Google Sign In with Firebase in SwiftUI app

While working on my latest iOS app, I had quite a few difficulties finding an up-to-date guide on how to correctly set up Firebase Authentication in Swift 5. I decided to write my first blog post in an attempt to fill this gap.

In this guide, I will show you how to build a simple app that lets users sign in using Google OAuth2.

Setting up Firebase

From the Firebase console create a new project, then head to “Authentication” and click on Get started.

Then navigate to Sign-in method and enable the Google authentication flow.

Setting up the SwiftUI app

Add the app to the Firebase project by clicking the button with the iOS logo and compiling the form.

When prompted, download the GoogleService-Info.plist and add it to the root of your Xcode project.

Now search for the Google Sign in package by typing https://github.com/google/GoogleSignIn-iOS and add the “GoogleSignIn” package product to your app.

We have now finished setting up the project, let’s move on to the coding part…

In the ExampleApp.swift file we need to initialize Firebase and we have to handle the URL that your application will receive at the end of the Google authentication process.

import SwiftUI  
import Firebase  
import GoogleSignIn  
  
@main  
struct ExampleApp: App {  
    init() {  
        // Firebase initialization  
        FirebaseApp.configure()  
    }  
  
    var body: some Scene {  
        WindowGroup {  
            ContentView().onOpenURL { url in  
                //Handle Google Oauth URL  
                GIDSignIn.sharedInstance.handle(url)  
            }  
        }  
    }  
}

In the ContentView.swift file we need to check if the user is already logged in, and we will to listen for changes in the login status.

import SwiftUI  
import Firebase  
  
struct ContentView: View {  
    @State private var userLoggedIn = (Auth.auth().currentUser != nil)  
  
    var body: some View {  
        VStack {  
            if userLoggedIn {  
                Home()  
            } else {  
                Login()  
            }  
        }.onAppear{  
            //Firebase state change listeneer  
            Auth.auth().addStateDidChangeListener{ auth, user in  
                if (user != nil) {  
                    userLoggedIn = true  
                } else {  
                    userLoggedIn = false  
                }  
            }  
        }  
    }  
}

Let’s now build an Authentication class in Authentication.swift

import Foundation  
import Firebase  
import GoogleSignIn  
  
struct Authentication {  
    func googleOauth() async throws {  
        // google sign in  
        guard let clientID = FirebaseApp.app()?.options.clientID else {  
            fatalError("no firbase clientID found")  
        }  
  
        // Create Google Sign In configuration object.  
        let config = GIDConfiguration(clientID: clientID)  
        GIDSignIn.sharedInstance.configuration = config  
          
        //get rootView  
        let scene = await UIApplication.shared.connectedScenes.first as? UIWindowScene  
        guard let rootViewController = await scene?.windows.first?.rootViewController  
        else {  
            fatalError("There is no root view controller!")  
        }  
          
        //google sign in authentication response  
        let result = try await GIDSignIn.sharedInstance.signIn(  
            withPresenting: rootViewController  
        )  
        let user = result.user  
        guard let idToken = user.idToken?.tokenString else {  
            throw "Unexpected error occurred, please retry"  
        }  
          
        //Firebase auth  
        let credential = GoogleAuthProvider.credential(  
            withIDToken: idToken, accessToken: user.accessToken.tokenString  
        )  
        try await Auth.auth().signIn(with: credential)  
    }  
      
    func logout() async throws {  
        GIDSignIn.sharedInstance.signOut()  
        try Auth.auth().signOut()  
    }  
}  
  
  
extension String: Error {}

Finally we’ll build a simple Login screen in Login.swift

import SwiftUI  
  
struct Login: View {  
    @State private var err : String = ""  
      
    var body: some View {  
        Text("Login")  
        Button{  
            Task {  
                do {  
                    try await Authentication().googleOauth()  
                } catch let e {  
                    print(e)  
                    err = e.localizedDescription  
                }  
            }  
        }label: {  
            HStack {  
                Image(systemName: "person.badge.key.fill")  
                Text("Sign in with Google")  
            }.padding(8)  
        }.buttonStyle(.borderedProminent)  
          
        Text(err).foregroundColor(.red).font(.caption)  
    }  
}

As well as a basic home page in Home.swift

import SwiftUI  
import Firebase  
  
struct Home: View {  
    @State private var err : String = ""  
      
    var body: some View {  
        HStack {  
            Image(systemName: "hand.wave.fill")  
            Text(  
                "Hello " +  
                (Auth.auth().currentUser!.displayName ?? "Username not found")  
            )  
        }  
        Button{  
            Task {  
                do {  
                    try await Authentication().logout()  
                } catch let e {  
                    err = e.localizedDescription  
                }  
            }  
        }label: {  
            Text("Log Out").padding(8)  
        }.buttonStyle(.borderedProminent)  
          
        Text(err).foregroundColor(.red).font(.caption)  
    }  
}

GitHub

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