Authenticate flutter app with keycloak and openid_client

Question

I'm trying to authenticate my flutter app to keycloak through openid_client

following the repo example, I've wrote an authentication function like this

authenticate() async {

  // parameters here just for the sake of the question
  var uri = Uri.parse('https://keycloak-url/auth/realms/myrealm');
  var clientId = 'my_client_id';
  var scopes = List<String>.of(['openid', 'profile']);
  var port = 4200;
  var redirectUri = Uri.parse('http://localhost:4200');

  var issuer = await Issuer.discover(uri);
  var client = new Client(issuer, clientId);

  urlLauncher(String url) async {
    if (await canLaunch(url)) {
      await launch(url, forceWebView: true);
    } else {
      throw 'Could not launch $url';
    }
  }

  var authenticator = new Authenticator(client,
      scopes: scopes,
      port: port,
      urlLancher: urlLauncher,
      redirectUri: redirectUri);

  var c = await authenticator.authorize();
  closeWebView();

  var token= await c.getTokenResponse();
  print(token);
  return token;
}

when I call the function, a webview popup appears and I can login through keycloak, but when the popup closes I get this error at the c.getTokenResponse():

Exception has occurred. NoSuchMethodError (NoSuchMethodError: The getter 'length' was called on null. Receiver: null Tried calling: length)

inspecting the Credential c, I can see that the TokenResponse has only "state", "session_state" and "code" fields

what am I missing?

Answer 1

I've been answered on github (link), so I'll copy the solution here:

---

On mobile devices you should use the PKCE flow. This is automatically selected when you omit the redirect uri in the Authenticator constructor.

So, it should be:

var authenticator = new Authenticator(client,
      scopes: scopes,
      port: port,
      urlLancher: urlLauncher,);

Make sure you add the uri http://localhost:4200/ (including the trailing slash) to Valid Redirect URIs in keycloak.

Make sure you add the uri http://localhost:4200/ (including the trailing slash) to Valid Redirect URIs in keycloak.