Returning from threading/ GCD/ completion handler

Question

I have some logic to sign a user in from a login screen. If the login fails, I want to display a message to let the user know. If the user logs in successfully, I trigger a segue. When I test it with invalid credentials, the error alert displays as expected but the segue is still being triggered even though it's nested in an if-else statement. Why is this? How can I return out of it and avoid the else block...? I tried adding 'return' under the DispatchQ/ show alert code and that didn't have any effect.

self.login(username: usernameTextField.text!, password: passwordTextField.text!) { (error) in
        if error != nil {
            DispatchQueue.main.async {
                self.showAlert(msg: error ?? "error")
            }
        } else {
            DispatchQueue.main.async {
                // segue code
            }
        }
    }

login:

func login(username: String, password: String, completionHandler: @escaping (_ error: String?) -> ()) {
    SessionHelper.shared.logUserIn(withUsername: username, andPassword: password) { (error) in
        if let err = error {
            completionHandler(err)
        }
        completionHandler(nil)
    }
}

Answer 1

You're running the completion handler either way; if you get an error back, you're calling it, but then you fall through and run it with nil. Try this:

func login(username: String, password: String, completionHandler: @escaping (_ error: String?) -> ()) {
SessionHelper.shared.logUserIn(withUsername: username, andPassword: password) { (error) in
    if let err = error {
        completionHandler(err)
    } else {
        completionHandler(nil)
    }
}

}

Answer 2

Your login function calls completionHandler twice in the error case. The if falls through to the following statement. You should either put the following statement in an else block, or return from the true block.