How do I rerender my view in swiftUI after a user logged in with Google on Firebase?

The firebase and swiftUI combination is kinda tricky at first, but you will figure out that the same pattern is used in every single project, no worries. Just follow my steps and customise on your project, here is our strategy.

- This might be a long answer, but i want to leave it as a refrence to all Firebase-SwiftUI user Managing in Stack OverFlow. -

1. Creating a SessionStore class which provides the BindableObject, and listen to your users Authentification and Handle the Auth and CRUD methods.
2. Creating a Model to our project ( you already did it)
3. Adding Auth methods in SessionStore Class.
4. Listening for changes and putting things together.

Let s start by SessionStore Class: import SwiftUI import Firebase import Combine

class SessionStore : BindableObject {
    var didChange = PassthroughSubject<SessionStore, Never>()
    var session: User? { didSet { self.didChange.send(self) }}
    var handle: AuthStateDidChangeListenerHandle?

    func listen () {
        // monitor authentication changes using firebase
        handle = Auth.auth().addStateDidChangeListener { (auth, user) in
            if let user = user {
                // if we have a user, create a new user model
                print("Got user: \(user)")
                self.session = User(
                    uid: user.uid,
                    displayName: user.displayName
                )
            } else {
                // if we don't have a user, set our session to nil
                self.session = nil
            }
        }
    }

    // additional methods (sign up, sign in) will go here
}

Notice that we’ve declared that our session property is an optional User type, which we haven’t yet defined. Let’s quickly make one:

class User {
    var uid: String
    var email: String?
    var displayName: String?

    init(uid: String, displayName: String?, email: String?) {
        self.uid = uid
        self.email = email
        self.displayName = displayName
    }

}

Now, adding signUp, signIn and signOut methods

class SessionStore : BindableObject {

    // prev code...

    func signUp(
        email: String,
        password: String,
        handler: @escaping AuthDataResultCallback
        ) {
        Auth.auth().createUser(withEmail: email, password: password, completion: handler)
    }

    func signIn(
        email: String,
        password: String,
        handler: @escaping AuthDataResultCallback
        ) {
        Auth.auth().signIn(withEmail: email, password: password, completion: handler)
    }

    func signOut () -> Bool {
        do {
            try Auth.auth().signOut()
            self.session = nil
            return true
        } catch {
            return false
        }
    }
}

Finally, we need a way to stop listening to our authentication change handler.

class SessionStore : BindableObject {

    // prev code...

    func unbind () {
        if let handle = handle {
            Auth.auth().removeStateDidChangeListener(handle)
        }
    }
}

Finally, Making our content view:

import SwiftUI

struct ContentView : View {

  @EnvironmentObject var session: SessionStore
  var body: some View {
    Group { 
     if (session.session != nil) {  
      Text("Hello user!") 
     } else {
        Text("Our authentication screen goes here...")   
    }  
   }
  }
}

@Ghazi Tozri thanks again for your answer, while it wasn't what I wanted to do exactly it pushed me in the right direction. I just want to put here what I finally did so if anyone wants to not use Email / Password sign-in but Google sign-in they can benefit from it too.

I used the Combine Framework + the @Publisher Syntax to make it a bit more readable and I also don't need the Signing in and out Methods because Google Provides them.

The SwiftUI Button for Google sign-in would look something like this:

struct GoogleSignIn : UIViewRepresentable {
    @EnvironmentObject private var userData: SessionStore

    func makeUIView(context: UIViewRepresentableContext<GoogleSignIn>) -> GIDSignInButton {
        let button = GIDSignInButton()
        button.colorScheme = .dark
        GIDSignIn.sharedInstance()?.presentingViewController = UIApplication.shared.windows.last?.rootViewController

        //If you want to restore a session
        //GIDSignIn.sharedInstance()?.restorePreviousSignIn()
        return button
    }

    func updateUIView(_ uiView: GIDSignInButton, context: UIViewRepresentableContext<GoogleSignIn>) {
    }
}

And the Used SessionStore like this:

import SwiftUI
import Combine
import Firebase
import GoogleSignIn

final class SessionStore: ObservableObject {
    @Published var showFavoritesOnly = false
    @Published var qrTags = qrTagData
    @Published var session: User?
    var handle: AuthStateDidChangeListenerHandle?

    func listen() {
        handle =  Auth.auth().addStateDidChangeListener { (auth, user) in
            if let user = user {
                self.session = user
            } else {
                self.session = nil
            }
        }
    }
}

In a view that checks for the authentication state I use the .onAppear() function like this:

struct UserProfile: View {
    @EnvironmentObject private var session: SessionStore

    var body: some View {
        VStack {
            if session.session != nil {
                SignOutButton()
            } else {
                SignInView()
            }
        }.onAppear(perform: {self.session.listen()}) 
    }
}

To sign out this function will do (from the Firebase Docs):

func signOut() -> Void {
        let firebaseAuth = Auth.auth()
        do {
            try firebaseAuth.signOut()
        }
        catch let signOutError as NSError {
            print ("Error signing out: %@", signOutError)
        }
    }

Hope this can help somebody else too.