Q:

How to change email address in Firebase? 

I have some problems with changing email address in firebase authentication. 
My code looks like this now:
func changeEmail(withEmail email: String, completion: @escaping ((Bool) -> Void)) {
    guard let currentUser = Auth.auth().currentUser, let email = mail else { return }
    currentUser.updateEmail(to: email) { [weak self]
        error in
        guard let self = self else { return }
        let title: String
        let message: String
        if let error = error {
            title = "alert.error.title".localized()
            message = error.localizedDescription
        } else {
            title = email
            message = "auth.confirm.email.popup".localized()
            currentUser.sendEmailVerification()
        }
        self.navigator.showAlert(title: title,
                                 message: message,
                                 bottomLeftTitle: "general.got.it".localized(),
                                 bottomLeftHandler: { completion(error == nil)
        })
    }
}

So it is okey, and working, and user can actually change email. 
But problem occurs when user stayed too long and needs to re-login.  Everyone knows that it is disturbing user experience in app. 
Auth.auth().reload() //not working in this situation. 

So how to change email, without asking user to logout and login again? 

A:

There is a reauthenticate method exactly for this purpose. 
https://firebase.google.com/docs/auth/ios/manage-users#re-authenticate_a_user
What you need to do is ask the user for its login credentials again.  No logout - login needed. 
Possible code for that:
if (self.newPassword == self.newPasswordConfirm) && (!( self.newPassword.isEmpty) || !( self.newUserName.isEmpty)) {
    reauthenticate(email: self.accountEmail, password: self.oldPassword) { isSucceeded in
        //Successfully authenticated
        if isSucceeded == true {
            if ! self.newUserName.isEmpty {
                // update username
            }
            
            Auth.auth().currentUser?.updatePassword(to: self.newPassword) { (error) in
                // Alert user that it didn't work
            }
            
            self.editProfile.toggle()
        }
        // Failed to reauthenticate
        else if isSucceeded == false {
            // Alert User
        }
    }
}

