Laravel: How to let user login with Email or Username

<?php

namespace App\Http\Controllers\Auth;

use App\Http\Controllers\Controller;
use Illuminate\Foundation\Auth\AuthenticatesUsers;

class LoginController extends Controller
{
    /*
    |--------------------------------------------------------------------------
    | Login Controller
    |--------------------------------------------------------------------------
    |
    | This controller handles authenticating users for the application and
    | redirecting them to your home screen. The controller uses a trait
    | to conveniently provide its functionality to your applications.
    |
    */

    use AuthenticatesUsers;

    /**
     * Where to redirect users after login.
     *
     * @var string
     */
    protected $redirectTo = '/home';

    /**
     * Create a new controller instance.
     *
     * @return void
     */
    public function __construct()
    {
        $this->middleware('guest')->except('logout');
    }
}

<?php

namespace Illuminate\Foundation\Auth;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Auth;
use Illuminate\Validation\ValidationException;

trait AuthenticatesUsers
{
    use RedirectsUsers, ThrottlesLogins;

    /**
     * Show the application's login form.
     *
     * @return \Illuminate\Http\Response
     */
    public function showLoginForm()
    {
        return view('auth.login');
    }

    /**
     * Handle a login request to the application.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\RedirectResponse|\Illuminate\Http\Response|\Illuminate\Http\JsonResponse
     *
     * @throws \Illuminate\Validation\ValidationException
     */
    public function login(Request $request)
    {
        $this->validateLogin($request);

        // If the class is using the ThrottlesLogins trait, we can automatically throttle
        // the login attempts for this application. We'll key this by the username and
        // the IP address of the client making these requests into this application.
        if (method_exists($this, 'hasTooManyLoginAttempts') &&
            $this->hasTooManyLoginAttempts($request)) {
            $this->fireLockoutEvent($request);

            return $this->sendLockoutResponse($request);
        }

        if ($this->attemptLogin($request)) {
            return $this->sendLoginResponse($request);
        }

        // If the login attempt was unsuccessful we will increment the number of attempts
        // to login and redirect the user back to the login form. Of course, when this
        // user surpasses their maximum number of attempts they will get locked out.
        $this->incrementLoginAttempts($request);

        return $this->sendFailedLoginResponse($request);
    }

    /**
     * Validate the user login request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return void
     *
     * @throws \Illuminate\Validation\ValidationException
     */
    protected function validateLogin(Request $request)
    {
        $request->validate([
            $this->username() => 'required|string',
            'password' => 'required|string',
        ]);
    }

    /**
     * Attempt to log the user into the application.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return bool
     */
    protected function attemptLogin(Request $request)
    {
        return $this->guard()->attempt(
            $this->credentials($request), $request->filled('remember')
        );
    }

    /**
     * Get the needed authorization credentials from the request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    protected function credentials(Request $request)
    {
        return $request->only($this->username(), 'password');
    }

    /**
     * Send the response after the user was authenticated.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\Response
     */
    protected function sendLoginResponse(Request $request)
    {
        $request->session()->regenerate();

        $this->clearLoginAttempts($request);

        return $this->authenticated($request, $this->guard()->user())
                ?: redirect()->intended($this->redirectPath());
    }

    /**
     * The user has been authenticated.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  mixed  $user
     * @return mixed
     */
    protected function authenticated(Request $request, $user)
    {
        //
    }

    /**
     * Get the failed login response instance.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Symfony\Component\HttpFoundation\Response
     *
     * @throws \Illuminate\Validation\ValidationException
     */
    protected function sendFailedLoginResponse(Request $request)
    {
        throw ValidationException::withMessages([
            $this->username() => [trans('auth.failed')],
        ]);
    }

    /**
     * Get the login username to be used by the controller.
     *
     * @return string
     */
    public function username()
    {
        return 'email';
    }

    /**
     * Log the user out of the application.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\Response
     */
    public function logout(Request $request)
    {
        $this->guard()->logout();

        $request->session()->invalidate();

        return $this->loggedOut($request) ?: redirect('/');
    }

    /**
     * The user has logged out of the application.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return mixed
     */
    protected function loggedOut(Request $request)
    {
        //
    }

    /**
     * Get the guard to be used during authentication.
     *
     * @return \Illuminate\Contracts\Auth\StatefulGuard
     */
    protected function guard()
    {
        return Auth::guard();
    }
}

/**
* Get the login username to be used by the controller.
*
* @return string
*/
public function username()
{
    return 'email';
}

/**
* Get the login username to be used by the controller.
*
* @return string
*/
public function username()
{
     $login = request()->input('identity');

     $field = filter_var($login, FILTER_VALIDATE_EMAIL) ? 'email' : 'username';
     request()->merge([$field => $login]);

     return $field;
}

/**
     * Validate the user login request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return void
     *
     * @throws \Illuminate\Validation\ValidationException
     */
    protected function validateLogin(Request $request)
    {
        $messages = [
            'identity.required' => 'Email or username cannot be empty',
            'email.exists' => 'Email or username already registered',
            'username.exists' => 'Username is already registered',
            'password.required' => 'Password cannot be empty',
        ];

        $request->validate([
            'identity' => 'required|string',
            'password' => 'required|string',
            'email' => 'string|exists:users',
            'username' => 'string|exists:users',
        ], $messages);
    }