonize the script! Thank you very much!
$ python app.py

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