Firebase+Extension.swift

import Firebase
import FirebaseAuth

extension FirebaseApp {
    private static var didConfigForAppExtension = false

    static func configureFirebaseForAppExtension(
        extensionPlistName: String,
        accessGroup: String,
        force: Bool = false
    ) {
        func configure() {
            // Shared firebase auth state for extension
            copyContainerFirebaseData()

            configureFirebase(plistName: extensionPlistName, accessGroup: accessGroup)
        }
        if !force {
            guard !didConfigForAppExtension else {
                return
            }
            configure()
            
            didConfigForAppExtension = true
        } else {
            configure()
        }
    }

    static func configureFirebase(plistName: String, accessGroup: String) {
        func configure(plistName: String) {
            guard
                let configURL = Bundle.main.url(forResource: plistName, withExtension: "plist"),
                let options = FirebaseOptions(contentsOfFile: configURL.path)
            else {
                return
            }

            FirebaseApp.configure(options: options)
        }

        configure(plistName: plistName)

        // In each app, set the access group to the keychain access group or app group
        Auth.switchToUsingAppGroup(accessGroup)

        Auth.migrateFirebaseAuthToAccessGroupIfNeeded(accessGroup)
    }

    private static func copyContainerFirebaseData() {
        do {
            let fileManager = FileManager.default
            guard let localFirebaseURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent("firebase") else {
                assertionFailure("Unable to get shared container")
                return
            }
            try PersistenceSyncManager.shared.copyContentsFromSharedTarget(to: localFirebaseURL)
        } catch {
            assertionFailure(error.localizedDescription)
        }
    }
}

private extension FirebaseAuth.Auth {
    static func switchToUsingAppGroup(_ accessGroup: String) {
        do {
            try Auth.auth().useUserAccessGroup(accessGroup)
        } catch let error as NSError {
            print("Error changing user access group: %@", error)
        }
    }

    // it basically checks if Auth is already using the correct userAccessGroup and
    // if so, nothing needs to be done.
    // If not, the existing user is temporary saved,
    // then Firebase Auth is configured to use the Access Group and then the user is moved to it.
    static func migrateFirebaseAuthToAccessGroupIfNeeded(_ accessGroup: String) {
        let auth = Auth.auth()

        // get current user (so we can migrate later)
        guard let user = auth.currentUser else {
            print("currentUser is nil")
            return
        }

        if auth.userAccessGroup == accessGroup {
            print("Firebase Auth is already using the correct acces group \(accessGroup)")
            return
        }

        print("Firebase Auth is not yet using correct access group \(accessGroup). 🔄 Migrating...")

        // for extension (widget) we want to share our auth status
        do {
            // switch to using app group
            try auth.useUserAccessGroup(accessGroup)

            // migrate current user
            auth.updateCurrentUser(user) { error in
                if let error = error {
                    print("Auth.auth().updateCurrentUser \(error.localizedDescription)")
                } else {
                    print("Firebase Auth user migrated")
                }
            }

        } catch let error as NSError {
            print("❌ Error changing user access group \(accessGroup): \(error.localizedDescription)")
        }
    }
}